OK this is a weird one.

I have a 1985 v-6 camaro (2.8 motor)

The motor has 180,000 miles on it (car has 248,000) Anyways, the motor was partially rebuilt about 15,000 miles ago. (new timing chain, rebuilt heads, new oil pump/pickup, water pump and rod bearings).

The car just passed NJ state inpection and seemed to be running fine....EXCEPT for a vibration at idle 700-900 rpms. The vibration is so bad it will shake the coffee out of your hand. The vibration is not temperature dependent. It happens whenever I am at a light or in park idling.

When i get on the gas, the vibration goes away and it runs fine at highway speeds with no miss or detectable problem.

I did do a compression test: All cylinders were 165-175 psi

I also did a leakdown test. On the driver side bank, the front cylinder leaked about 40% where the other two were about 20-25%

On the passenger side, the middle cylinder leaked about 40% and the other two were 20-25%

I could hear the air coming from the dipstick tube so I am lead to believe the rings are leaking but why a vibration only at idle?

I tried changing the plugs. The old plugs looked fine. no obvious misses or oil coated plugs. After the plug change, no difference.

Any suggestions on what I can try next?

 

Something's whacky. I'm thinking that either the flywheel/flexplate is cracked or the harmonic dampener on the front of the motor might be slipped. Something that is causing a physical imbalance that you are feeling in that RPM range. Make sure it's not an engine accessory, though. Yank the belt(s) and run it just for a minute or so to see if it's any different.

 

Vacuum leak

 

My 388 had a real bad shake at idle - fix; replaced motor mounts. I used the Poly type instead of the stock rubber.

 

I checked the Vacuum leak idea....pulls 19 inches at idle while shaking like hell.

I replaced the plugs, cap/rotor...still no difference.

Any other ideas? I am going to try the belt idea tonight.

Also forgot to mention, there is an audilbe "tick" heard through the tailpipe about the same frequency as the miss.

so when car shakes, you hear a tick sound. Does this imply anything?

 

Mine does the same thing except mine began to do it after i accdently put it in park when i was doing 45MPH

 

I looked from under the car when the motor was running and even though the motor mounts are old, I don't think they are the culprit. The driver side looked worse. I put a jack under the oil pan with a block of wood and raised the motor a little to get the tension off of the motor mounts.....no difference.

The flexplate is not missing any counter weights, nor is it cracked.

Would an Idle Air control valve do this?

Even though the vacuum read 19 inches from the back of the intake, could other vacuum lines still be leaking?


What about a bad convertor?

 

Have you tried cleaning out the throtle body with TB cleaner maybe it has carbon buildup?

 

Not to say that your driver's side mount is bad, but that's the one that was bad on my car, and I don't know if they have ever been replaced, if not they were 17 years old, yours would be like 20 years old.

 

Yes the motor mounts are original to the car...20 years old.

But when I lifted the motor, the vibration did not go away.

Also when you put the car from park to gear, the motor does not jump like it would with a broken motor mount.

Does anyone know if the Idle Air Control could cause something like this?

 

IT might cause it not 100% sure though.