When I came home last night, my wife informed me that her '90 100
would not start. When I tried to start it, I noticed that the fuel
pump was not priming at ignition on time.
I then went ahead with the diagnostic procedure outlined in Bentley,
using #52 terminal on the relay panel (fuse box), where the fuel pump
relay normally resides. When I introduced 12 volts (from the battery)
to the terminal, 12v was present at the fuel pump connector. I then
reconnected the fuel pump and tried the same experiment. The fuel
pump would not work. It made a knocking sound when I connected it to
the battery. After several more attempts, I reinstalled the fuel pump
relay. Then got into the car and turned the ignition on and of
several times just to make sure I was not imagining things, and
interestingly on the second try, the fuel pump started working.
This morning, I decided to repeat the test procedure, and sure enough,
when I connected it to the battery, it hummed along with no problems.
Not too long before this failure, I was noticing that the pump
generated some knocking noise for a short time after cold start (about
5 knocks approx. 1/2 sec apart). Past that, the pump worked without
the "buzz", and without irregular noises typically observed.
It felt like the pump hit a "dead" spot, and by introducing spikes of
voltage it made it past the dead spot. I suspect that the fuel pump
needs to be replaced as soon as possible.
Can anyone recommend a good source for a Bosch pump?
Hope this info can be helpful to some of you.