odd fuel pressure/long crank issue - SOLVED

E28 technical advice asked and given! Troubleshooting, modifications and more.
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iamcreepingdeath
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Joined: Dec 28, 2008 10:03 PM
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odd fuel pressure/long crank issue - SOLVED

Post by iamcreepingdeath »

So I am doing my research on the forums, but I figured i'd start a thread because my symptoms aren't the usual ones.

Basic background on the car: 1985 535i with M30B35 swap out of an E32. I upgraded the in-tank pump to a TRE performance 255 LPH pump, and deleted the in-line pump. For a long time, this worked great. Then recently, my car started losing fuel pressure at high load, so I replaced the fuel filter, which seemed to help.

However, there are a couple problems that have cropped up more recently, maybe the same issue?

1. Long cranking when starting, esp when engine is already warm. This is a more common problem that I am researching. I will be trying a check valve next and see if that does anything.

EDIT: Installed a check valve, did not seem to help. I also installed a fuel pressure gauge. Watching the gauge while starting the car, I notice that as the starter is cranking, no fuel pressure is present, then after a while suddenly fuel pressure builds and the car immediately starts. So why does the fuel pressure not come on for a while? I decided to hard wire the fuel pump relay to a throw switch to eliminate that problem (I've been planning to do that anyway, I love throw switches, it's kind of a weird fetish). So now, when I turn on the fuel pump w/ the switch and then start the car, the car starts in like 5-6 cranks, which is a lot faster, but still seems like a long time. Also of note, there only seems to be like 24 psi of fuel pressure, which is reeeeally low.

2. This is the odd one: Total fuel cutoff when aggressively "turning left", and by that, I mean when the tail end kicks out or I am doing a cookie/donut/whatever they are called. Going right, no problemo, but going left, total loss of fuel.

EDIT: After I installed a check valve, and this issue seems to have gone away. I just went and ripped some left-hand donuts.

3. Less noticeable of an issue: Every once in a while I detect a slight drop in fuel pressure under high load, like for example accelerating hard in the higher rpm range.

What is going on? Could it just be fuel starvation of the pump when the fuel sloshes to the back of the tank when doing a "cookie"? Then why not when I turn right? Besides a check valve, what should I try next? One thing I did is I detached the fuel return line from the regulator and started the car, but no fuel came out. Shouldn't there be fuel coming out?

EDIT:
SOLVED! Turns out the rubber connector between the pump and the in tank pickup line developed a tear in it. I replaced that with a bit of 3/8" hose, and fuel pressure is back to normal!
Last edited by iamcreepingdeath on Jul 05, 2022 11:35 AM, edited 6 times in total.
Federico
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Re: odd fuel cut off issue when turning left aggressively

Post by Federico »

I would pull the modded pump assy out & verify nothing came loose
Shawn D.
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Re: odd fuel cut off issue when turning left aggressively

Post by Shawn D. »

Note that the tank is not symmetrical in any direction and that there's a little "bucket" the pickup should be in. It could easily be sloshing fuel out.
iamcreepingdeath wrote: Jun 07, 2022 11:24 AM One thing I did is I detached the fuel return line from the regulator and started the car, but no fuel came out. Shouldn't there be fuel coming out?
Yes, there absolutely should be fuel flowing out of the regulator (NOTE for folks who don't know: the regulator lets fuel out of the rail, not into the rail).
iamcreepingdeath
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Re: odd fuel cut off issue when turning left aggressively

Post by iamcreepingdeath »

Shawn D. wrote: Jun 07, 2022 6:02 PM Yes, there absolutely should be fuel flowing out of the regulator (NOTE for folks who don't know: the regulator lets fuel out of the rail, not into the rail).
Right exactly! I tested it again just now. With the car running, I removed the fuel return line off the FPR. Zero fuel, not even wet. Revved it, etc, nothing. Just to make sure I wasn't going insane, I removed the FPR entirely and went to start the car, and sure enough, Old Faithful erupted out of the now vacated hole in the fuel rail.

WTF? Do FPRs fail like this? If so, could that be the culprit? If the FPR is not flowing fuel through, the in-tank pump must be just pushing against nearly stagnant fuel at an idle. that can't be good for it. Sheesh...
Mike W.
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Re: odd fuel cut off issue when turning left aggressively

Post by Mike W. »

My guess would be more that the pump isn't pushing hard enough to open the relief on the FPR, I had something like that on my E36 lately that drove me crazy as it caused me to run out of gas with 1/4 tank. It might be a leak within the tank plumbing or on the suction side, if it's not sucking full fuel, it can't generate full pressure. In other words, check your fuel pressure first before buying any parts and just swapping them out. A hardware water pressure gauge and a couple of fitting and bit of hose can make an inexpensive short term tool, nothing I'd leave in place for an extended time, but fine for testing.
iamcreepingdeath
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Re: odd fuel cut off issue when turning left aggressively

Post by iamcreepingdeath »

Mike W. wrote: Jun 07, 2022 10:57 PM My guess would be more that the pump isn't pushing hard enough to open the relief on the FPR, I had something like that on my E36 lately that drove me crazy as it caused me to run out of gas with 1/4 tank. It might be a leak within the tank plumbing or on the suction side, if it's not sucking full fuel, it can't generate full pressure. In other words, check your fuel pressure first before buying any parts and just swapping them out. A hardware water pressure gauge and a couple of fitting and bit of hose can make an inexpensive short term tool, nothing I'd leave in place for an extended time, but fine for testing.
True story. I did just order a pressure gauge fitting and a gauge that I can install permanently in line in the fuel line feeding the rail. We'll see what the gauge says when I get it in the mail and get it installed.
Philo
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Re: odd fuel cut off issue when turning left aggressively

Post by Philo »

I had this issue and it was a loose wire at the FP relay. Not saying this is your problem but check it to rule it out. The wire that was loose was a 12v lead going into the relay base.
iamcreepingdeath
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Re: odd fuel cut off issue when turning left aggressively

Post by iamcreepingdeath »

I couldn't mess with my car for the last two weeks, hence the time gap between my last post and this one, but just today I did two things:

1. Installed a check valve right after the fuel filter
2. Installed a fuel pressure gauge right before the rail.

The long cranking problem has not been resolved. The check valve did not fix that issue. Also, the fuel pressure seems to hover at 28 psi, and drop to 22-24 psi when I rev the engine. Does that seem kinda low? Would that explain why the FPR isn't letting any fuel out of the return? Would that also explain the long cranking issue?

