'87 740 fuel issue

[James Sweet]

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.

 

[Mr. V]

Make certain the fuel pressure regulator is operating correctly.

Perhaps it is failing and allowing out inadequate fuel pressure.

 

[James Sweet]

Make certain the fuel pressure regulator is operating correctly.

Perhaps it is failing and allowing out inadequate fuel pressure.

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.

 

[Alistair Ross]

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

 

[Mike F]

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.

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.)

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

[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

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.