Three things I know and have seen to cause a surging idle:
1) air in the cooling system - bleed the system to purge all the air out. Depending on the engine, the bleeder screw may be on the thermostat housing or on the head where the upper radiator hose connects.
2) bad or clogged 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.
1) air in the cooling system - bleed the system to purge all the air out. Depending on the engine, the bleeder screw may be on the thermostat housing or on the head where the upper radiator hose connects.
2) bad or clogged 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.