Bosch L-Jetronic: Technical Details
The purpose of this page is to acquaint the surfer / VW enthusiast who happens upon it with the name, general appearance, and function of each of the components used in the L-Jetronic EFI system in Type I Volkswagens, as well as to educate the surfer / enthusiast as to the operation of the L-Jetronic system as a whole. The page consists essentially of two parts, the first of which will walk you through the system as a whole and the second of which will provide more of an in-depth look at each of the system's components. Unfortunately, my web-authoring skills are extremely crude, so there are no "html#" indexes here. Sorry.
Part One: An Overview of the Bosch L-Jetronic System
The Bosch L-Jetronic electronic fuel injection system is an air flow control (AFC) system, meaning that the primary variable it uses to determine the amount of fuel that is to be delivered with each injection pulse is...you guessed it: air flow. With every half-cycle of the engine's cylinders (ie, every 360-degree rotation of the crank, or 180-degree rotation of the distributor), the electronic control unit (ECU) re-evaluates the volume of air entering the system and, after figuring in other important variables such as engine temperature, engine speed, and throttle position, decides whether to admit an amount of fuel greater, lesser, or equal to that which was delivered during the previous injection pulse. To put it another way, all things being equal, air flow, and not the driver's right foot, is what determines fuel flow in the L-Jetronic AFC EFI system.
Of course, the driver's right foot does have a lot to do with it. After all, when you step on the gas pedal of an AFC EFI car, you open the engine's throttle butterfly, which in turn admits more air into the system, which then triggers the ECU to admit more fuel into the system. However, the fact remains that in an AFC EFI automobile, when you step on the gas, all you are really doing is opening a butterfly. The ECU handles the rest. What this means is that, depending on engine load, ambient air temperature, and a number of other factors, the amount of fuel which enters the engine of an AFC EFI-equipped car might vary from pulse to pulse, even if the amount of air entering the system remains the same. In other words, by severing the direct link between the driver's right foot and the fuel delivery system, AFC EFI systems are able to ensure that the air/fuel ratio delivered to the engine's cylinders is as precise as it can possibly be.
(As a brief aside, it's worth mentioning that this differs markedly from the way in which carbureted engines handle the task of air/fuel delivery. In a carbureted automobile, when you step on the gas, you open not only the carb's throttle butterfly, but also its fuel metering device, the accellerator pump. What this means is that the movement of the driver's right foot directly controls both the amount of air and the amount of fuel entering the engine. Cycle-to-cycle fuel variations are therefore out of the question, which means, in turn, that the air/fuel mix delivered to each of the engine's cylinders will be far less precise in a carbureted automobile than on a comparable fuel-injected model. But, I digress. We're here to talk fuel injection, not carburetion. So, without further ado, let's get back to the task at hand...)
In a fuel-injected motor, fuel is delivered, as you might have suspected, through a set of electromechanical valves known as fuel injectors. The L-Jetronic system is a multi-point fuel injection system, meaning that there is one such injector for each of the engine's cylinders. However, these four injectors do not operate independently; rather, at every other break in the distributor's points (ie, once per 180-degree rotation of the distributor rotor, or once per 360-degree rotation of the engine's crankshaft assembly), the ECU, having already calculated the amount of fuel needed by the engine at that particular moment, sends an electrical signal to the injectors, opening all four of them simultaneously, just long enough to allow the predetermined amount of fuel to pass through them and join the stream of air being sucked into the cylinders.
