Purpose of fast idle selenoid?

[Mike R.]

What is the purpose of the fast idle selenoid on a L89 with automatic and no AC?

Thanks

[Duke W.]

Re: Purpose of fast idle selenoid?

It's probably there to prevent engine runon.

What's the recommended idle speed in Drive?

With the big cam it must be pretty high, and then when you put it in Neutral revs probably go up over 1000.

What does the CSM say about adjusting the solenid?

The ported vacuum advance and low initial timing spec for emissions heats up the combustion chamber surfaces and for this reason these early emission controlled engines tended to run-on when shut down.

These "anti-dieseling" solenoids were pretty common on GM engines in the seventies. My '72 Vega GT had one. The recommended idle speed in neutral (It was a manual trans) was 1200 with the solenoid energized and 700 de-energized. The high idle speed cause the revs to hang on upshifts and made the car difficult to drive smoothly. When you shut off the engine the solenoid de-energized closing down the throttle plate so it would stop.

The high idle speed was for emissions. I just disconnected the damned thing and set the idle at 700.

Duke

[Mike R.]

Re: Purpose of fast idle selenoid?

Thanks Duke, I didn't even think of the difference in idle speed between neutral and drive. That must be it. My previous corvettes have all been 4sp.

The body is off for paint on Wednesday so hopefully I will be setting the idle sometime in March.

It's probably there to prevent engine runon.

What's the recommended idle speed in Drive?

With the big cam it must be pretty high, and then when you put it in Neutral revs probably go up over 1000.

What does the CSM say about adjusting the solenid?

The ported vacuum advance and low initial timing spec for emissions heats up the combustion chamber surfaces and for this reason these early emission controlled engines tended to run-on when shut down.

These "anti-dieseling" solenoids were pretty common on GM engines in the seventies. My '72 Vega GT had one. The recommended idle speed in neutral (It was a manual trans) was 1200 with the solenoid energized and 700 de-energized. The high idle speed cause the revs to hang on upshifts and made the car difficult to drive smoothly. When you shut off the engine the solenoid de-energized closing down the throttle plate so it would stop.

The high idle speed was for emissions. I just disconnected the damned thing and set the idle at 700.

Duke