Fuel Pressure Mystery -- need help diagnosing an infrequent/intermittent problem.

[Aresby]

Hi,

I've an '89 E30/M20 in well-maintained condition.

Yesterday, driving along, the engine simply died. No fault light, no prior indication, just -- choked off.

@ the side of the road, try to start. Turns over, but doesn't start.

Roadside diagnostics:

1/2 tank gas. Check.
Battery -- OK. Check.
Check fault codes @ port -- nada. Check.
Starter turns over -- OK. Check.
Fuel pressure at gauge right before FP regulator -- Zero!

Check FP fuses -- OK. Check.
Swap in spare FP relays -- OK. Check.
Listen for FP -- whirring, tho not entirely sure if the sound's different than usual. Check.
Whack fuel filter a few times -- no change. No pressure.

I suspect(ed) a clog in the fuel filter, a dead FP sender, or a seized fuel pump. Need to diagnose where I have my tools :-/

Call for a tow ... Car's in the carport overnight.

This morning, start the process -- sanity check, try to start the car.

Starts right up!

Fuel pressure's right back to normal.

Since it's working, I'm not sure what/how to diagnose. Does this sound familiar to anyone? Can a fuel-pump/sender intermittently function, or is dead really dead?

Looking for advice on what to check (for).

Thanks!

Aresby

[Jared at Pelican Parts]

Fuel pressure regulator. Had the same thing happen to me.

take the vacuum hose off the regulator. If gas dribbles out or if you smell gas, it's bad and must be replaced.

[Aresby]

Hi Jared,

Tried that. No gas dribble, no bad smell. Clean as a whistle, actually ...

It's been ~ 2 years since replacing the fuel pressure regulator. With no dribble/smell, what do you think? Replace anyway -- or something else in the fuel delivery chain?

I've been running the car a bit (alas, just in/near my parking lot ... Murphy says the INSTANT I take it on the hwy, It'll die off again ;-)), to see if it's heat-related. Honestly not sure if heat's ever a problem for FP's. Digging further atm ...

Aresby

[Jared at Pelican Parts]

try hooking up the pressure gauge, running the car hard and shut the car off. See if it hold residual pressure for an extended period with the car running. I dont remember the exact time, but It should hold pressure for at least a half hour or so. If it bleeds off the second you shuit off the car, the regulator is bad. Heat can affect the regulator, btw.

[Aresby]

'However (tm) ...'

I'd checked the for smell and dribble, by removing the vacuum hose with the engine off. Then replacing it and restarting. Starts trght up.

I'd also disconnected to the hose while running -- to monitor the characteristic rise in FP (my gauge is a permanent mount ...), and saw no issues.

Moments ago, however, I disconnected while running, and ...

... the car instantly choked off, as before. And won't re-start, despite ignition turn over. AND, fuel pressure reads -- zero.

Per your suggestion, sounds like (at least)? the FPR!! nice catch. other suspicions? or start here?

Aresby

[Jared at Pelican Parts]

if the car instantly shuts off when ou disconnect the line, it's the FPR.

[Aresby]

Yes, it does. Well, did -- now it won't RE-start.

I was thrown by the zero-pressure reading *before* the FPR -- figured that it was then something earlier in the chain.

In any case, FPR it is !!

Thanks a bunch :-)

Aresby

[Aresby]

Turns out it's not (just?) the FPR :-(

Swapped in a new FPR, and started right up. Then disconnected the FPR's vaccum host while running, and ... choked.

Ran the electrical diagnostics -- 12 V across the FP relay socket's terminal pairs.

Popped the rear seat and the FP hatch, and checked voltage across the FP connector with the FP relay jumper in place -- 12 V! So the pump's, apparently darned-close to dead.

Ordering ...

[Frajo]

Sounds like itʻs going around. Had the same symptoms, even swapped fuel pumps with no change...after swapping the ECU it slowed down the frequency of the problem but occasionally it wonʻt start. after a few minutes it will start again.

[Aresby]

Replaced the fuel pump, and -- so far -- no stall/choke issues anymore.

Starts up fine, survives the detach of the FPR vacuum hose, behaves on a short run to the grocery ;-) ...

One minor issue ... prior to the FP replacement, my typical idle RPM was ~ 800. now, after new FP install, it's at about ~500 rpm, and, looking at my exhaust fumes, is running a bit rich.

I suspect that different flow rate/pressure due to the new pump need to be 're-learned' by the Motronic? or, perhaps, an issue with the oxygen sensor?

[BMWpark.org]

sounds to me like your battery hit ground.
could be ignition / spark ie= cap n rotor. say the rotor got wet, or wobbled. (easy check)

imo a fuel pump 'fail' light would be nice, but they are more common if you leave your tank almost empty during the extreme cold and hot months.... just my experience with the posts on the boards...

[Aresby]

Originally Posted by BMWpark.org

sounds to me like your battery hit ground.
could be ignition / spark ie= cap n rotor. say the rotor got wet, or wobbled. (easy check)

battery? is that comment meant for me, or for Frajo ?

[Jared at Pelican Parts]

Check your spark plugs.