94 stock LT1, about 80,000 miles, 6 spd trans,never raced or run real hard, runs great otherwise,Engine has developed a vibration/noise when the RPM approaches 3300 while accelerating normally through the gear ranges. It seems to smooth out if I take it over 4000 RPM. I don't often rev the engine that high, and it pulls great despite the vibration. However, I am concerned there is an issue that should be found and fixed before something lets go. Observation of the harmonic balancer indicated it is "OK". I'm not sure how to properly test it,other than to look with the naked eye for out of round condition? in 2006, when I bought the car, the clutch was replaced with an after market one. Not sure how to tell if it is causing the vibration, or whether a bad clutch or flywheel would only vibrate in an certain RPM range.
Electronically, in 2016 I had a no start, and the plugs, wires, coil pack,and ignition control module was replaced. The vibration might be related to this job, but I can't recall if it started around the same time. I realize these are low end torque engines, and seldom had run the RPMs over 3,000. The noise seems to start suddenly when it happens, so wondering if it is possible to something like a solenoid somewhere not working properly at higher RPM. Looking for similar experiences or ideas on what to look for.

 

Pull the flywheel inspection plate and see if it has the stock dual mass flywheel or an aftermarket one

 

 

Does it do it in neutral while sitting in the garage revving to 3K? If so, it is the motor, accessories or clutch assembly. If not, check the rest of the drivetrain; driveshaft, wheel balance etc.

 

Could maybe be the engine ignition breaking up - Bad Optispark?

 

btw 3k is nothing for that engine doesnt hurt it a bit. You must be short shifting it a lot?

 

Yup, or a cylinder or two down on compression.

 

Is there a way to diagnose an optispark issue.

 

I have tried revving it up in neural. I think it got into the same vibration. but hard to tell as there is no load on the engine.We had not considered the drivetrain, such as U joints, because the vibration is related to the engine RPM, not the vehicle road speed. Are the u joints not behind the tranny on the C4?

 

is it possibly related to accessories such as the AC compressor. Surely there must be a solenoid or similar device that releases the compressor at high RPMs? When the vibratioin/noise commences, it comes on very suddenly on approaching the mid 3000 RPM range.

 

Not sure if there is a diagnoses procedure for the opti.. Its a pretty common element to crap out and your milage is about right if it hasnt been replaced or serviced recently. The Opti and ICM are notoriously trouble prone. I like the idea of checking the cyl compression also (id do this first).

 

Yes, the ujoints are behind the tranny.

 

Don't overlook bad or loose motor mounts as a possible cause

 

Does it have a boom sound along with the vibration?

 

I will send a video out in the next couple weeks.

 

I have been reading about harmonic balancer symptoms. Although it does not appear to be wobbling,should start with replacing it? The car is 25 yrs old, 80,000 miles, from some of the comments on other threads, it seems it is a common for the rubber to to deteriorate.

 

The rubber can come apart on a balancer and is visible. The outer pulley can rotate about the rubber or inner hub causing engine accessories not to turn at proper speed. Or, the outer pulley can actually displace toward or away from the engine causing the belt to be miss-aligned with the engine accessories. I have had both of these happen

There are some good suggestions in this thread, but can only comment on my personal experience.

I did a clutch replacement and right after that I had a vibration. It was bad parts not balanced properly. This is not uncommon, if you did the work I would say take the parts back. However 13 years later I'd say you're starting fresh replacement.


How long have you had the vibration….from 2006?