Slow return to normal idle

[RNajarian]

Hi. Gang,

My 1983 SC motor has 135K miles on it. After it was transplanted into my 1975 S I noticed the following problem;

At cold idle I rev the motor, it goes to the higher RPM then returns normally to the idle of 850 RPM. Driving it when cold also gives me a simular response.

However, once the motor is warm I get a different response. At warm idle I rev the motor, it goes to the higher RPM and stays at that higher engine speed well after my foot is released from the gas pedal. After 4-5 seconds the RPMs slowly decrease and fall to the warm idle of about 1000 - 1100 RPMs. This happens when stationary or when driving.

When the motor was in the SC I did not have this problem. As I stated earlier it only seems to have sthis problem when warm.

Thanks for any thoughts or suggertions.

[Christien]

Could be the pedal cluster - my car is similar to this, and it's just a case of the gas pedal not returning to normal resting position. If I reach down and pull it up, the engine returns immediately to proper idle rpm.

[RNajarian]

Yea I thought it was the pedal cluster as well but would that explain the cold/hot differences?

[Superman]

On my '83 motor, there is a hanging idle problem when the Decel Valve is working. The Decel Valve is one of the saucer-shaped brass thingies just to the right of the air box, as you are looking into the engine compartment. Above the #5 and #6 cylinder heads. They are about 3" or so in diameter. One is Auxilliary Air Valve. This has an electrical plug. The Decel Valve does not have an electrical plug, but does have a small vacuum hose to it. I plug the vacuum hose, which disables the Decel Valve and allows the engine to quickly return to idle when the throttle snaps shut.

Actually, I suppose the problem could be the other thingie, the Aux Air Valve. The electrical plug is supposed to close this valve off once the engine is warm. If it were to stay open, it would create a too-high idle when warm.

[911st]

What Superman said.

I'd plug the decel valve for crisper throttle response with throttle lift as it is a smog function only on an SC. Keep the AAV, that is a cold start function. They typically fail open with fixed elevated RPM but they can get funny before a full failure.

Also, you might have disturbed something and could have a vacuum leak. Put a vac gauge on it and see if it flutters. Or spray some carb cleaner around the intake runner connections or other suspect locations before motor gets to hot.

[prebordao]

Superman, what did you use to plug the decel vacuum hose ?

[kodioneill]

There is a repair for a hanging decel valve. Remove the valve, use a socket to cover the pipe that sticks out, put it in a vice and crush it down about 1/4 inch. Use the search function to find the adjustment procedure and testing of the aux air valve.