Trying to sort an idle issue

[ian974]

Hi guys,

I'm trying to get an issue sorted out, S1 elise with a rebuilt standard engine.
Engine was recently replaced, while the car seems to start up and run fine for a while, and runs fine once driving it properly, when it gets warmed up the engine seems to idle roughly with the revs jumping up and down with the idle valve catching it, it doesn't stall, but occasionally almost does. Revs don't jump or stick too high either, probably up to 1300 rpm max. It seems to rev right through the range properly. I'm not sure if it might be using more fuel as well, I think the mpg might be lower than usual but I've only been keeping more of an eye on it recently so might be ok :paperbag:
The IACV has recently been replaced. I've tried a couple of throttle bodys so I don't think it would be the throttle sensor, but guess its still possible.
Other things I was going to check is air intake sensor at the engine end of the manifold, and coolant temp sensors. I've searched about and apparently the lambda sensor can cause issues like this too? Other thing I've heard it can be is possibly ECU but obviously want to check through other options first. Have checked about and doesn't seem to have any air leaks.
Any ideas what the likely culprit would be?

Cheers,

Ian

[Dominic]

I'm far from expert, but mine was having similar issues recently. There were several problems, but the most significant was the (after market) air filter that was getting clogged up. A quick clean out made a big difference. It would be nice to think you could solve your issue with similar ease.

[mwmackenzie]

I'll have a stab and say coolant temp sensor.... can't remember the colour but the one under the housing that screws in up the way if that makes any sense?