These fuel injectors don't simply 'dump' the fuel into the airstream; instead, they actually 'spray' the fuel into the stream, vaporizing the fuel in preparation for the combustion process. In order to do so, however, the fuel must exit the injector under pressure--it must, in other words, be forced through the injector's rather narrow nozzle opening. In order to pressurize the fuel, the L-Jetronic system uses a high-pressure electric fuel pump to scavenge fuel from the tank and deliver it to each of the engine's injectors under pressure. Unless the engine is idling, this pressure is kept more or less constant so that the only factor determining the amount of fuel which passes through the injectors during each half-cycle is the length of time the ECU signals the injector to remain open (this length of time is known as the 'injection pulse'). Think about it: if the fuel pressure were allowed to fluctuate, then the injection pulse would hardly matter--a hypothetical pulse of 1 second at 35psi, for example, would deliver a helluva lot more fuel than a pulse of 1 second at 10psi. At all engine speeds above idle speed, this pressure is maintained by a fuel pressure regulator, but at idle, the pressure regulator redirects more fuel back to the tank, lowering fuel pressure levels so that a leaner mixture (ie, less fuel per unit of air) can be delivered to the cylinders (at idle, a leaner mixture makes for a cleaner-running engine, and since the load on the engine at idle is minimal, a leaner mixture is OK).
In summary, then, the basics are as follows. Air entering the system is measured, and, based primarily upon that measurement, the ECU determines how much fuel should accompany it into the engine itself. Stepping on the gas admits more air into the system, a change for which the ECU compensates, all things being equal, by admitting more fuel into the engine. Letting off the gas reduces the air entering the engine, a change for which the ECU compensates, again, all things being equal, by admitting less fuel into the engine. The ECU is able to control the amount of fuel entering the engine by varying the length of the injection pulse. An electric fuel pump delivers pressurized fuel to the injectors themselves, and a fuel pressure regulator ensures that adequate pressure is maintained throughout the rpm band.
The theory is simple, but the details are a bit more complex. Read on to find out exactly how each of the L-Jetronic system's individual components work--and how exactly it is that they manage to work together--to realize the simple induction plan I've outlined above. Trust me--it's cool.
Part Two: A Detailed Look at the Bosch L-Jetronic System
Perhaps the best way to introduce you to the specifics of the L-Jetronic system is to throw up the dreaded schematic. If you're a VW buff, chances are you've seen this one before, as it has been reprinted in about a million different places (catalogs, magazines, manuals, etc) since first appearing in the Bentley Manual years ago. Rarely, however, have those who have chosen to reprint it bothered to adequately label and explain it. So, what I'm going to do here is to show the schematic, accompanied by a numbered index of the parts shown in it. Then, I'll take an in-depth look at each of the components listed in the schematic (including real-world photos of them) and an in-depth look at a couple of the components conveniently left off of the schematic. So, here goes...
1. Fuel Filter
2. High-Pressure Electric Fuel Pump
3. Fuel Pressure Regulator
4. Cold Start Valve (aka "the 5th injector")
5. Fuel Injection Valve
6. Auxiliary Air Regulator (aka "the air bypass valve")
7. Air Flow Sensor (aka "the air flow meter")
8. Throttle Valve Housing (aka "the throttle body")
9. Intake Air Distributor (aka simply "air distributor")
10. Temperature Sensor I (1976 and later cars only)
11. Thermo-Time Switch
12. Potentiometer with Fuel Pump Switch (part of the Air Flow Sensor, really...)
13. Throttle Valve Switch (not used in 1977 and later California models)
14. Resistor Unit
15. Temperature Sensor II
16. Electronic Control Unit (aka "ECU," "the Computer," "CPU," "the Black Box," etc.)
17. Ignition Distributor (the unnumbered unit below and to the left of #17 in the diagram is, of course, the ignition coil)
1. The Fuel Filter
The fuel filter used in the L-Jetronic system does just that: it filters fuel. Duh, right? Anyway, the filter mounts just below the fuel tank on the passenger side of the car, filtering from the fuel any and all particles larger than 10 microns before--and this is important--before the fuel is allowed to enter the electric fuel pump. In other words, the sequence is as follows: tank --> filter --> pump. Perhaps the most curious feature of the filter (as with the electric fuel pump, as we'll see) is that, while the fuel enters the filter through a 7mm high-pressure line from the tank, it exits the filter through a 13mm high-pressure line. That's right...the fuel line fittings on the filter are different! (See Rebuilding the Bosch L-Jetronic EFI System for more on this curiosity.)
