240 Fuel Pump Troubleshooting
Interruption of fuel supply is one of the most common no start problems in fuel injected cars.
If you are somewhat mechanically inclined, familiar with the use of a 12 volt test light, and have a good wiring diagram available, you can quickly trouble shoot most fuel pump related problems. Better repair manuals like the Bentley 240 owner’s manual give excellent step by step diagnostic procedures for both K and LH Jetronic systems.
Fuel pump failure is rarely immediate and is almost always preceded with several months of intermittent growling noises from the pump. Often the growling noise is caused by a failed in-tank pre-pump or a crack in the rubber line connecting the pre-pump to the fuel lines inside the tank. If the noise seems related to fuel level in the tank, you can almost be sure that the problem is related to the pre-pump or rubber line mentioned above.
You should also be aware that on most models 1977 and newer, if there is a problem in the primary side of the ignition system, the fuel pump relay does not engage to supply power to the fuel pumps. This is a safety feature to prevent flooding of a car with a no-spark problem. Further testing of the system requires pressure gauges and advanced understanding of electrical circuits and for safety reasons should be left to an experienced technician.