vibration problems
 

Just bought this 2001 Cobra and I have a vibration between 55/65 mph if you take it out of gear it does not do it, i was told the motor mounts are bad so ordered from and will install thursday,but when you put it in 1st gear or rev, gear with park brake the motor moves very little very little.and you can let off the gas and it vibrates. so needs some help or was they right on the mounts.this car has 91k miles on it.trans shifts perfect.thanks

With 91k miles, it would be a good idea to replace the motor mounts...might also want to check the balance of the driveshaft...

I would try rotating the drive shaft.

Before any of that, I would check to see if maybe the bearings are worn out on the driveshaft u-joints. I don't think motor mounts would be the culprit. If you had bad mounts, the engine would move up when you hit the gas.

Could be one of the following:

Transmission bearings
Drive shaft bearings
harmonic balancer
warped driveshaft
bad bearings on the IRS short axles
Bad driveshaft indexing
U joint bearings

Probably something worn out on it. The driveshaft could be out of round or incorrectly indexed.

Definitely check the driveshaft for that type of vibration.

I had that issue once on a Chevelle that I built and I found out after the fact that a weight had come off the driveshaft at some point, thus putting it out of balance. :eek: However, I didn't find this out until the driveshaft broke in half on the freeway and actually cracked the bellhousing of the trans in half. Ya, not a happy camper day. :mad:

So, best to check it now before anything like that happens! :thumbsup:

Under load, vibration. No load, no vibration. Almost sounds like a bad U joint.
IF the shaft was out of balance the vibration would continue when you take it out of gear since the shaft is still spinning, but under no load conditions it may not happen. It could still be drive line related.

Is there any particular gear it happens with? (3, 4, 5).

The monkey wrench is the vibration you get when unloading the engine when the cobra is not in motion. Do you get vibration when throttling the engine out of gear or is it only occuring when going from a load to no load state?

Before spending money on having the drive shaft balance checked or having parts removed that may not be the cause, a basic (low cost) approach would be the following:

Try replacing the PCV valve: If the vibration occurs randomly under no load (at idle or when the RPMS fall after taking the trans out of gear) it may be related to the PCV valve sticking. (should be replaced or cleaned every 30k miles).

This is assuming it is sensor/ECU related and probably not the cause of the vibration: The ECU stores data in load tables and there is one for no load at various RPMs. I recall hearing about issues when you punch it and hit the rev limiter may cause some data to become corrupt in the load tables. This may not be the issue you are having but if it is, you can clear the load tables by disconnecting the battery for a minimum of 1/2 hour (check for any ODBII codes before disconnecting the battery). after doing that, the ECU will relearn and rebuild the load tables. Note the engine will run a bit erratic for a short time while it rebuilds the load tables (you will need to drive the car after doing this, also wait for the guages to reset before turning on the ignition).