Passat (B5) :: Car Quit On The Road, Bad Fuel Pump Diagnosed

The car quit on the road and I diagnosed a bad fuel pump. Multiple tests confirmed the pump had failed. While we were doing the diagnosis, I noticed some strange behavior when monitoring voltage to the pump. While supply voltage was present during cranking, none was applied to the pump when the key was first turned to accessory. I decided to chase one thing at a time and replaced the pump (and fuel filter, since it had been too long since last replacement and I wanted to "cover all bases").

After pump replacement, I opened the coupling in the supply line before the fuel rail to confirm proper operation. When the key was initially turned to accessory (multiple times to prime everything), there was NO fuel flow. When we then cranked it, fuel was present very quickly. After reconnecting the line, a couple of rounds of cranking got the fuel pressure up and the car started and ran perfectly. A quick VAG-COM check showed no codes. I test drove it and then shut it down. Immediately after shut down, I switched the key to accessory and the fuel pump started right up. After doing this 4-5 times with a few minutes pause in between, I pronounced the car "good" and finished up.

This morning, I jumped in, turned the key to accessory and heard......nothing. No fuel pump running for the normal 2-3 seconds. I cranked it and it took two rounds of 4-5 sec cranks for it to fire and run (normally). I assumed it needed time to build fuel pressure. I then drove it for 20 minutes, parked it for 45 minutes and when I restarted it, it took a longer than normal crank period (maybe 3 seconds) to fire and I did NOT hear the pump run in accessory position. The rest of the day was with short stops and I could always hear the pump start in accessory and the car always fired up instantly. I have the Bentley CD and analyzed the wiring harness. It looks like the fuel pump relay is only commanded to operate by the ECU - there is no external input to the relay coil I can see (ground or B+).

The bottom line is that SOMETIMES the pump is commanded to run in ignition switch accessory position and sometimes it isn't. The "isn't" always seems to be after the car has sat for a while. Is there any combination of failed sensors that would cause the ECU to NOT command the pump to run for it's few seconds in accessory position ? Obviously, the next step is the FP relay but I hate to just throw parts at the car without some sort of troubleshooting and the repeatability of the problem with sitting time leads me to believe it may be more of a relay DRIVE problem then the relay itself.