Abnormal Engine Vibrations

[Firehawk94]

My car is a 94 Camaro with the 3.4L V6. The engine only when at idle with the ac on vibrates like a mother. It idles around 800 rpm which is where it is supposed to be. Does anyone else have this problem? I gave the car a tuneup and all. I'm all out of ideas to fix it! Please any help will be absolutely appreciated.

[slow6]

motor mounts?
is it misfiring?

[98 3.8]

Did you say that's only with the AC on?

[braindeadracefan]

Maybe the a/c compressor is going bad...?

[Firehawk94]

No it isn't the motormounts. The motormounts wouldn't cause the vibrations. The engine is causing the vibrations. And it only does real bad when the ac is on. The motormounts are tight and NOT worn out. The engine does misfire a little, but only when
I let off of the gas when I'm going down a hill. I'm guessing that is normal though since there is no cat and my exhaust is breathing through a flowmaster. My ac compressor is in good shape, I do know that when I turn on the ac the rpms go down to about 700-800. Now I do know that when u turn your ac on the engine is supposed go to a fast idle.....my car doesn't do that it idles down. This sounds real puzzling don't it?! Gonna be hard to fix this problem.....

[Scarano]

How about a crack in one of the manifolds or a breach of some sort at the EGR valve.

[TraceZ]

Did you ever fix this?

I can 95% guarantee to you it is the motor mounts.

I just went through the exact same thing with my 94 3.4 Camaro. It would vibrate like a mother at 600 RPM idling in drive. Idling in nutral at 800 RPM it was almost gone. 900 RPM, almost undetectable. 1000 RPM completely gone.

I replaced the motor mounts and she is smooth as glass now. They were $35.00 each at dealer discout price and it took me 2 hours to change them out.

Seriously, for as cheap and simple as this fix is, give it a shot. It sure it will fix it. It is not uncommon for 12 year old mounts to dry up and distort inside. The rubber in mine was noticable shrunken compared to the new ones.

I know i'm a noob on this forum, but trust me here. I joined this forum just for this issue, but I had the problem solved before my membership was approved. I figured I might as well post the solution to the problem anyways.

I'm going to go ut on a limb and guess your car is a 5-speed. Your used to the higher idle. If your car was an auto and idled in drive at 600 rpm you would of noticed the problem a long time ago.

[Antiricer4u]

Oh and just to clear it up....

A misfire will not happen when your off the gas...at least not noticibly...any popping and or banging on deceleration (or letting off the gas in anyway) is a backfire. A misfire is charecterized by a noticible loss in performance as well as jerking and sputtering since the engine has to crank on a piston that is'nt hitting when it should...if at all.

You'll know a misfire when you have one... A backfire is preatty normal occurance no matter what whenever you take an engine from high to low RPM's jsut due to the sheer amount of fuel being burned.

[FunkZ]

You're correct in that when you turn the A/C on the car should go to a fast idle to handle the additional load of turning the compressor. If there's a compressor bearing going out then it could be causing a bigger drag on the motor, making it idle lower than it should. Or it could be that it's just not going to fast idle.

I'm thinking a simple test might be to remove the serpentine belt, start the car, take note of what rpm the engine is idling at, and then turn on the A/C. Since the belt is off the engine rpm's should go up without the load of the compressor.

BTW - my car idles around 1050-1100 rpm normally, and up to 1200 with the A/C on.