2. The High-Pressure Electric Fuel Pump
By means of a series of rollers spun by a permanent-magnet electric motor, the fuel pump delivers a steady supply of pressurized fuel to the L-Jetronic fuel system. Gasoline from the tank, having passed through the fuel filter, enters the electric pump through a 13mm fitting and is expelled under pressure from a 7mm fitting at the opposite end of the pump. The pump mounts on a bracket just below the fuel tank on the passenger side of the automobile, and is controlled not, as is the case with most low-pressure electric pumps, by current from the ignition circuit, but rather by an electrical signal from the air flow meter. When the air flow meter tells it to pump, it pumps; when it tells it to stop, it stops (more later). When the fuel exits the pump, it is sent through a short stretch of 7mm line to a steel fuel line which runs through the car's backbone tunnel to the rear of the vehicle, where the engine is located. At the rear of the car, adjacent to the transmission, the steel line ends, and another short stretch of 7mm line takes the pressurized fuel through the firewall and into the engine compartment, where it enters another steel fuel line, the fuel rail (represented, but not labelled, in the diagram above). The fuel rail delivers fuel to each of the motor's four fuel injection valves as well as to its cold start valve; the rail also delivers fuel to another device, the fuel pressure regulator.
3. The Fuel Pressure Regulator
The fuel pressure regulator's job is to maintain the fuel pressure within the fuel rail at approximately 30 psi at idle and at approximately 35 psi at all other engine speeds. It consists of two chambers, a fuel chamber and a vacuum chamber. The fuel chamber has two fittings, one through which fuel enters the device from the fuel rail and one through which it exits it to the vehicle's fuel-return lines. The vacuum chamber has a but a single fitting, a vacuum connection which hooks up to the intake air distributor. At idle, when vacuum levels in the intake air distributor are higher than they are at other engine speeds, the vacuum chamber of the fuel pressure regulator will contract so that the spring-loaded flap in the fuel chamber will allow enough fuel to pass through the regulator to maintain 30 psi in the rail (the fuel which passes through the regulator is, of course, sent back to the fuel tank at the front of the car through a return line). When you step on the gas, vacuum levels in the intake air distributor will fall, and the spring-loaded flap in the vacuum chamber will open up. This partially closes the fuel chamber of the pressure regulator, allowing less fuel to be sent back to the tank. This means, of course, that the fuel pressure in the fuel rail will rise--to approximately 35 psi. Step on the throttle a little bit more, and the vacuum chamber of the pressure regulator will open even more, allowing even less fuel to be returned to the tank, but--and this is important--the pressure in the rail remains at 35 psi, in spite of the greater return restriction, because of the simple fact that the fuel injectors deliver a helluva lot more fuel to the motor's cylinders at higher engine speeds than they do at lower ones. Release the throttle, and as the engine slows, vacuum levels in the air distributor will slowly rise, gradually decreasing the volume of air in the pressure regulator's vacuum chamber until, at idle, it is once again closed and the spring-loaded flap in the fuel chamber once again is able to allow enough fuel to be returned to the tank to maintain the idle-speed fuel pressure of 30 psi. It's simple, really.
4. Cold-Start Valve (aka "5th Injector")
As its nickname implies, the Cold Start Valve is essentially an additional or "fifth" fuel injector, a valve which, like all of the others, is connected to the engine compartment fuel rail. It is an electromechanical valve which delivers additional fuel (ie, in addition to that delivered by the four standard injectors) directly into the air distributor, but only when the engine is being started, and started cold (hence "cold start" valve). Essentially, the cold start valve operates at the behest of the thermo time switch (discussed in detail in section 11, below): if the thermo time switch tells it to open, it opens, admitting additional fuel into the system; otherwise, the valve remains closed. Unlike the engine's other four injectors, however, the cold start valve does not pulse--it does not, in other words, deliver one squirt of fuel at a time. Rather, the cold start valve emits a steady spray of fuel into the system for as long as the thermo time switch signals it to do so. The ultimate purpose of the cold start valve, of course, is to provide additional fuel to the system during cold start-ups, when the engine requires a richer fuel mixture. For more info, scroll down to section 11, which explains the operation of the thermo time switch and, in turn, the operation of the entire cold start system in greater detail.
