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'87 740 fuel issue

Usually I'm the one answering these questions but this one has me a little stumped. Car is an '87 740 Turbo, ran great until one day I noticed the idle was stumbling a bit and then I could hear the main fuel pump whining.

Figured ok, car has close to 300K mi on it, pre-pump hose is probably shot. Pulled that and indeed there was a hole in the bellows. Replaced it with a short piece of fuel line and verified the pump to be functional. Put it back together and found that after driving around for a few minutes the same symptoms were back.

Thought the main fuel pump must be failing from the hole in the pre-pump hose so I went to the junkyard and grabbed a main pump from a '90 760. Popped that in, replaced the filter while I was down there and now I still have exactly the same symptoms.

Let the car sit for about a week, not having time to mess with it. Tried to start it the other day and it would crank but not fire, fuel pump sounded odd. Popped the cap off the relay and held it closed manually, pump sounded funny for about 30 seconds then loaded down and sounded normal. Turned the key and the car fired right up.

Thought perhaps the pre-pump was failing intermittently, stuck an amp meter on it and drove around, current draw remains between 2.7A-3.1A regardless of whether the main pump is whining or not.

Had one more idea, perhaps the tank vent was clogged causing a vacuum to build up. Removed the gas cap and drove around again but no change.

So here I am, car starts and runs just fine, idles in the driveway, but if I go drive it around, within about 5 minutes the idle gets lumpy and the main fuel pump starts whining. Sometimes it comes and goes in cycles as I'm driving. Both pumps seem to be running just fine, I'm not sure where to go from here.

Reply to
James Sweet
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Make certain the fuel pressure regulator is operating correctly.

Perhaps it is failing and allowing out inadequate fuel pressure.

Reply to
Mr. V

I thought about that, but would it fail intermittently and cause what sounds like cavitation in the main pump? I wouldn't expect so, but then I've been wrong before. Next time I'm at a yard I'll probably grab one just to have a spare on hand.

Reply to
James Sweet
J

Might be worth a mention......I had similar symptoms on a car some years ago....Turned out to be a piece of plastic bag that had got into (?) the petrol tank, when the tank was relatively full (plastic bag floated to the top of the fuel) the car went well and when it had been standing for a while it went well. But after the car had been running for a while the flow from the tank to the pump would drag the piece of plastic onto the filter inside the tank restricting the flow.

I had even fitted a clear plastic pipe the fuel line to help find out what was happening, this showed bubbles (cavitation) in the fuel. This made me realise that the problem was in the tank. Took about a weeks worth of hassle to find this, even bought another pump in the process.

ttfn.....Alistair

Reply to
Alistair Ross

Any time the main pump is noisy like that, there's a problem getting fuel to it. Since you covered everything else, then your tank pump is dying or not getting enough power, or something is restricting the hoses from the top of the tank to the main pump. Pull off the inlet hose to the main pump, with the tank pump running, fuel should gush out that hose in enormous quantities. (I don't have to tell you to have a large can to collect it.)

Reply to
Mike F

The fuel level doesn't seem to make any difference. I like the filter idea though, I may add one so I can see what's going on, can't hurt to have a filter there anyway.

Reply to
James Sweet

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