The erratic idle is normally due to a faulty IAC(idle air control) valve, which is located on the throttle body:
Fig. Fig. 2: Throttle body unit on an SFI vehicle
Before installing a new idle air control valve, measure the distance that the valve is extended. This measurement should be made from motor housing to end of the cone. The distance should be no greater than 11/8in. (28mm). If the cone is extended too far damage to the valve may result. The IAC valve pintle may also be retracted by using IAC/ISC Motor Tester J-37027/BT-8256K. It is recommended not to push or pull on the IAC pintle. The force required to move the pintle of a new valve should not cause damage. Do not soak the IAC valve in any liquid cleaner or solvent as damage may result.
Fig. Fig. 1: Model 300 TBI unit Idle Air Control valve; there are two types, one with a collar and one without
Fig. Fig. 2: The Model 700 TBI unit's IAC valve is basically the same as that for Model 300
Shiny spots on the pintle or seat are normal and do not indicate misalignment or bend in the shaft.
Be sure to clean the IAC valve O-ring sealing surface, pintle valve seat and air passage. Use a suitable carburetor cleaner (be sure it is safe to use on systems equipped with an oxygen sensor) and a parts cleaning brush to remove the carbon deposits. Do not use a cleaner that contains methyl ethyl ketone. It is an extremely strong solvent and not necessary for this type of deposits. Shiny spots on the pintle or on the seat are normal and do not indicate a misalignment or a bent pintle shaft. If the air passage has heavy deposits, remove the throttle body for a complete cleaning. Replace the IAC O-ring with a new one.
Yes, it is entirely possible for the IAC valve to be failing and not set a fault code to turn on the check engine light. I have replaced many of them that never set any fault codes that were bad
The IAC valve has a pintle that moves in and out and helps control the engine rpms when revving the engine. When the pintle is not moving correctly or sticking, the engine rpms become erratic when revving the engine.
Yes, I am here everyday until about 8pm MST, and you can re-open this question at any point, even after accepting, and it is directed straight to me.
Ok, sorry about that. Your description of the symptoms is dead on for a faulty IAC valve.
If you still have the valve, look at it and tell me if the pintle( the end of it) is black with carbon deposits.
Is this just an issue when revving the engine in park, or is it noticable when accelerating in drive(under a load)?
did you measure the distance the new pintle extended and make sure it didnt extend beyond 1 1/8" from motor housing to end of pintle cone, per above directions?
As long as it wasnt extended beyond 1 1/8" you are ok on that.
I am checking a couple of other things on this and will get back with you shortly. havent forgotten about you, just had a couple of family issues to take care of
Sorry it took so long, but my older manuals are all tucked away as I dont come across many of the pre 96 models anymore.
if the #3 cylinder has low compression, that is going to be a separate issue.
Do you know anyone that has a scanner with live data capability? I would like to get the IAC counts, as we could be looking at a pcm not sending correct voltage or info to the IAC. Also, the O2 readings would be helpful in knowing what the ratio is as to whether it is running lean or rich when this occurs.
In the meantime, spray the base of the intake manifold and the throttle body base with carb cleaner while someone is revving the engine. If the idle smooths out, let me know