Idle speed and air con ?

[stswmc]

I own a 2000 1.8T , 99k , and have just noticed that the idle speed repeatedly increases from 800 rpm to 2200 rpm over about 20 secs then falls back down again . Unless however the air con is on! the harder this works ( ie the lower the temp setting ) the lower the idle revs are held near 800 with no hunting. Have cleaned the throttle butterfly. Is there a faulty temp sensor that the air con overides in the engine management? Ideas gratefully recieved before submitting to the main dealer diagnostics.

[Lex Luthor]

Ideas gratefully recieved before submitting to the main dealer diagnostics.

With the cost of a registered VAGCOM at £50 plus a cable for a tenner and an existing laptop you really should not be paying the dealer £70-£90 per hour for things you can do yourself.

The fault you describe is probably caused by a bad coolant temp sensor.
Costs not a lot and is easy to change so do that first.

[stswmc]

Thanks for the reply. So the sensor is telling the computer the engine is cold , just like pulling the choke out in a bygone era . Will try that , thanks again