Rough Idle Problems

This diagnostic takes you through the possible causes for a carbureted GM product to idle roughly or "hunt" for a stable idle RPM . The diagnosis can be used in general for your car - just skip the areas which don't apply. A lot of the items even apply to fuel injected systems.

Perform "Diagnostic Circuit Check". This check has three purposes. First, it determines that the "service engine soon" light is functional. Turn the ignition switch on and observe the light. Second, it checks to see if the computer is capable of storing "faults codes". Third, it checks to see if there any "fault codes" stored in the computer. Short out the A and B terminals on the ALDL and observe the flashing light. It should flash a "12" indicating that the ECM is functional. Write down all stored codes and send them to me for further analysis if you don't have the service manual.

Visually (physically) check:

Vacuum hoses for splits, kinks and proper connections as shown on the Vehicle Emission Control Information label.

Ignition wires for hardness, cracking and proper connection at both distributor cap and spark plugs.

Check throttle linkage for sticking or binding.

Check carburetor for flooding - look down the throat and see if there is gasoline flowing from internal seams or jets.

Check float level - most modern carburetors have a way to check float level externally thorough the vent hole (tube)

Check the Idle Control operation - on most carburetors this is an electrically controlled servo-motor.

Check the engine idle speed, both base and ECM controlled. See the vehicle Emission Control Information label.

Check the EGR operation - there should be NO EGR at idle. You may have to remove the EGR Valve to inspect the internal valve. Even a slight carbon build-up can cause an extreme vacuum leak internally to the EGR Valve.

If the engine idles roughly when it is hot, then check the following:

Check the PCV Valve for proper operation by placing your finger over the opening (while running). You should feel vacuum on your finger however when released the valve should snap shut. If it hisses at all, replace it. Also, remove the valve from the tube an shake it. It should rattle like there is a loose marble inside. If it doesn't rattle, replace it.

Check the canister purge and vent system. It should NOT apply vacuum to the system at idle. There are many types of control systems for this device however if you remove the hose labeled "tank" and feel vacuum at the port during idle then you need to service the system and probably replace the control valve. If that is the cause, blocking the "tank" port will stop the rough idle.

Check the spark plugs for proper gap.

Run a cylinder compression test.

Check the ignition timing. See the Vehicle Emission Control label for information.

Check the exhaust system for restriction or blockage. A clogged catalytic converter will cause high speed as well as idle problems.

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