Forum:
My 1.3's engine rises and falls constantly when at idle - but rpm changes when the heater fan is put on. This is driving me mad - it sounds like I'm revving the engine when sitting at junctions/lights. I've fitted new raditor cap, have checked for air leaks and am sure there's no air in the system. An injection cleaner has been run through the tank, the thermostat seems to be ok, thinking it may be a sensor, but which one? Thoughts please?
Engine coolant temp sensor,
Engine coolant temp sensor, But I think car would be driving bad with check engine light on.
Or there is a inlet air leak somewhere introducing too much air at idle. This could be from the throttle body air bypass that controls the idle for auto choke air.
This could be from the throttle body auto choke mechansim sticking open. As soon as engine starts to warm up, if the choke does not close of airflow, the hunting at idle starts.
The speed changing with heater on, this is due to the extra load on alterator, so it affects engine speed.
I presume there are water pipes running through the throttle body? Take off the main air inlet pipe so you can see the throttle body butterfly flap. In front of it somewhere hopefully there should be a small air bypass hole. Stick your finger over this with engine running, to see if it lowers idle speed and stops the hunting.
If it does then it's the auto choke bypass valve is sticking open. You may find lightly tapping the base of throttle body with a mallet may free in up.