5. The Fuel Injection Valves
In many ways, the fuel injection valves represent the core of the system. They are the electromechanical (ie, solonoid) valves which are responsible for delivering fuel to the engine. At the tip of the valves, there is a small curcular opening which is plugged, when the valve is closed, by a small needle. When the valve opens, it does so by retracting the needle away from the injector tip, opening the end of the valve and allowing fuel to pass through it. The Bosch L-Jetronic system, like any other multi-point fuel injection system, makes use of one such valve per cylinder. That means, of course, that on an aircooled VW Type I motor there are four such injectors, but, as I mentioned in Part One of this page, these injectors do not operate independently. Instead, the ECU provides an electrical signal to all four injectors once per 180-degree rotation of the distributor telling them how long they should open; all four receive this signal at the same time, and all of them open at the same time. When open, the valves introduce a spray of pressurized gasoline into the intake airstream; accordingly, they are mounted just above the engine's intake valves and are connected to the pressurized engine compartment fuel rail. (A word is perhaps in order regarding the fact that all of the injectors open at once rather than independently. My guess is that VW did this in order to make the system less complex, reducing the number of calculations the ECU needs to perform every second and eliminating the electrical complexity of individually-operated injectors. This was, after all, the mid-1970s. At any rate, if you sit down and figure it through, this means that each cylinder receives two squirts of fuel per cycle--and by cycle I do mean the full intake, compression, fire, and exhaust cycle of the piston--since each of the cylinders does one full cycle per 360-degree rotation of the distributor (720 on the crank).)
6. The Auxiliary Air Regulator (aka "bypass valve")
OK. So far, the components I've discussed have all had something or another to do with the engine's fuel supply. With this device, which I prefer to call the air bypass valve, we shift our focus to the components of the L-Jetronic air supply system. In section 4, above, I mentioned the fact that, when cold, the engine requires something of a richer mixture in order to get started, a richer mixture which the cold start valve dutifully supplies. This is true, but it's only part of the story. Once the engine has started, it's only a matter of seconds before the thermo time switch signals the cold start valve to close; the engine, of course, will still be in the process of warming up. In other words, the richer mixture provided by the cold start valve is only needed for a brief period during and immediately following a cold engine start-up. After that brief period has ended, the engine actually requires additional air in order to maintain an acceptable, steady idle while the car is completing its warm-up period. Remember--we're only talking about a cold, idling engine here...it's important to keep this in mind. Anyway, any time the engine is idling, the throttle butterfly is closed, admitting very little air into the system. When the engine is cold, however, this device--the air bypass valve--opens up, allowing additional air to enter the engine. A bimetallic strip inside the device opens and closes the valve according to engine temperature: if the engine is cold, it swings the valve open, and if the engine is warm, it swings the valve closed. In addition, a ceramic heating element inside of the air bypass valve helps to ensure that the bimetallic strip heats up more or less in synch with the rest of the engine. Air enters the bypass valve through a small diameter air hose attached to the air cleaner, and if the valve is open, it then passes into the intake air distributor, bypassing the closed throttle butterfly. Once the engine has warmed up, the air bypass valve closes. It's really a lot simpler than I've made it sound--just remember this: when the engine is cold, the air bypass valve lets more air in, and once the engine has warmed up, the bypass valve stops letting more air in.