Here is a quick vid I took of the pressure gauge. Let me know if this link doesn't work.

https://drive.google.com/file/d/1UcscW3 ... sp=sharing

And funny enough, the fuel cutting off while doing left-hand cookies seems to have been resolved, maybe the check valve fixed that?
Mike W.
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Re: odd fuel cut off issue when turning left aggressively

Post by Mike W. »

iamcreepingdeath wrote: Jun 30, 2022 5:18 PM I couldn't mess with my car for the last two weeks, hence the time gap between my last post and this one, but just today I did two things:
No worries, there's bigger things in life than cars. Maybe... :laugh:
The long cranking problem has not been resolved. The check valve did not fix that issue. Also, the fuel pressure seems to hover at 28 psi, and drop to 22-24 psi when I rev the engine. Does that seem kinda low? Would that explain why the FPR isn't letting any fuel out of the return? Would that also explain the long cranking issue?

Here is a quick vid I took of the pressure gauge. Let me know if this link doesn't work.

https://drive.google.com/file/d/1UcscW3 ... sp=sharing
Definitely low, if it's a 3.0 Bar, it's roughly 45PSI not running or the throttle wide open. Seems like idle with full vacuum is about half a bar below that, or ~38PSI. So if it's not hitting those numbers, you're not going to see anything in the return, no fuel will bypass into the return below the rated pressure. Now why your fuel pressure is low I don't know. Might be a weak pump, or it might be sucking air.
And funny enough, the fuel cutting off while doing left-hand cookies seems to have been resolved, maybe the check valve fixed that?
Dunno on that one. But I bet it comes back unless the pressure goes up to spec.
iamcreepingdeath
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Re: odd fuel pressure/long crank issue

Post by iamcreepingdeath »

Another update.

So I tried starting the car while watching the fuel pressure gauge, and I noticed this: The car cranks for a long time, and there is no fuel pressure. Then after a while of cranking, the fuel pressure suddenly builds, and the car starts immediately. However, the fuel pressure remains low at like 24 psi.

To eliminate the late fuel pressure build issue, I hard wired a throw switch to the fuel pump, bypassing the fuel pump relay. I've been planning to do this anyway, I love throw switches, it's kind of a weird fetish. I want everything to be hooked up to throw switches.

So anyway, when I turn on the fuel pump w the throw switch, the car now starts within 5-6 cranks, which is way faster than it has been, but still slower than it should be.

However, here is an interesting observation: I removed the fuel pump access panel and listened to the pump (just turned on via throw switch, engine not running), and it sounds odd, like the fuel pump is sucking on foam or froth, rather than liquid gasoline. Next step is to pull out the fuel pump and inspect for leaks or any abnormalities, and then submerse the pump in a bucket of gasoline and run the pump again (which may be tricky to do, I'll need to use wire leads, which is super sketchy with the open gasoline nearby). But my suspicion is something is going on w the pump, as it sounds really weird.
Galahad
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Re: odd fuel pressure/long crank issue

Post by Galahad »

Maybe there's a leak in the hose connection between the pump and the sender unit - I could see that making a frothy noise and it would explain the low fuel pressure as well. Normal fuel hose isn't rated for submersion in gas, you need to use special stuff
gadget73
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Re: odd fuel pressure/long crank issue

Post by gadget73 »

when I've had problems with things cutting out in turns, it was usually something related to the pickup sock. Either it fell off, wasn't the right one, or in one case it had a crack in it so it would suck air when the fuel ran to the other side if it was below a certain point.

and +1 on the split in-tank hose. Had that happen before too, and it was from using standard fuel hose, not in-tank hose.
iamcreepingdeath
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SOLVED!

Post by iamcreepingdeath »

Pulled the in-tank pump out (I had previously done the in-tank pump upgrade to the TRE pump), and herein lies the problem.

Image

Sheesh. Such a tiny problem for such a large thread...

Moral of the story: It's usually something stupid, take a look at things before freaking out.
gadget73
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Re: odd fuel pressure/long crank issue - SOLVED

Post by gadget73 »

yeah but if I don't jump to conclusions I won't get any exercise

but I know what you mean, having massively over-thought things and started diagnosing from the most complicated solution backwards more than once.
Mike W.
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Re: odd fuel pressure/long crank issue - SOLVED

Post by Mike W. »

gadget73 wrote: Jul 06, 2022 8:51 AM yeah but if I don't jump to conclusions I won't get any exercise
I like the way you think, I may start using that line. :up:
BDKawey
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Re: odd fuel pressure/long crank issue - SOLVED

Post by BDKawey »

mine experienced similar failure. using the cek method of putting the smaller diameter section in some real metric fuel hose would be the best way to be sure it doesnt happen again.

an in take upgrade on my other car may be in my future. I plan to cek it as well as maybe using a fuel clamp that doesnt have exposed worm gear track on the inside or using a fuel injection (nut bolt) style clamp because I feel like it puts less weird squish on the hose and leaving the marks which could lead to rips.
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