Decreasing Fuel pressure/stall

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Decreasing Fuel pressure/stall

Post by adika » Sat Apr 04, 2020 9:47 pm

Hello everyone, Im having an issue with the e12 that wont allow me to drive it. Considering I have never been able to drive it, im hoping I can fix it as soon as possible. I removed the old fuel pressure regulator (I suspect it may have been broken) and put an adjustable ebay one from a freind of mine as a temporary measure. Since this one has a gauge I can hopefully better diagnose any issues I may have.

The car starts great and idles (which needs some work), but when I give it gas the pressure begins to drop, and if I hold the gas down the pressure drops all the way down to near zero and the car stalls. Will then start up perfectly, I give it gas, and the same thing happens again :? . At Idle the pressure goes up and down, about 10psi in either direction, but also not sure why.

I am hearing a new vacuum leak, but I cant seem to pinpoint where its coming from. This issue was happening even before the leak.
The previous owner said he had intalled a new fuel pump, but I am unsure if the filter has been changed.

I really want to fix this issue but unsure how to pinpoint the issue. Any help would be appreciated as always.


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Re: Decreasing Fuel pressure/stall

Post by Lock » Sun Apr 05, 2020 1:38 pm

Sounds like a fuel flow problem, not the regulator. Replace the fuel filter, they're cheap, and while you're there check the condition of the fuel hoses under there. You're supposed to use special 90* hoses but most garages would probably use normal fuel hose.

Check the intake hose from the tank to the fuel pump. This hose doesn't have any supports, is vaguely routed from the top of the tank, down the side of the tank and then around to the pump and can easily get a bend in it. If this happens, the suction will pull the bend tighter and restrict flow more. That the PO replaced the pump sounds like they've chased this same issue. There's also an in-tank lift pump, but I've seen no difference when it's working vs not. However if its impeller broke it might be restricting flow.

In this scenario it'll idle okay, but when the engine needs more fuel the flow is not there, pressure will drop and it'll stumble and stall. One sign is the pump. You should only hear a soft consistent buzzing. If it's loud, it's struggling.

Mine got a minor bend in the intake hose above the tank, and it would cut out at WOT and the pump was really loud. I fixed this temporarily by grabbing it from the wheel well and wriggling it to undo the bend until the pump quietened down, since replaced the hose.
1979 528i 5-speed conversion G260/6, Motronic 1.3 179
1980 Honda CX-500

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