7. The Air Flow Meter
The air flow meter is the device responsible for measuring the volume of air which enters the system. Accordingly, all of the air which enters the system passes through this device first. Air for the air bypass valve (as above) and the decel valve (not in the schematic, but see below, section 18 for more info on it) is sent to these devices only after first being measured by the air flow meter, and it is only after having passed through this meter that air enters the intake air distributor through the throttle butterfly. Inside of this device, there's a springloaded, counterweighted flap attached to a potentiometer contact. As air enters the system, this flap is swung open; the more air that enters the system, the wider it opens up. Through the potentiometer, the device itself sends a continuous (ie, uninterrupted) electrical current to the ECU, a current which the ECU interprets as a measure of incoming air. The voltage of this current varies according to the degree to which the counterweighted flap in the air flow meter is opened at any given time, so that, as the flap opens and closes, the ECU "knows" how much air is entering the system and can adjust the fuel supply (ie, by adjusting the length of the injection pulse) accordingly. In 1976 and later cars, this device also contains an engine temperature sensor ("Engine Temperature Sensor I"), and in all cars, the air flow meter's potentiometer is also directly responsible for controlling the operation of the electric fuel pump. On aircooled Type I motors, this device is mounted between the air intake boot and the air cleaner, just above the alternator.
8. Throttle Valve Housing (aka "throttle body")
Basically, this is just a round tube with a round flap in the middle. The round tube is the throttle valve housing (or "throttle body," as I prefer to call it), and the round flap is the throttle valve (which I like to call the "throttle butterfly"). The butterfly is linked to the throttle cable, which is connected to the gas pedal. When you step on the gas, the pedal pulls on the throttle cable, and the throttle butterfly opens, admitting more air into the system. When you let off the gas, the butterfly closes, and less air enters. It's that simple. Well, almost that simple. You see that screw on the front of the throttle body? That's the bypass screw, a screw which controls the extent to which a tiny air bypass passage in the throttle body is opened. This tiny air bypass passage allows a small amount of air to go around the butterfly, so to speak, when the butterfly is closed (at idle, primarily). Accordingly, this screw is used to adjust the engine's idle speed. If you turn it clockwise, you allow less air to bypass the throttle butterfly at idle, slowing the engine's idle speed. If you turn it the other way, more air is able to bypass a closed throttle, and the engine will idle faster. The throttle body mounts on top of the intake air distributor, right next to the alternator.
9. The Intake Air Distributor
The intake air distributor is a hollow box through which the engine's intake air passes on its way from the throttle body to the left and right intake manifolds, which in turn deliver it to the engine's cylinders. Integral with the air intake distributor are the oil filler tube and the alternator stand. Rubber boots link it to the left and right intake manifolds, and both the cold start valve and the throttle body are directly attached to it. In addition, the intake air distributor has a single vacuum fitting (to which the EGR valve, the decel valve, and the fuel pressure regulator are attached in series) as well as a set of small air intake passages through which air from the air bypass valve and the decel valve pass on their way into the engine. Thus, although the intake air distributor has no moving parts of its own, it is in many ways the physical hub of the L-Jetronic system.
10. Temperature Sensor I (1976 and later cars only)
Sorry, but I haven't got a picture of Temperature Sensor I to post since it's mounted inside of the air flow meter, adjacent to the potentiometer. Basically, Temperature Sensor I measures the temperature of the air entering the engine, and, based upon those measurements, it influences the voltage of the current which the air flow meter's potentiometer sends to the ECU. In other words, it mixes a second variable--engine air temperature--with the air flow meter's air volume measurements so that the temperature of the air entering the engine (and not just its volume) can be taken into account by the ECU when it decides how much fuel the injectors should deliver.
11. The Thermo Time Switch
The Thermo Time Switch works in conjunction with the cold start valve, but it mounts to the bracket that holds the air bypass valve onto the engine, smack-dab in the middle of the engine. The thermo time switch contains a tiny electric switch governed by a bimetallic strip of metal, a strip of metal heated both by the heat of the engine and by means of an internal heating element. Basically, it works like this: if you start the engine when it's already hot, the bimetallic strip in the thermo time switch will have severed its internal connection, breaking the cold start valve's ground circuit so that the cold start valve does not open. If you start the engine while it's cold, on the other hand, the bimetallic strip will have turned such that the switch inside the thermo time switch is closed--completing the ground circuit and allowing the cold start valve to operate. Once the engine has been started, the switch's internal heating element will heat the bimetallic strip so that the electrical switch will cut off after a predetermined number of seconds have passed (a number of seconds marked on the outside of the thermo time switch itself), cutting off the cold start valve. In a nutshell, then, the thermo time switch governs the operation of the cold start valve, plain and simple.
