# idle revs dropping too low... then back up



## RadoGTST (Jul 5, 2010)

Here's my problem:

After starting a cold engine revs stay in region of ~950 for about a minute, then slowly start to drop gradually for another minute or so to about 600 or even less so the engine stalls sometimes if I leave it alone.

If I sit in the car and help it a bit keeping the revs at about ~900 it will be just able (650revs) to keep itself running after the temp. of water reach some 40-42C (may be just a coincidence, I don't know if it has anything to do with it)

As the temp. rises (or engine warms up, don't know) so do the revs, so...

45deg.C - 700revs
50deg.C - 750revs
52deg.C - 800revs
56deg.C - around 900revs

When fully warm they stay around 950 so as it should be.
Problem is that I have to keep an eye on it on traffic lights after driving off after say 2minutes when not fully warm.

Any ideas?

Thanks
Radek


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## mattysupra (Oct 31, 2008)

if you have after market ecu it may need a slight adjustment. 

If not then under the inlet plenum there is a screw in the end of a cast block thing. Give it a turn and it should higher or lower the revs. 

O and check for air leeks.


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## freakazoid3 (Jan 19, 2008)

I'd say the AAC valve (idle control) as above needs adjusting and/or cleaning


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## GTR RGT (Aug 29, 2006)

maybe coolant temp sensor, if you have an aftermarket ecu then maybe the calibrator didnt set up the warm up enrichment


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## Darren-mac (May 13, 2009)

I had the same problem and it turned out to be my o2 sensor. Replaced it and it was sorted


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## RadoGTST (Jul 5, 2010)

It was all ok until about 2mths ago when it started, didn't adjusted or touch anything so it's definitely not a case of adjusting or changing settings I think.
More like something's clogged, dirty or just gave up...

How do you clean AAC valve? Never did that before...


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## RadoGTST (Jul 5, 2010)

Darren-mac said:


> I had the same problem and it turned out to be my o2 sensor. Replaced it and it was sorted


Both of my o2 sensors show some 3.88v right after start, then after some 30sec on my powerfc o2h comes on and both start to drop slowly to 0.1 - 0.15v, so do the revs. Anyone know if this is right?
Cheers


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## GTR RGT (Aug 29, 2006)

Both should continue to drop as the car warms, same for the revs. They should both read almost identical to each other. They should fluctuate and 0.1 is normal. 

They're usually not used by the powerfc until closed looped so you don't need to monitor them until car is at operating temps. hope that make sense


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## RadoGTST (Jul 5, 2010)

GTR RGT said:


> Both should continue to drop as the car warms, same for the revs. They should both read almost identical to each other. They should fluctuate and 0.1 is normal.
> 
> They're usually not used by the powerfc until closed looped so you don't need to monitor them until car is at operating temps. hope that make sense


Good. So it looks they're both fine.
My revs continue to drop, but to a point when the car is all shaking and stalls. Help it a bit with acc.pedal and it's all sweet.
Still have a minor misfire though.
I want to get to the idle valve next and try to clean it. I will have a look into workshop manual, someone has a picture of it?  (unless it's the adjusting screw under the plenum  )
Thanks


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## Swobber (Oct 8, 2006)

The problem you have is very typical for a coolant temperatur sensor gone bad.
If possible, have a look at the temp sensor input to ECU when starting up, and during warmup.
This is normally a different sensor than the one used to show temp in the instrument.


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## GTR RGT (Aug 29, 2006)

The aac isn't the easiest job, especially if you have big arms and a fuel filter in the way  
As I said before, Id check or change the coolant temp sensor first as its easier


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## RadoGTST (Jul 5, 2010)

Swobber said:


> The problem you have is very typical for a coolant temperatur sensor gone bad.
> If possible, have a look at the temp sensor input to ECU when starting up, and during warmup.
> ...


Ok, this morning it was showing 3.96v when cold and started to drop as the engine was warming up. Guess that's ok...?
Other thing I noticed was the o2h thing on my powerfc commander, as soon as this comes on, both readings from o2 sensors start to drop to 0-0.1v. That happened exactly at 20degreesC. 
Is this when the thermostat opens the flow of coolant? (pardon my technical english ;-) 
Have to confirm this but I think the temperature rise was a bit slower after this, will confirm later


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## GTR RGT (Aug 29, 2006)

Yes them readings are normal. The 02 sensors only work accurately once warmed up, hence why the powerfc ignores them until warm. The calibrator would of put in a switch over at a desired temp based on coolant temp. 

Id think your thermostat would open alot later aswell


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## RadoGTST (Jul 5, 2010)

Hmmm....

The temp rise is constant actually.
When water temp reach 20degC revs drop by about a 100 immediately (thats when the o2h thing on commander turns on) and this is when it begins to struggle and dies after another minute or two


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## RadoGTST (Jul 5, 2010)

Does it mean that my sensors are ok? 
When faulty, can they take inacurate readings or should be completely dead?


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