I've got an 89 firebird formula 350 with 69000 miles and is completely stock. It does have an after market muffler but I'm not sure what kind. It's not any thing special. When I take the car up over 3000 rpm's which does not take alot to do, I get alot of vibration in the car. I can even feel it in the gas pedal. This happens if I'm driving or just sitting in park and reving it up. I have done a complete tune up on the car but did not solve the problem. Could it be the exhaust or something internal. I also checked the motor mounts and they were fine and for the heck of it I verified the transmission was bolted tight to the engine. Any thoughts. I've got 3 3rd gens now and have had 5 over my life and have never seen this before.

 

you might want to take a look at the harmonic balancer. The purpose of the balancer is to smooth out the vibrations of the engine so that you never hit the critical node of the engine. There are fractional multiples of the node that are referred to as harmonics.

IIRC, the balancer should be a 2 piece assembly w/ a rubber gasket between the two. Over time the rubber will wear away or become stiff and brittle, effectively changing the damping of the balancer.

I can't guarantee that this is the case for you, but from what you have described w/ the vibrations both driving and in park it definitely sounds like you are hitting a harmonic node at or around 3k rpm.

 

Check Your U Joints for signs of wear. How about Your tranny mount, has it ever been changed?

 

I have checked the trans mount and it looked good but I'm not sure if it's the original one. As far as the u-joints go, no I have not looked at that but a friend of mine said the same thing today. He also said the same about the harmonic balancer. I will check both out this weekend and let you know what I found so please check back in a few days in case I have some more questions. Thanks. To every one else please keep the ideas coming.

 

If you are getting the same vibration sitting still, it could not be u-joint related.

 

Your right. It makes the vibration even sitting still. I'm going to check out the car on sunday. What would happen if I took off the drive belt to eliminate all accessories and ran the motor for say 30-45 seconds. Other than the fact the battery will not get charged is there any harm going to be done with out the air pump and water pump moving?

 

your waterpump wont run but for that short of time it shouldnt hurt anything. heres another angle, does it vibrate right away when everything is cold or is it heat related?

 

It doesn't matter weather the car is cold or hot. The vibration is always there over 3000 rpm's. I took the belt off and only ran the motor and the vibration went away. Put the belt back on and the vibration came back. I don't want to rule out the harmonic balancer just yet, because what if the belt tension is enought to throw off the harmonic balancer if it is bad? What do you think?

 

i'll look at mine in prson and ponder for a bit, there has got to be a logical reason for this. does the pulley spin true? look at it during idle and see what it does, im developing a theory.

 

Yes, the pulley looks good at idle. I had a friend help me, and he brought the car up to 3000 rpm while I looked at it and still looks fine. I'm comparing this to my other 2 cars and they all appear to be spinning and looking the same. My friend thinks the vibration could be under the car. He was thinking maybe the flywheel or torque converter, but when I take the drive belt off it goes away. I'm not ready to give up yet. This does not affect the performance of the car at all. It's just pissing me off because I know it should not be like this. I have talked to a guy I work with who use to be a gm mechanic ( now semi retired ) and he is completely stumped!

 

maybe I missed something, but is it a serpentine car or v-belt?

 

The car has a serpentine drive belt system. I think they all did starting in 1988. I could be wrong on that.

 

Is the flexplate a neutral balance or is it weighted like my 89 305 was? Maybe that weight came off of the flexplate causing the vibes. Pop the inspection cover off and look at the flexplate to see if it's missing, also check the torque converter bolts.

 

I will check both items, but if the weight came off of the flex plate then does that have to be removed or can you just put one back on. I'm sure this is really a dump question.