uneven idle when warm

[blease]

I wonder if I can ask the sages if there is a sensible place to start with this problem. There is lot of advice out there but perhaps this story points to a specific problem.
Got an 89 S2, stock motor AFAIK.
Starts from cold perfectly and idles properly for a minute or two until it warms then idle becomes progressively rougher. If it had carbs I would say it is 'hunting'. Smoothes out over 2,000 rpm and pulls well although it smells a bit overfueled. Once fully warm, roughness makes it very difficult to drive at town speeds. Never actually cuts out but feels like it will.
Have swapped DME relay with no effect.
Car does burn oil but no smoke visible on start-up, idle etc.

With the problem occurring when warm is there a good place to start fault finding? Cheers all,
Simon

 

[TTM]

I would check the 02 sensor and clean the idle stabiliser valve with some WD40 or something similar.

 

[zcacogp]

As jamface said. And think about taking the rear cover off the AFM and cleaning the tracks (or swapping with a known-good one.)


Oli.

 

[Mike_Pollock]

I would look at the temp sensor.

 

[blease]

Great stuff. Thanks gents. Will check Clarke's for the location of all these bits and bobs!

 

[J4CKO]

Mine occasionally does a bit of hunting, I was thinking Idle valve or O2 sensor, is your MPG rubbish as well ?

 

[alwigley]

ORIGINAL: A9XXC

I would look at the temp sensor.

+1 very likely culprit and multi meter is not likely to tell you the truth about it.......

 

[blease]

As with all things, life intervened and I really did not have the time to go into this myself. However, I really wanted to get the head gasket replaced (no idea of its provenance!) so I checked the car into my local trusted mechanic to do this and check out the idle at the same time. Turns out that the pipes to and from the ISV were pretty rotten and probably leaking so hopefully replacing those and cleaning the ISV will cure the idle. Glad I got the head gasket changed...it was also pretty rotten. pics to follow.

 

[blease]

Here's my old head gasket.

Pretty rotten around 1 and 4 and moth eaten everywhere else. I suspect I was not far away from the dreaded cooling short circuit situation.
Replacing the rubber pipes to and from the ISV seems to have cured that problem. Car feels great! Typically, the pipes had split on the underside and there was no way of knowing it was them until the inlet manifold was off allowing access to the ISV. At least fixing that problem made me also pay attention to the head gasket. If you don't know how old it is, replace it!