Decreasing Fuel pressure/stall

[adika]

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.

Thanks

[Lock]

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.

[RandyM]

I know its been a while.... but i'd start by Checking the fuel flow at the rail by disconnecting the 7th injector hose and getting a clear plastic container to contain fuel. Measure the flow at idle and under throttle.
I had an issue similar that turned out to be a loose wire at the fuel pump.

[autonoob]

Typical causes for low fuel pressure include a dirty fuel filter, weak pump, incorrect tank venting, restricted fuel lines, a clogged pump inlet strainer and faulty electrical control. But first, let's look at a common fault.

[528i-1981]

I might start by jumping the fuel pump and seeing if that changes anything. There are two pins on the AFM connector that you can bridge to turn the pump always-on. If the symptom is resolved, then you can eliminate a restriction as the cause and focus on electrics. Besides the connections to the pump itself, I wouldn't rule out the AFM pump switch, the combo relay and even the temp-time switch.

I also tend to think this is a line-kinking situation. This could be made worse by a weak in-tank lifting pump. I would take that out and clean the screen and bench test it. You'll need a new o-ring when you put it back in, they swell to seal and cannot be reused. Don't be tempted to reuse it because your car and trunk will smell badly of gas. AMHIK!

A couple of details that would be helpful to know - what are the pressure measurements you're obtaining? Does this symptom happen each and every time? If you only go slightly off-idle and open the idle switch at the throttle body, does the fuel pressure drop?

Good luck - getting an E12 sorted out is a rite of passage that makes driving a well-running one very satisfying!

[528i-1981]

... getting a clear plastic container to contain fuel. Measure the flow at idle and under throttle.

If you do this, make sure it's a big container, as the pump delivery rate at spec is about 16 oz. per minute.