Erratic Idle Speed

[Robert S.]

Configuration: 74', L48, 4-speed. Correct part number Quadrajet which has been rebuilt. Thermal vacuum system hooked up as part of vacuum advance system. Electrical switch in vacuum advance system is present but is not connected as part of the vacuum advance system. Everything that connects to the Quadrajet is as it came from the factory except AIR system is missing so diverter valve vacuum hose fitting is capped off on Quadrajet. Idle speed is controlled by Idle Stop Solenoid which works as intended.

What is right: Normal engine operation is exceptional at all throttle conditions and all ranges of ambient air temperatures from 32F to 95F. No stumbling, stalling - basically almost as good as a modern computer controlled injected engine. Starting is fine. Car has normal high idle when cold and comes down normally as it warms up

What is not right: Car, once warm, will idle at times at 1200 rpm and on the same drive a few miles later will be at 900 rpm but mostly wants to idle at 1200 rpm. If I adjust the 1200 rpm idle down to 900 then it tries at times to idle at 600 rpm. No matter which idle speed it is running the idle speed has a slow fluctuation which can be heard more than see on the tach. - I am guess 50 rpm variation - it slowly rolls up in speed and then down in speed and then up in speed in a rythmatic pattern.

I do not see any obvious vacuum leaks.

Any other ideas or T/S tips? has anyone ever experienced something like this?

I am thinking that I should bypass the thermal vacuum system temporarily to see if that affects the idle

[Michael W.]

Re: Erratic Idle Speed

A Qjet that sticks temporarily at a higher than normal idle speed may have a defect where the primary throttle shaft has worn an elliptically shaped hole in the baseplate. This due to extensive use and the constant pressure of the return springs.

With the engine shut off, grab the throttle bellcrank and attempt to move it radially. If any play is felt a rebuild is in order.

This is a common item on Qjets and is relativley easy to repair.

[Jack H.]

Re: Erratic Idle Speed

Such can also be caused by the distributor. Two issues come to mind: (1) centrifical advance weights 'sticking' and not allowing the return springs to crisply pull the weights back to their zero advance position, or (2) the vacuum advance unit being defective and/or replaced with a 'similar' but NOT the correct original unit.

If #2 is suspected, a quickie test is to take the car for a drive, get the engine fully warmed, run 'er up through the gears and decel to a stop to re-create the higher than normal idle RPM condition.

Now, with the engine running at elevated idle RPM, shift into 1st gear, keep your foot on the brake and slowly let out the clutch to put a load on the engine, without stalling, and momentarily FORCE a lower engine speed.

Push the clutch back in and shift into neutral again. Did the idle speed drop and return to it's normal curb idle RPM? If so, that suggests one of the distributor advance components (centrifical and/or vacuum) isn't fully releasing from its emission controlled elevated idle position...

It's not atypical to see this kind of 'Jeckyl/Hyde' curb idle behavior on emissions controlled cars (1971 and later) and it's one of the things savvy PV judges look for during test drives...