12. Potentiometer with Fuel Pump Switch
I haven't got a picture of this one either, since, like Temperature Sensor I, it is a part which resides within the air flow meter itself. Basically, what the potentiometer does is to translate the movement of the air flow meter's air metering flap into a continuous electrical signal of continuously varying voltage, a signal which the potentiometer sends to the ECU and which the ECU uses to determine the engine's cycle-by-cycle fuel needs. The extent to which the air flow meter's air metering flap is swung open by the incoming intake air at any given moment determines the voltage of the signal which the potentiometer sends to the ECU at that moment; hence, the potentiometer does indeed "translate" the mechanical movements of the air flow meter's parts into the electrical signal used by the ECU such that varying amounts of intake air can be "read" by the ECU in terms of varying voltage levels. The potentiometer also contains a set of contacts which govern the operation of the electric fuel pump. When the air flow meter's air metering flap is closed (ie, when the engine is turned off), the potentiometer cuts off power to the fuel pump; when the flap opens, even very slightly (as at idle), the fuel pump once again begins to operate. This prevents the fuel pump from operating when you have the key turned to "on" but the engine turned off--in other words, it ensures that the fuel pump operates only when the engine itself is running.
13. The Throttle Valve Switch
Sorry about the photo, folks...the throttle valve switch is the black box attached to the throttle body in this photo. It just never occurred to me to take a picture of the throttle valve switch by itself. Anyway, the throttle valve switch attaches to the throttle body (as in the picture), and it contains an internal rotary switch which moves with the movements of the throttle butterfly (it is physicaly linked to the throttle butterfly). In 1975 and 1976 California model Type I VWs and in 1975, 1976, and 1977 49-state model Type I VWs, the throttle valve switch has three functions. The first, quite simply, is to communicate to the ECU the extent to which the throttle butterfly is open. Its second function is to control the operation of the EGR valve such that when the throttle butterfly is either completely closed (as at idle) or wide open (as in, when the driver floors it), the EGR Valve shuts down; only at partial throttle does the throttle valve switch permit the EGR valve to open. The third and final function of the throttle valve switch is to provide full-load enrichment, which simply means that if the driver floors it (ie, if the throttle butterfly swings fully open), the throttle valve switch signals the ECU to provide a bit more fuel to the cylinders in order to achieve the maximum possible amount of all-out top-end power. On 1977 California models, VW switched to a fully-mechanical EGR Valve, dropping the throttle valve switch entirely and installing a tiny switch--called, appropriately, the "microswitch"--to take over the throttle valve switch's full-load enrichment function (the microswitch was dropped the following year). On 49-State 1978 and 1979 engines, the throttle valve switch continued to control the operation of the EGR Valve, but the switch's other functions were dropped, according to the Bentley Manual, simply by leaving a couple of the terminals in the switch empty.
14. The Resistor Unit
I haven't got a real-world picture of this one either, so I cheated and scanned one out of a Rocky Mountain Motorworks aircooled catalog. The resistor unit mounts inside the engine compartment, on the passenger-side firewall, adjacent to the point at which the evap system hoses pass into the engine compartment. The resistor unit's only job is to reduce the voltage that the ECU supplies to the fuel injectors. Very simple.
15. Temperature Sensor II
Contrary to what its name implies, Temperature Sensor II is the L-Jetronic engine management system's primary temperature sensor. On aircooled Type I motors, temperature sensor II screws into the driver's side cylinder head, next to the intake manifold, where it tracks the engine's temperature and reports it to the ECU. It's a very small, very simple, but also very important part of the L-Jetronic system.
