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I have a 1999 Chevrolet Lumina with a 3.1L engine and 200K miles. At idle the engine runs slightly rough. It’s not that bad, but noticeable. Also, when the engine is cold and running it will occasionally “stumble” like it is going to stall. The car runs great when driving. I’ve done the following:
*Changed the spark plugs
*Changed spark plug wires
*Cleaned EGR valve
*Cleaned EGR air passageThe car passes smog inspection and there are no trouble codes on the OBDII. Could this be an issue with the intake manifold gasket going bad? Thanks…
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Should be pretty easy to check for any leaks during those conditions with carb cleaner to find out for sure.
i was just going to suggest the possibility of an intake leak, but collegeman beat me to it!
this video might also be helpful:
I agree with the suggestions above especially about looking for a vacuum leak. Well done on starting with the basics like the plugs and wires. The absense of a check engine light is also an important clue as it means that the computer does not detect a fault. This would mean that it’s likely something basic. If it were me I might check the operation of the idle control valve next. This is a common failure point that can cause issues with idle but not set a check engine light. One test I do is to tap on the valve while the engine is running. If the idle changes or stumbles then I normally replace the valve. Many people try and clean them but personally I haven’t had a lot of luck with that.
Keep us posted on your progress
I tried finding the vacuum leaks with the carburetor cleaner technique, but I couldn’t find any leaks – idle kept steady.
I also checked the idle control valve by tapping it with the handle of a screwdriver. Nothing happened – idle also kept steady.
I think this might be related to another issue involving the intake manifold and a coolant loss issue. I’ll continue to research the forum for more info… Thanks to everyone who helped with suggestions! 🙂
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