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nasty self-revving engine

Posted by nomis3613 
July 02, 2010 07:12AM
Hi, I suspect this is gonna mean a trip to the mechanic on Monday, but if anyone can help me solve it myself, I'd be most grateful!

The engine won't idle, without any throttle pressed it spikes to 1900rpm, drops to 1500, spikes again, etc etc. It's happened a couple of times in the past, but came good after a restart, this time it did it the whole way home (30 mins, including various restarts trying different theories). The weather has been damp every time it has happened. Other possible clues:
- disconnecting Idle Control Valve did nothing
- disconnected battery for 10 mins, no change
- disconnected throttle switch, caused the car to idle at a steady 3000rpm

Any ideas?

Many thanks,
Simon
July 02, 2010 07:26AM
Is the check engine light showing any codes?:

http://www.unofficialbmw.com/repair_faqs/motronic.html

__________
Dave
'91 325iX
July 02, 2010 10:02AM
Quote
Dave_G
Is the check engine light showing any codes?:

http://www.unofficialbmw.com/repair_faqs/motronic.html

If his car is Aussi spec, it won't have the 'check engine' feature; that is unique to North America thumbs down
July 02, 2010 10:03AM
Vacuum leak? Is the boot from the MAF to the intake manifold torn like mine was (underneath where you couldn't see it)? Oil dipstick complete seated? If it
isn't, it will cause the engine idle to "hunt".

alan
July 02, 2010 10:13AM
Quote
nomis3613
Hi, I suspect this is gonna mean a trip to the mechanic on Monday, but if anyone can help me solve it myself, I'd be most grateful!

The engine won't idle, without any throttle pressed it spikes to 1900rpm, drops to 1500, spikes again, etc etc. It's happened a couple of times in the past, but came good after a restart, this time it did it the whole way home (30 mins, including various restarts trying different theories). The weather has been damp every time it has happened. Other possible clues:
- disconnecting Idle Control Valve did nothing
- disconnected battery for 10 mins, no change
- disconnected throttle switch, caused the car to idle at a steady 3000rpm

Any ideas?

Many thanks,
Simon

From your clues, it would appear that there may be a problem with the throttle position switch. That would be my first place to look.
The TPS usually gets filled with dirty oil that seeps down the post that the throttle valve is mounted to. This is oil that comes from the return from the valve cover. Over time the oil builds up inside the TPS unit and eventually pollutes the micro switch that indicates 'closed throttle'. When this is out of commission, the Motronic unit doesn't know the engine is supposed to be at idle and assumes throttle is being applied when it isn't.

First think to check is that the TPS is closing properly at idle; clear instructions are found in the Bentley manual on how to test and set it. If that fails to correct the problem then the TPS will have to be replaced or cleaned. Since it is a sealed unit, surgery will be needed to open it. A simpler method of cleaning is to just submerge the whole thing in solvent for a few hours or over night then let it drain out the solvent and dirt. That may or may not be successful depending on the amount of crud that has gotten inside.
July 03, 2010 01:16AM
Thanks everyone!

Yeah, I wish I could do the Stomp Test. Very jealous of your NA spec cars!!

Doing the Bentley tests, I found that the Throttle Position Switch is actually getting 11v (Bentley says it should be 5v) with the ignition on and, even more strangely, 2v with the ignition off. I think this is the problem. What could be causing this?

Continuity across the TPS was fine, as was the test across the Motronic pins.

And now the car is just idling at a steady 1800rpm.

On a different matter (although it could be related), my battery is measuring 12.2v with the ignition off, dropping to 12v with the ignition on (without the engine running). Is this ok? The battery is only 1 year old, but sometimes the car doesn't get driven for a few weeks at a time.

Thanks,
Simon
July 03, 2010 10:23AM
A bad battery could be causing problems and it should be at 12.8v with nothing turned on. But the important thing is what the voltage levels are when it is running; in which case it should be closer to 13.4 volts.

Since the Motronic unit supplies the voltage to the TPS, I would double check the center pin tin to make sure it is at 0(zero) volts since it is the grounded pin. If it isn't then check the ground cable that connects the engine block to the chassis; it' on the same side of the car as the TPS.
July 03, 2010 09:29PM
Thanks Peter. I checked the centre pin and it's 0v compared to the earthing point, so that's ok.

The +2v on the outer pins with the ignition off is really strange. I think I need professional help with this one! So I'll take it to the mechanic tomorrow.

Thanks,
Simon
July 21, 2010 10:00PM
Just thought I'd let you know what the problem turned out to be: clogged up Idle Control Valve.

I'd discounted it early on, because I replaced it only 2 years ago. Mechanic diagnosed it, cleaned it out and now it's fine. Actually it's better than fine! The low throttle inputs are much less snatchy than before. Bonus!
July 21, 2010 10:15PM
Quote
nomis3613
Just thought I'd let you know what the problem turned out to be: clogged up Idle Control Valve.

I'd discounted it early on, because I replaced it only 2 years ago. Mechanic diagnosed it, cleaned it out and now it's fine. Actually it's better than fine! The low throttle inputs are much less snatchy than before. Bonus!

It sure feels good when you can put one of these gremlins to bed thumbs up

I never get tired of how beautifully smooth the M20 runs; it's like a sewing machine and if you plant your foot, it's like being in the pouch of a slingshot or crossbow just loosed B)
July 21, 2010 10:37PM
Quote
Archeo-peteriX
I never get tired of how beautifully smooth the M20 runs; it's like a sewing machine and if you plant your foot, it's like being in the pouch of a slingshot or crossbow just loosed B)
Amen!
July 21, 2010 10:43PM
I was at a car show last Saturday night and came across a 1600 with an M-20 in it. What a rocket that must be.I almost peed myself.

grinning smiley

alan
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