16. The ECU (Electronic Control Unit)
The ECU is the L-Jetronic system's "brain." Signals from the air flow meter, throttle valve switch, engine temperature sensor, ignition distributor, and other parts of the system are all sent to this central box, which is mounted in the car's rear luggage compartment on the passenger side. The ECU uses the data from these signals to determine the injection pulse, continuously adjusting the amount of fuel that the system's four main injectors deliver to the engine. Adjacent to the ECU is another part which, though not labeled on the L-Jetronic schematic diagram, certainly merits a few words here: the Double Relay, whose job it is to regulate the battery-circuit voltage supplied to the various parts of the L-Jetronic system.
17. Ignition Distributor
Folks, do I really need to show you a picture of this one? And do I really need to go into detail about what it does and how it works? I think not. But I do think a few brief words about the distributor and its role in the L-Jetronic system are in order. The distributor is vital to the system not only because it supplies the spark that fires the engine, but also because it supplies the ECU with a measurement of the engine's rpm; a simple wire connection to the ignition coil supplies this vital signal. It's also worth mentioning that the L-Jetronic system uses--and must use-- a double-vacuum distributor. In other words, you cannot use a "high-performance" 009 or 010 distributor with the system. It just won't work.
18. The Decel Valve (aka "Vacuum Limiter," not shown in the schematic)
The Decel Valve is a part of the L-Jetronic system's integrated emissions control system. If it helps, think of it as a second air bypass valve, since what it essentially does is just that: it allows additional air to bypass a closed throttle--but only under certain conditions (or, if you're familiar with carbureted-model VW emissions control devices, you might want to think of the decel valve as an EFI-version of the tried and true "throttle valve positioner"...their roles are the same, and the ways in which they operate don't really differ all that much). Here's how it works. If you're driving along (let's say...at 65mph in 4th gear) and you suddenly let off the gas, the throttle butterfly will suddenly close, depriving the engine of much of its air supply even though it's still running way above idle speed (65 in 4th is roughly 3,500 rpm). Assuming of course that you haven't also pushed in the clutch, what you've managed to do by letting off the gas at speed is to create a lot of vacuum in the intake air distributor (since the engine's trying to suck in a lot more air than it can get either through the throttle body's bypass screw or by forcing the throttle butterfly to remain partially open). Since there's a lot of vacuum, the fuel pressure regulator will still be supplying roughly 35 psi to the injectors, and in the absense of sufficient air with which to burn it, a lot of fuel is not what you want. Not unless you like breathing unburned hydrocarbon emissions, that is. Anyway, my point is that it is much more difficult for the system to lean out the mixture under these circumstances than it is at idle, when fuel pressure is able to drop to roughly 30 psi. So, instead of attacking the problem from the fuel angle, VW instead chose to attack it from the air supply angle with the decel valve. Thus, when you let off the throttle in this way and create the levels of vacuum I'm considering, the decel valve snaps open, admitting additional air into the intake air distributor with which the engine is then able to burn the excess fuel--reducing the engine's drop-throttle hydrocarbon emissions. The decel valve consists of two chambers, a vacuum chamber--connected by a small-diameter vacuum line to the intake air distributor--and an air chamber. When vacuum levels in the intake air distributor become too great, the vacuum chamber within the decel valve will contract, which in turn opens a passage in the air chamber through which the additional engine air passes. The valve takes its air from the same fitting (on the intake boot, downstream of the air flow meter) as does the air bypass valve, and it admits its air into the intake air distributor in roughly the same place that the air bypass valve admits its air into the intake air distributor. The decel valve mounts on the driver's side engine compartment hinge, and was only fitted to manual transmission cars (since Autosticks automatically return to a lower rpm when you let off the gas at speed).
