Odd engine idle behavior

[Andy E]

Hello. Ref my1997 MGF MPI.

On a recent drive to the MOT station the engine idle became very erratic. Idle speed varied between 900 and about 2500 rpm but thankfully the fault cleared by the time I got to the testing station. The trip home was fault free as have been a few other drive outs since then. On other occasions the idle has varied between about 900 and 1300 rpm. A TPS reset made no difference.

Yesterday morning I started the engine and it went straight to 2000rpm. I shut it down and restarted it and the same thing happened. I shut it down without letting the engine get warm. This morning I started it and it went straight to a normal approximate 1300 rpm and everything seemed fine as the engine got to temperature and the idle settled at about 900 rpm.

The throttle cable is very free moving and the throttle butterfly valve is also free moving and the spring easily returns it to its stop. I removed the IACV and this seems to be working as with it still connected to the loom the plunger moves in and out slightly when the ignition is turned on and off. The IACV was clean as was the hose feeding it.

Does anyone have a suggestion as to what else I could check? Some years ago I had a similar problem with the idle speed not reducing below about 1300 rpm. This was cured by replacing the ECM coolant temp sender. Is there a failure mode for this sender which could cause the above issues?

[Charless]

Maybe try cleaning the coolant sensor connector and check the wiring back to the ECU?

[Andy E]

Thanks Charless, I'll give that a go.

[Rob Bell]

Yes, fast idle when warm can be related to the coolant temperature sensor failing (the engine ECU will think the engine is cold and keep the engine in cold-start conditions - it will over fuel and maintain a fast idle).

Two other things to consider:
1. A vacuum leak - given that this problem seems intermittent, the most likely explanation for this is that the idle air control valve is sticking, letting more air in (equivalent to opening the throttle) - thus causing a fast idle
2. the throttle itself is sticking open - sometimes found on cars with plastic throttle bodies (many have been changed to the later metal type) - usually caused by someone in the past replacing the original hose clip with a jubilee clip that has then been over-tightened, causing distortion of the plastic body...

[Andy E]

Hi Rob

I've checked for mechanical sticking, the throttle cable and the butterfly valve in the throttle body all seem to be clear of any undue friction. The throttle body is the earlier plastic bodied item but it does still have the original air hose clip. Although I have stripped and checked the Idle Air Control Valve, maybe I need to try another one to eliminate it.

Could a failing temperature sender cause an idle speed significantly above the normal 1300 ish rpm cold start idle speed? I've seen speeds of up to approx 2500 rpm.

Thanks for the response.

[talkingcars]

In my experience this is probably an air leak in or around the inlet manifold.

[Andy E]

Thanks, something else for me to check. Having said that, the car has behaved itself since I carried out the last checks!