Idle Problem Revisited - Opinions Welcome

[repenttokyo]

1991 M50.

So last week my idle started to surge between 1,200 and 1,500 rpm while stopped, as well as pin there at times as well. Occasionally it would drop back down to normal, but these occasions grew fewer and fewer.

I removed and cleaned my idle control valve, and I also replaced the rubber elbow between the MAF and the throttle because it was full of holes. I cleaned the MAF with CRC cleaner.

Now I still have an idle problem, but it is different. If the car is moving and I put it in neutral, it idles at 1,300 rpm. If I come to a complete stop and wait about 7 seconds, it drops back down to normal idle. Also, if I rev the engine while stopped, it drops back down to normal idle, pausing briefly just above the 1,000 rpm mark.

I don't know what could be causing this. Is it possible I need a new idle control valve and that cleaning it wasn't enough? Theories? Thanks!

[BennyM]

I'm assuming that your didn't have any error codes stored in the computer?

You might want to check the temp sensors. A temp sensor could make the engine think it's really cold outside and force it to run faster. The Bentley has a good guide for testing these. It'll also show you how to test the MAF and TPS electrically.

There are numerous vacuum components that could let air in causing it to surge at idle. It sounds like you've only addressed a few of them so far. You started with the right ones, though.

[repenttokyo]

I'm assuming that your didn't have any error codes stored in the computer?

You might want to check the temp sensors. A temp sensor could make the engine think it's really cold outside and force it to run faster. The Bentley has a good guide for testing these. It'll also show you how to test the MAF and TPS electrically.

There are numerous vacuum components that could let air in causing it to surge at idle. It sounds like you've only addressed a few of them so far. You started with the right ones, though.

the thing is, the car isn't surging at idle. The surge problem disappeared after I cleaned the ICV. Now I have two idles - one when the car is in motion, and one when the car is stopped. That makes it difficult for me to consider a temperature sensor as being the problem.

[repenttokyo]

Anyone? Bueller? Bueller?

[Ferret]

I'd be digging further for more vaccuum leaks - though have you tried seeing if unplugging the maf gets it to settle down?

Thing is a tiny leak can cause a high idle because the ecu has the ability to iron out the mixture issue but not start a heavy misfire.

The temp sensor suggestion could also be a good one? They're not horrendously difficult to get at on the m50 - maybe testing the air and coolent sensors with the old bentley manual will give you a hint?

[repenttokyo]

when the car is at rest, the idle is fine. i cannot unplug the maf while I am driving to see if it reduces the high idle while the car is in motion.

Once again - with regards to temperature sensors, I don't understand how a temperature sensor can cause a high idle only while the car is rolling in neutral.

The vacuum leak seems to be the most likely culprit, from what you have mentioned. More air getting into the system as the car moves down the road makes sense, versus less air when it sits.

[BennyM]

when the car is at rest, the idle is fine. i cannot unplug the maf while I am driving to see if it reduces the high idle while the car is in motion.

I think you can try driving the car with the MAF unplugged and see if the idle becomes more consistent. It will obviously run rough, but with no reading, the ECU will substitute some factory default number so you can limp around.


Once again - with regards to temperature sensors, I don't understand how a temperature sensor can cause a high idle [B]only while the car is rolling in neutral.

If you're moving, then air is rushing through and changing the temperature of things. This may be most true for the intake manifold temp sensor, but you're probably right, 'cause that's the only way I could see it having an effect.


The vacuum leak seems to be the most likely culprit, from what you have mentioned. More air getting into the system as the car moves down the road makes sense, versus less air when it sits.

Does the idle change if you're holding the brake pedal down? That system is another potential vacuum leak zone.

[repenttokyo]

I think you can try driving the car with the MAF unplugged and see if the idle becomes more consistent. It will obviously run rough, but with no reading, the ECU will substitute some factory default number so you can limp around.

If you're moving, then air is rushing through and changing the temperature of things. This may be most true for the intake manifold temp sensor, but you're probably right, 'cause that's the only way I could see it having an effect.

Does the idle change if you're holding the brake pedal down? That system is another potential vacuum leak zone.

i will try driving with the MAF disconnected and see what difference it makes. With regards to the brake - it doesn't change the idle, so at least we can rule that vacuum leak out.

I suppose I should start hunting around the engine bay with a can of WD40 and see if I can affect the idle with a few carefully placed sprays?