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Volkswagen
Volkswagen: Fuel Injection System Service
edited from an article by Larry Bailly, ImportCar, April 2000
Over the past 10 years, Volkswagen has repositioned itself in the U.S. auto market as a major player in the import sector with the introduction of several sporty, updated models. This resurgence has fueled the popularity of older models, as well. What this means to the aftermarket is that import service specialists can expect to see an increasing number of VWs coming in for repairs, that can potentially represent a significant part of your business.
While the first Rabbits arrived in 1975, there are still a multitude of water-cooled VWs models in service — some are more evident in certain parts of the country than others. The first fuel-injected cars were introduced in 1977 and, since then, all but a very few models have been exclusively fuel-injected.
With the variety of models, engine configurations and engine management systems offered by VW, it’s important to know how the vehicle you’re working on is equipped: 4-cylinder or VR6 powerplant, with 2, 4 or 5 valves per cylinder, with SOHC or DOHC, supercharged or not, and with anywhere from 1.5 to 2.8 liters.
Then there are the various fuel-injection configurations — Continuous Injection System (CIS), CIS-E, CIS-E Motronic, Digifant I and II and various California versions. Which type is on your customer’s vehicle? You need to check the VIN data on the door post for country of origin, and date of production and model, and then check the EPA sticker under the hood before referring to a service manual (I prefer the Robert Bentley/factory-authorized manuals) or your computerized repair system. It’s also important that you carefully check for the engine code, as this will usually narrow the choices of engine management systems.
Beyond basic diagnosis, you will need specs for fuel pressures, temperature sensors, pin charts and wiring diagrams, all of which are available in the Bentley books. They are broken down by vehicle series — the 1985-’92 Golf and Jetta are in one book, the Cabrio and Scirocco in another, Corrado and Passat are in their own book, as well.
FIRST THINGS FIRST
For basic diagnosis, you can get by with a couple of jumper wires, a test light (preferably an LED-type) and a low impedance volt/ohmmeter for computer-equipped cars. VW has available special tools to conduct the tests, but the Bentley manuals give readily available substitutes and even some hints at easily fabricated test equipment.
The most common problem you’ll encounter with a VW is the "tow-in, no start car." Should you push it into the shop? Not yet. First, I like to take a few moments with test jumper in hand and check the car in the lot before trying to move it. Too often, just moving the car fixes it and, without OBD, you may not find the problem until the next time the car is towed in.
I usually get into the car, wait for some quiet time (when the frequent freight train passes) and then turn the key to the run position, listening for the click of the relay under the dash and a sometimes loud, sometimes soft "buzz" noise from the rear seat/trunk area. The buzz should last only for a few seconds then stop, until the key is turned briefly to the start position, which should again energize the relay and the fuel pump(s).
If there is no click or buzz, this is the time to get down on your knees to locate the fuel pump relay. It is usually on the fuse/relay board under the driver’s side of the dash. The manual will help you locate the correct relay, but don’t always assume you’ll find it where it’s supposed to be, because the factory relocated them and issued service bulletins for the early models.
Once you’ve located the relay, use your fused jumper to "jump" the circuit after removing the relay, and again listen for the fuel pump(s). With the circuit jumped, the pump(s) should run continuously. Then, if the car starts, you may just need to replace the relay and send the car on its way, but don’t.
Although the relays are known to fail, many other factors can cause them to fail prematurely. I always perform a circuit load (amp draw) test to make sure the load wasn’t too much for the relay. This can be done with a pair of jumpers and an amp tester that will read at least 20 amps (more is better, as I’ve seen spikes as high as 30 amps from bad pumps) connected across the relay terminals.
Normal draw for a single-pump system is anywhere from 4 to 8 amps running, and up to 12 amps as the pump starts. (Early relays have a 16-amp fuse attached to the top. I don’t recommend substituting a larger one because a bad pump can take out the relay and the wiring to the panel and more!)
On later two-pump systems, the running draw should not exceed 10 amps even though the relays have heavier circuitry. If the system checks OK and the car seems to run fine, road test it and, if the buzzing noises aren’t excessive from the back of the car, you can usually let the car go after offering a bit of advice to the customer. Tell him or her that because the fuel pumps on these cars are lubricated and cooled by the fuel that passes through them, they should be conscious of the quality of the fuel they use and shouldn’t consistently run the tank low. Loss of fuel at the pump can cause very high load spikes that, over time, may cause the pump or relay to fail.
Another common cause of relay and fuse board problems is water leaks onto and into the relays and fuse panel. In this case, repairs are pretty straightforward after addressing the source of the leaks.
FUEL PUMPS
Starting in 1985, almost all models had two pumps: a "pre-pump" in the tank and a "main pump" under the car. The main pump is mounted under the vehicle near the right rear wheel, along with the filter and, on some models, other system components such as the accumulator or a damper to quiet the pump’s pulses.
The in-tank pump is accessible through a cover in the trunk or rear cargo compartment, and is either held in with a tab/twist system or a screw-on cap, which is the case on plastic tanks (the pump and sender are usually spring-loaded and may "pop-up" when removed).
