Questions and Answers
Audi Fuel Injection
Q. My brother's 1988 Audi 90 will start but quits almost immediately. The car had been parked for several months, he got it running and drove it about 90 miles before it started running roughly, and then quit about a mile from my house. Fuel pressure seems to be good, although I have no way to check accurately.
By accident, I discovered if some gas is poured into the air intake, the car will start and run, but it is necessary to "pump" the accelerator pedal. If the pedal is held steady, the engine dies as if fuel starved; if pumped, it can be kept running for several minutes. It will do this in any given RPM range - 2,000, 4,000, 6,000 etc, but only if the pedal is pumped.
I've added injector cleaner to the fuel, and have even taken off the main injector body to spray carburetor cleaner in/on it. Under the main injector body is what appears to be a cam of some sort, which works a plunger coming out of the bottom side of the injector body. When pushed, that plunger creates a sound of air/gas being pumped in the injector body. This plunger seems to be free - it will slide in and out if the body is turned upside down and back again.
If I try to start the engine with one of the injector line banjo bolts off, I don't see any pressure - i.e., no fuel comes out, but yet when I "prime" the system it runs long enough that I'm sure that on occasion the fuel does flow. Any suggestions?
Thanks,
TimA. That car has a Bosch K-Jetronic fuel injection, Mechanical Fuel Injection. That cam and plate you discovered is the Airflow Sensor Plate. As the volume of air entering the engine increases, the air flow sensor plate rises, the control plunger opens and fuel delivery to the injectors is increased. When air flow decreases, the sensor plate and control plunger drops and less fuel is delivered to the injectors. When the engine is stopped (no air flowing past sensor plate), the control plunger closes off the fuel distributor metering slot and no fuel is supplied to the injectors.
Fuel pressure is very important in an MFI system since it is what actually opens the injector. If the fuel pressure is even a little bit too low, the engine will not run. I would do a fuel pressure test and make sure it is within specification. Fuel pressure is supposed to be 88 psi to 95 psi and fuel output should be 130 - 150 cc/minute. It is very likely you have a bad fuel pump.
Once we fix the fuel problem we're going to have a problem with the air box you took apart. They were not meant to be taken apart and Robert Bosch is very secretive about how his parts are put together It will now need to be adjusted and they are very tricky to adjust. I have adjusted many of them and I can tell you it takes a certain feel to adjust it properly. But let's get it started and worry about that later if we have to.
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