Won't Idle Low and Hesitates at High RPM

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Microang
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Won't Idle Low and Hesitates at High RPM

Post by Microang »

Hi,

So I'm experiencing a strange issue with my 1986 628CSi, basically it refuses to idle properly below 1000 rpm, even though I understand it's meant to idle around 700-800 rpm, if I lower the idle to where it should be it will stall out the second I push the clutch in and let me foot off the accelerator after I move off, the rpm will dip to around 400 rpm and die so I have to keep it around 1000 rpm all the time to be able to drive it. Any ideas what this could be? I'd really like to fix it.

Also another intermittent issue is when driving full throttle when it gets to around 4000 rpm it will splutter and lose all power for about a second and then come back on power, like the fuel supply was cut briefly or the throttle closed. This doesn't happen all the time but when it does, like when joining a motorway and accelerating in 2nd gear it can be extremely annoying and this issue just seems very strange to me so any ideas would help. It also only happens under load, free reving it's fine and I've not noticed it in 1st gear either, and well I'm rarely full throttle in 3rd gear, but I imagine it isn't a gearbox issue as otherwise it definitely wouldn't be intermittent.

Any help would be greatly appreciated.
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tschultz
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Re: Won't Idle Low and Hesitates at High RPM

Post by tschultz »

Ground cable issueS?

Faulty AFM track?
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Microang
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Re: Won't Idle Low and Hesitates at High RPM

Post by Microang »

I don't think it's a grounding issue. I checked and adjusted the timing and noticed that cylinder 1 is dropping in and out, especially as the RPM increases. The plugs are new, and the rotor and cap are also fine, changed the leads over to another set and still the same issue. Could it be the ignition module?
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Brucey
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Re: Won't Idle Low and Hesitates at High RPM

Post by Brucey »

on a 628 CSi sparks are generated by the L-jet ECU, the ignition mapping unit, the amplifier and the coil, and distributed to each cylinder by the, er distributor. An intermittent ignition fault on one cylinder only has to be the plug, the plug lead, or some fault in the distributor.

Hesitation at ~4000rpm as you describe may be fuel starvation. A very common cause is that the in-tank pump isn't able to pick fuel up. This happens if

- the in-tank pump isn't working
- the fuel pickup inside the tank is faulty
- the fuel level is too low

in combination with brisk acceleration that is sustained for more than a few seconds and/or a blocked fuel filter. Some models only have one fuel pump anyway and these are always bad when the fuel tank is less than 1/4 full.

The throttle will only work properly at small openings and the idle will only work if you have

a) no air leaks
b) the correct setup in the TB and TPS
c) a functioning idle control system.

There is a whole procedure that you have to go through to make this work properly. But it sounds like the TPS opens when the main throttle butterfly isn't admitting enough air to the engine; this is a common cause of a stall as you describe, due to faulty setup.

cheers
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Microang
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Re: Won't Idle Low and Hesitates at High RPM

Post by Microang »

Do you know where I could read on the procedure you describe? Is it possible to be the ignition coil or module? Also I've noticed that although it runs at idle going up and down when you put load, like power steering is when it will stall without upping the idle speed? The power steering and brakes work fine when the engine is able to keep running but would that help narrow down what it is?

Thanks.
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Re: Won't Idle Low and Hesitates at High RPM

Post by Brucey »

the TB setting procedure can be found in Haynes, BMW, and other manuals.
If the TB is set too far closed (it tends to close itself through wear BTW and needs resetting quite often) then the idle control will have to work overtime to keep the engine running and may stall when the PAS is used etc.

If you have unmetered air leaks the engine won't idle well and may stall; the mixture strength will be all wrong and again the engine will stall easily.

Could be either or (more likely) both of the above.

cheers
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Dmckim
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Re: Won't Idle Low and Hesitates at High RPM

Post by Dmckim »

Had a similar issue. Ended up being that the wires were switched on the TPS. At low idle was getting a lot of fuel, choking it out. At WOT, it starved it for fuel, stopping it for a minute until the throttle came off of WOT.

Ron on the board here helped me diagnose this on a late night phone call!

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Microang
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Re: Won't Idle Low and Hesitates at High RPM

Post by Microang »

So I have a bit of an update, the car was tuned for it's MOT as it failed on emissions last month. The idle is now a little better but it needs more work.

However I have since noticed an annoying issue, which is when it is in higher gear (4th and 5th) cruising around 1500-2500 rpm it hesitates and the whole car jitters. I have also noticed that the strange rustling sound I get at high RPM (5000+ rpm when I'm on the accelerator) up is caused by the clutch springs, at least I'm guessing as it makes a similar noise at idle, but it goes away when the clutch is depressed, just as the noise at higher rpm goes away when I let off.

Does this still sound like a tuning issue or would these combined problems mean imminent clutch failure? The rpm doesn't seem to rise abnormally when you floor it in high gear though, but the issue may just be beginning?

Thanks for all your help guys.
86 BMW E24 628 CSI Zinnoberrot - 260/6 Manual Conversion (Saved from the grave)
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