Heu posted this a while back on the Resource:
dohcrxl writes: I noticed that the surging idle question gets asked from time to time and thought we could use my lil write up as a service to the community.
Three things I know and have seen to cause a surging idle:
1) air in the cooling system - when hot coolant comes in contact with the AICV (idle air control valve) it gives the ECU a reference as to what to set for an idle. The ECU then lowers the idle because it senses that the engine is hot. When it does that the waterpump of course slows down with the rest of the engine and is not spinning fast enough to pump a sufficient and constant flow of coolant because there are air pockets in the system preventing it. When that happens, the coolant that used to be in contact with the AICV now retriets, the ECU then senses that the engine is cold, raises the rpms, the waterpump surges and speeds up, pushing coolant back onto the AICV, and the cycle becomes endless.
2) bad Idle Air Control Valve - on the back of the IM and should have a blue w/yellow and a black w/yellow wire going to it. Cleaning the filter screen inside the AICV will help restore a more steady idle. To do so, you must remove the valve assembly from the throttle body and clean out with brake cleaner.
3) A leak in the throttle body or intake manifold gasket - with the engine running, carefully spray brake cleaner on the mating surface edge by the gaskets. If the engine bogs when you spray, there is a leak where you sprayed.
There is of course the possibility of leaking vacuum lines. If you don't hear or see any obvious leaks, unplug all the vacuum hoses one at a time. Each time after you unplug one, plug the exposed nipple at the manifold or throttle body with your finger. If the surging goes away you know it involves that line and or any component that line goes to. After you do this, put that hose back on and proceed to repeat that for each and every vacuum hose and it may uncover the problem. Another way to help isolate a pinhole leak in a vacuum line is to put your hand over the throttle body with the engine running. A leak will give itself away by the whistling it will make. Note* cover up only as much of the throttle body opening as needed to allow the engine to stay running yet force the leak to reveal itself. _________________
- Heu