Some pumps on certain model year Golfs, Jettas and Cabrios were recalled, so if the original crimp-type clamps are still intact, a call to the local VW dealer may save the customer some dollars that also "buy " good will for you. Earlier cars with one pump usually have the pump encased in a foam insulator and mounted on rubber mounts. This setup is for good reason, as they can be quite noisy if the parts are not reinstalled as designed.
Over the years, we’ve had many cars towed in because of a situation where the customer just had the pump replaced with a "new’ one and the car still wouldn’t run. In most of these cases, either the relay was the real problem or a "cheaper" substitute pump (one that wouldn’t produce the pressures needed for the system) was used. With that said, it’s important to use quality, model-specific replacement parts.
The mid-’80s models have their main pumps mounted in a plastic reservoir under the car. Use extreme care when replacing the filter, which is also mounted to this box. It’s easy to crack the housing, which is not repairable and not cheap, either.
Another learned tip is to always replace the filter when replacing the pump. One reason is because a plugged filter may have caused the pump failure in the first place and, another is that when the pump failed, some small pieces may have made their way to the filter and could cause a repeat failure of the new pump.
FUEL MANAGEMENT/DELIVERY
In all CIS systems, fuel delivery is varied mechanically by manipulating pressures and metering the fuel flow through tiny passages in the fuel distributor and mechanical injectors. In straight CIS systems, this process is handled strictly mechanically, while some electronics and other controls are added for CIS-E and CIS-E Motronic systems. Digifant systems, on the other hand, use the typical air flow meter with electronic controls to direct fuel delivery through electrical port injectors.
Common failures in early CIS systems are usually related to corrosion or contamination of the mechanical parts, failure of the components used for cold starting and warm up, or vacuum leaks that rob flow from the sensor plate that controls fuel flow. Failure of check valves at the pump or at the filter outlet can be located by following the procedures in the manuals. Failure of the pressure control (warm-up) regulator can also be tested with a pressure gauge.
Look at the intake hoses and the auxiliary air valve/hoses for leaks or obstructions. (Note that a missing restrictor in a breather hose, between the valve cover and intake manifold, can cause all kinds of diagnosis problems.) Mice seem to like the air filter housings when the preheat hoses, that run from the exhaust manifold to the air cleaner housing, have fallen off.
Later CIS-E problems usually relate to the electronic control of the fuel distributor. Look for missing or blown system power fuses, or a failed oxygen sensor or computer power relay. After checking basics on these systems, a pin test of circuits using the disconnected harness connector at the computer will usually locate a component or connection failure. The computer is often located in the heater intake plenum area just in front of the windshield, under a rain barrier.
Do your customer a favor any time the car is in for service by cleaning out the drains, to help ensure that the computer doesn’t fail due to water ingestion.
Also note that some models have separate control units for knock sensor, oxygen sensor or ignition, that again require the use of the manual to locate and test these components.
COMMON DRIVEABILITY PROBLEMS
One of the first components to check for a driveability complaint is the engine-to-body ground:
• Usually a braided strap from the firewall near the coil to one of the valve cover studs; or
• A collection of brown wires bolted to the intake manifold, or sometimes to the cylinder head at the heater hose flange.
A "wiggle" test with the engine running will usually indicate a problem that may cause a stumble, miss or die-out on acceleration as the engine moves. Sometimes you’ll find a bad coil wire this way, too!
Another component that can affect both cold and hot driveability is the coolant temperature sensor, easily tested with an ohmmeter. It is mounted on the cylinder head coolant flanges on most models, and hidden under the flange on some models. Quite often, a trail of coolant residue from the sensor will indicate a problem as well as make a mess of the connector.
Although there are specific resistance values corresponding to temperature of the engine, a basic rule of thumb will be 1,500 to 2,500 ohms under 50°F, and less than 300 ohms when the engine is at operating temperature. A sensor outside of these ranges will cause an obvious rich or lean problem with the resulting driveability problems.
Be aware that some of the cars have two coolant sensors on the same coolant flange. The one for the computer is blue. Since the connector attached to it should also be blue, they sometimes get connected wrong (though I don’t know how), and the gauge sender (usually black) doesn’t give the right information to the computer.
On Motronic and Digifant systems, the ECU that controls the fuel injection also handles the ignition and spark advance, so without the correct information from the ignition system, a fuel injection fault may be created. We find failed ignition distributors on 16-valve and Vanagon models, sometimes starting with "intermittent die outs" and then progressing to "tow in/no start problems." The fuel pump won’t run without an ignition signal. Replacement distributors appear to have been upgraded (most are superseded to later parts) and, once replaced, seldom do we see repeat failures.
FINISHING UP
If the previous suggestions don’t produce results, then you need to get into more in-depth diagnosis using the pin charts and troubleshooting charts by referring to the manual for the particular model on which you’re working. Remember to always first check the basics (timing belts, compression, fuses, and the like) before diving in too deep.
Volkswagens are generally reliable vehicles and a lot of time can be lost searching for a simple problem that has been overlooked. After working on the first few problems, whether they involve the fuel injection system or another area, you’ll notice that most Volkswagen models are very similar and diagnosing them will become almost routine.
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