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engine dies on acceleration - suggestions?

Posted by vvurdsmyth 
'86 M20- cold start- fine, but when warmed the engine dies when clutch is let out in gear; it idles rough but if it doesn't stall, can be sometimes be teased to reving in neutral; when the revs are high enough slipping the clutch can get it moving and keeping the revs high (running in the lower gears) it seems normal, stopping at a light I have to play with it again to get it moving; when idling often it will die or nearly die when the accelerator is pressed; often when I get it moving by slipping the clutch it will accelerate, then die.

It has a new AFM, new throttle position sensor, new temp sensors; new spark coil, ignition wires are fine; just checked the cold start valve hot, it's tight; the vacuum line to the fuel regulator is fine; there seems no diagnostic check list in Bently's..

.. any suggestions..?
Sounds like something to do with fuel delivery. Fuel filter(s) plugged or fuel pressure regulator not operating well?

Salut, Bob p.
thank you Bob P 325is 88, yah, that seems to be the consensus; respondents on other forums have suggested that as well; it is consistent with power at high revs.. more manifold vacuum increasing the fuel pressure, will let you know...
Manifold vacuum is highest at closed throttle, lowest at wide open throttle. The pressure regulator, via the vacuum line, responds to alter the fuel pressure as required. less fuel pressure at idle, higher fuel pressure at open throttle.

My 1990 325i had similar symptoms, occasionally running way too rich, stumbling and belching black smoke. That was diagnosed as a faulty fuel pressure regulator. Regulator and vacuum line was changed. Made no difference. Bummer.

It turned out to be the Bosch Motronic ECU. A previous owner had installed a cheap performance chip and that apparently kacked out. I found a used ECU for $100, swapped that in, and the car runs like new again. Eureka!
But the '86 M20 with a cold start valve could be a 320i with L-Jetronic.

Anyway, my guess is that the engine is breathing non metered air. That means air bypassing the AFM somewhere.
Search by spraying start gas on various parts of the engine while it is running, and listen for idle increase.
It could be as simple as the oil filler cap not being properly on, or the dip stick not properly in, but usually the
problem is a leaking air hose. The rubber cracks up when it gets old.
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Ove Kvam
But the '86 M20 with a cold start valve could be a 320i with L-Jetronic.

Anyway, my guess is that the engine is breathing non metered air. That means air bypassing the AFM somewhere.
Search by spraying start gas on various parts of the engine while it is running, and listen for idle increase.
It could be as simple as the oil filler cap not being properly on, or the dip stick not properly in, but usually the
problem is a leaking air hose. The rubber cracks up when it gets old.

That would be my guess as well.

There are half a dozen or more places where unmetered air can leak in. The flex hose between the throttle body and the AFM, the manifold vacuum hose that feeds back to the ICV, oil dipstick, the oil filler cap, the intake manifold gasket, the throttle body gasket, etc.

One additional thing to check out is the wiring to the fuel injectors. The connector under the diagnostic connector is a known trouble spot. If one set of injectors are not firing, the car will idle fine but stumble or stall when a load is applied along with throttle.
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Ove Kvam
Anyway, my guess is that the engine is breathing non metered air. That means air bypassing the AFM somewhere.
Sounds like a likely reason. If this is the case, then I think the check engine light should be on. On North American E30s, we can do the stomp test (e.g. https://www.youtube.com/watch?v=5GlBsrOSefg). An intake leak would give a code 1215. I had this happen on my E30, and I never did find the source of the leak until I removed and reinstalled the intake manifold for an unrelated problem.

__________
Dave
'91 325iX
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Ove Kvam
But the '86 M20 with a cold start valve could be a 320i with L-Jetronic.
The OP didn't say where he's from, but if it's North America, it's probably a 325. I don't think I've ever even seen a 320 here. Were they even sold in North America? If so, they're very rare compared to the 325.

__________
Dave
'91 325iX
In Europe, the 320i was a big seller, and I think there was no check engine light feature on these.
rkj
I'm making a smoke machine for testing for these air leaks. I need one for all the evap stuff on my newer cars too.
If the smoke gets drawn into the vacuum system you might have a hard time seeing it. Are you planning to put the smoke inside and pressurize the system with smoke?

Bob in Everett
rkj
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Bob in Everett
If the smoke gets drawn into the vacuum system you might have a hard time seeing it. Are you planning to put the smoke inside and pressurize the system with smoke?

Yes, I like this guys design. The glow plug seems like a good idea. With all these vacume leaks on todays (and yesterdays) motors this seems like a needed tool in my shop... youtube

[www.youtube.com]
Rick,

While this gizmo looks like fun to build, it would be a lot simpler to just light up a cigarette and use that.
I subscribe to the KISS principle.

If you don't smoke, any good insense stick or mosquito repelent stick would work just as well.
Checking for vacuum leaks might be easier to do with a propane torch. Just turn on the gas and do not light it. direct the gas to areas of interest that might have a leak. The engine should be running of course. a marked change to the idle speed would indicate leaking air was also drawing in propane fuel with the air.

Bob in Everett
Bob,

What happens if you are investigating the sparkplug side of the engine and old plug leads happen to arc???

Smoke might be safer winking smiley
Well, if the plug leads are arcing, that could explain the trouble with stalling on acceleration. But to answer your question, yes if the propane were to be ignited by the arcing wires, it would quickly indicate what to fix next. Might singe some eyebrows. I think the risk is low because the vacuum leak detection is on the opposite side of the engine compartment.

Bob in Everett
A tombstone:
"John Who" Went to see if he had gas with a match, and he had.

:boohoo:
rkj
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Archeo-peteriX
Rick,

While this gizmo looks like fun to build, it would be a lot simpler to just light up a cigarette and use that.
I subscribe to the KISS principle.

If you don't smoke, any good insense stick or mosquito repelent stick would work just as well.

There's a youtube with a guy using a butt to blow smoke into the system but a steady stream is needed to really source out hidden leaks, cracked T fittings and such.
Quote
rkj
Yes, I like this guys design. The glow plug seems like a good idea. With all these vacume leaks on todays (and yesterdays) motors this seems like a needed tool in my shop... youtube

[www.youtube.com]

I finally watched this video and really like this. This kind of thing is fun to build, and I could really use this to track down an elusive vacuum leak in my wife's Mazda. It's been defeating me for months. I think I might just try to build one of these.

__________
Dave
'91 325iX
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