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Loud idle in a 2003 Cavalier

My 2003 Cavalier idles loudly after a couple days of rain. My mechanic has changed the spark plugs, checked the wires, and replaced the muffler. The sound went away the last time after I drove it for approx 3 hours. It’s back again after this last rain. Any suggestions?

Is your check engine light on? Do you have a tachometer? Do you mean the car idles faster (at higher rpms) than usual, or do you really mean it is louder acoustically? How many miles on this car?

If the engine is idling at its normal RPMs, but is loud, I would suggest that you check your exhaust manifold. The manifold may be cracked.

Actually, the noise is coming from the front of the car, I don’t think it has anything to do with the exhaust - but I will ask my mechanic about that. I still think it’s related to moisture because it goes back to being silent after driving for a long drive. I don’t drive very far to work, so it only is driven over 40 mph maybe twice a month. I only have 58,000 miles on it, and the check engine light is not on.

Actually, it is possible for a long drive to silence the noise from a cracked exhaust manifold. As the manifold heats up, the expansion of the cast iron can temporarily seal a small crack. Then, after everything cools down again, the crack and its resulting noise become apparent again. I had this exact situation with a Volvo engine.

The noise goes away after a long drive, but does not come back until after it rains for a couple of days - would moisture have anything to do with the manifold? (The idle was fine for about a month.)

You still haven’t told us if the engine is idling at higher-than-normal RPMs when you hear the noise, or if it is idling at its normal idle speed when you hear this noise.

Do you have a tachometer on the dashboard?

It is idling at its normal idle speed, just louder - it usually idles almost silently. I can also feel some vibration in the steering wheel. This problem seems to be related to moisture somehow. It ran fine for about a month and a half after the last occurance.

Did your mechanic check the wires by spraying them with water?

I’m not sure about that – the car is idling at just under 1000 rpm -runs fine - would the exhaust manifold cost a lot to replace? I’m running out of money! Is this something that I could safely ignore for a while?
Thanks for all your replies by the way.

If the exhaust manifold is cracked (and we don’t really know if it is), this can lead to Carbon Monoxide leaking into the passenger cabin. So, as to whether this can be deferred or not–you will have to make this decision.

A used manifold could be obtained cheaply from an auto recycler (what we used to call a junk yard) and can be installed for not very much money.

Please keep us posted!

I put this in the wrong place, sorry. - Just wanted to say that the noise disapeared after accelerating quickly to 65 mph on an empty country road. The car is idling normally and quietly - it’s been three days now. I still think moisture is the problem - but can’t figure out where.