Idle Stop Solenoid-? for the Judges

[Kevin Morris]

Hello again all. My '72 350-200hp, 4-spd. car (w/o AC) has a Rochester Q-Jet from another car (a '69, I believe). This carb has the Idle Stop Solenoid mounted to it, on the left (driver's side) front corner of the carb, via a bracket. The solenoid has a single-prong, single tan wire, leading to it from the wiring harness. I have heard both stories...that this solenoid is correct for ALL '72's (and maybe other years, as well)...and I have heard that this solenoid was suppose to ONLY appear on air conditioned cars. Does anyone know for certain?

Kevin #39927

[Joe L.]

Re: Idle Stop Solenoid-? for the Judges

Kevin-----

I believe that all 72s used the carburetor idle solenoid which was mounted on the left (driver) side of the carb. However, different solenoids were used over the years. For 1969, the solenoid was used only with A/C applications and was GM #1114421. For 1972, the solenoid used was GM #1114444. Both are discontinued.

[Kevin Morris]

Re: Idle Stop Solenoid-? for the Judges

So Joe...what does this solenoid do? I thought it was only for A/C cars, so that when the A/C was turned on and the RPM's subsequently dropped, the solenoid would actuate and push the idler arm on the carb up a little to compensate? What would it do on a non-A/C car like mine?

[Joe L.]

Re: Idle Stop Solenoid-? for the Judges

Kevin----

I believe that the solenoid was added as part of the TCS and/or other emissions control systems. These early emissions control systems were, essentially, "desperate acts" on the part of the automakers to quickly deal with then-new and evolving emissions regulations. Most of these systems had "side affects" just like many medications. In order to "treat" the side effects, other systems had to be added to the cars. I think that the idle control solenoid was one of them. Also, I think that idle control variations which the solenoid, influenced by other components, could control, actually produced part of the emissions reduction and was, therefore, an integral part of the emissions control strategy.

[Michael W.]

If George Romano were here, or Patrick

Hulst, either could vouch for the presence and function of the solenoid. George is the proud father of a new baby boy, born just this morning, and Dr. Patrick is at the Nationals.

I believe the solenoid on a '72 is similar in function to a '73, in that its function is to eliminate dieseling after engine shut down. When properly adjusted, the solenoid plunger opens and holds the carburator at the normal idle speed. When the ignition switch is turned to the off position, the plunger retracts allowing the throttle blade to close completely which blocks off air flow eliminating dieseling.

A common condition seen today on early to mid 70s cars is for the idle speed to be erroneously adjusted with the set screw on the carburator body. This screw is to be set to gently allow the throttle blades to just touch the venturi body, but not allow the passage of air. Idle speed is then set by turning the hex head of the plunger - the shaft it is attached to is threaded inside the solenoid housing.

[Clem Z.]

Re: If George Romano were here, or Patrick

that is correct the solenoid was use to set the idle speed so the engine did not "run on" when you shut of the key. the carb idle screw was set not to completely close the butterflys but to be at a idle speed that would not allow "run on" but not enought to keep idling under all condititons so the solenoid "kicked" up the idle speed up to what was needed.

[Jack H.]

No need to guess....

you'll find a complete theory of operation description of the emission system in your copy of the 1972 Chevy Chassis Service Manual. Solenoid gave the carb two discrete curb idle settings and along with the TCS vacuum solenoid did the equivalent of the '71-only CEC (combined emission control) system....

[Patrick H.]

Re: If George Romano were here, or Patrick

Mike,

Great explanation.

In fact, it's the only thing that didn't function as intended during Ops check of the 72. I had removed and tested mine about a week prior, and forgot to re-set idle (I counted turns out and then in instead), and lost 2 points for idle too high.

Patrick