I have a 1974 1.8

while the car is idling I can hear the fuel pump running (it is a non-Porsche part, moved to the front trunk by PO), however it isn't running consistently. I hear it wax and wane over the period of a few seconds. Kind of like a rotating tornado siren, only faster (and quieter )

So here is my question... is this normal, or should it be running at a constant pitch whenever the car is running.

If it is normal for it to wax and wane, what signal is telling the pump to slow down, then speed up again, all while under a constant load (parked and idling).

The reason I ask is that the fuel pump hasn't run for a year, and I just recently got it running (high resistance connection on relay board, subject for another post). Last year the car ran well enough to take out on the highway, but I thought I was hearing a continuous pitch from the fuel pump. Now the car cuts out on acceleration and struggles to go around the block.

Thanks

As these Bosch FI system use a closed loop fuel system, with a regulator, the pumps should run at a constant pitch and rate, all the time.

Check the electrical connections on this one and continue trouble shooting from there.

On my L-jet with a stock pump (35 yrs old) and stock mounting, I cannot hear the pump, ever.

...btw, that is not because the car does not run,

Thanks, that was my understanding of how they should work too. I just wanted to confirm that.

I had a problem getting adequate voltage to the pump, and it wasn't running at all. I thought I'd solved that, but it appears I've just partially solved it.

I used to be annoyed at the fuel pump noise, but at this point it is proving extremely usefull in debugging this. As the software guys would say, "it's not a bug, it's a feature!"

I am glad I could help.

I find that when the L-jet is running poorly, it really sucks, but when it is running great, it is a VERY reliable FI system.

I guess that should be obvious as Bosch used that L-Jet system as the basis for almost all its Electronic injection systems going forward (Non CIS, which is mechanical).

heck, even the motronic systems can be traced back to the L-Jet.

[quote name='Cupomeat' date='Jun 22 2010, 01:43 PM' post='1333827']


On my L-jet with a stock pump (35 yrs old) and stock mounting, I cannot hear the pump, ever.


I think that signal's a problem(noise)

Just to correct, the CIS is a hydraulic fuel system, not mechanical. CIS and MFI are two completely different systems.

The L-Jet system evolved into the Bosch Motronic system used on the 924S/944s, some 928s (IIRC), the 911 3.2 Carrera, the 964, the 993, 996, and 997. A licensed Nippondenso copy of the L-Jet system is also used on almost all the Nissan and Toyota models built during the 80s and 90s.


Now addressing the problem. No the fuel pump should not vary in sound. I would suggest that you take your engine lid off, and put a fuel pressure gauge on that you can tape to the back window and see in your rear view mirror. Then go drive it and see if the pressure varies. Granted, and L-Jet system should vary from 28 to 34 PSI, but anything outside of that can indicate either a pump problem, a problem with the inlet side of the tank, a regulator problem, or a blocked return line.