19. The EGR Valve (not shown in the schematic)
The Exhaust Gas Recirculation (EGR) Valve (the round, brass-colored part in the center of this picture) is another component of the L-Jetronic system's integrated emissions control system, designed to reduce the vehicle's NOx emissions. NOx emissions are an unfortunate byproduct of the heat of the combustion process, but by recirculating a tiny portion of the engine's exhaust back into the throttle body (back into the incoming supply of fresh air, that is), NOx emissions can be kept in check. As far as I understand it, this is because the presence of exhaust gasses in the engine's cylinders during the combustion process somehow actually lowers the temperature of the combustion process itself, preventing the cylinders from heating to the point at which serious levels of NOx are generated. However, recirculated exhaust fumes are generally undesirable when the engine is cold (where they slow the engine warm-up process), when the engine is at idle (where they can cause the car to stumble or stall), and when the engine is at full throttle (where they can prevent the engine from achieving its maximum horsepower levels), so the trick with any EGR system on any car is to make sure that these exhaust gasses are only recirculated when the engine is both warm and operating at partial throttle. On all 49-State L-Jetronic Type I engines (and 1975 and 1976 California models), an electro/vacuum EGR Valve is responsible for regulating the recirculation of exhaust gasses under these constraints. When the engine is cold, a bimetallic strip within the valve prevents it from opening until the engine warms up. Once warm, the valve is regulated both by an electrical signal from the ECU and by a vacuum signal from the intake air distributor. At idle, the throttle valve switch sends a signal to the ECU indicating that the throttle butterfly is closed, to which the ECU will respond by cutting off the current sent to the EGR Valve (preventing it from opening at idle). At full throttle, the throttle valve switch sends a signal to the ECU indicating that the throttle butterfly is wide open (meaning that the engine is operating under full-load conditions), to which the ECU also responds by cutting off the current to the EGR valve (preventing the valve from opening at full throttle). At partial throttle, however, the EGR Valve receives sufficient current from the ECU to open, allowing a certain amount of engine exhaust to re-enter the incoming airstream through an opening in the rear (ie, next to the fan shroud) of the throttle body. The amount of gas which the EGR Valve allows to enter the engine in this way is regulated by the EGR Valve's vacuum apparatus, which is connected to the intake air distributor by a small vacuum line. The exhaust gasses which pass through the EGR Valve and enter the rear of the throttle body are scavenged from the driver's side of the exhaust system, adjacent to the muffler. The gasses then pass through a square-ish filter (to remove as much exhaust "soot" as possible) before entering the EGR Valve itself. (1977, 1978, and 1979 California models use a mechanical EGR Valve which, to the best of my knowledge, is operated by the throttle mechanism itself.) L-Jetronic EGR Valves are one of the few components of the L-Jetronic system which is no longer available new. (Trust me...if you call the dealer and ask for one of these, they'll just laugh. Out loud. At you.)
OK. I've now covered each of the major components of the Bosch L-Jetronic fuel injection system as thoroughly as I am able. Hopefully, my descriptions have helped you to get a sense not only of how these individual components work, but also of how they work together, as a system.
When faced with the decision of how exactly I should go about setting this page up, I struggled with a chicken-and-the-egg sort of dilemma: you cannot understand the system without first understanding its components, but you also cannot fully understand these components without first understanding the system. My point is simply this: before you go sprinting out to the garage to tinker with your Beetle's fuel injection system, I think that you really need to have a good handle on the system and its parts, plain and simple. That's why I've posted this page, and I sincerely hope that it helps.
To get a better idea of how these parts fit together spatially (ie, how the system as applied to the Type I motor actually goes together), take a look at Rebuilding the Bosch L-Jetronic EFI System. It's a page which documents the process of restoring the L-Jetronic system on an automobile which has at one point or another been converted to carburetion. Where I come from, at least, such conversions are very common--I'd even venture to guess that they're the norm. Unfortunately, as smog laws continue to tighten across the country, many of these ex-injected cars will be forced off the road unless their owners can manage to restore their often long-discarded Bosch EFI systems. Rebuilding the Bosch L-Jetronic EFI System is therefore intended to be a guidebook (or at least the first tentative steps toward the creation of a guidebook) for those who need to go through with such a project.
P.S.--I almost forgot! Here's an L-Jetronic system wiring diagram. The scan's kinda crappy, but it's all there. I hope it helps, too.
VWs - VW Tech - Bosch L-Jetronic EFI - 1978 Superbeetle Convertible - 1967 Microbus -1998 New Beetle
Why? - Type I - Type II - Type III