Can't seem to sort out idleing speed-its too high?!

[RubySport]

My Renault clio 1994 1.4cc idles at around 1500 when in neutral. It has been known to drop to around 1100 once or twice but is mostly very high. We have replaced the idle control valve but it hasn't made any difference. The car has only done 26,000 miles and is in good nick.
Any ideas what the problem could be?

[RubySport]

No other symptoms, we thought that seen as though it has gone down to 1100 occasionally then it would be unlikely to be an air leak because that wouldn't self correct. Would an electrical fault be possible? Thanks

[RubySport]

Yes its a single point fuel injection. We will check the coolant temp sensor in the way you described and see if that works, thanks.

[RubySport]

Coolant temp sensor seems to be working ok, we disconnected and reconnected it and it affected the revs. The car has a brand new idle control valve we bought in the last few weeks but today we read that it needs to be reset once it has been fitted so followed the resetting procedure as described in the haynes manual using a 5mm shim. Now the car won't idle at all and wants to cut out altogether unless the accelerator is pressed so we are now in an even worse situation! Could it be the rev counter itself is giving the wrong reading? Or could it be that the engine needs revs of 1500 in order for it to work? Any ideas greatly appreciated.