Vibration concern

[32valvZ]

Ok, I have pretty much all the small to medium jobs taken care of since getting my car a couple of weeks ago.......brakes are done, b/u light switch is done, extensive cleaning of the suspension parts done & many other small jobs also.

Now I am on to bigger problems. When sitting still at idle or while moving, clutch in or out........I get a very noticeable vibration at 900-950 rpm.....Thats pretty much the only time I get it.

I spoke to Bill Bordeux via Email & he suggested maybe an injector miss or a weak coil. After I elaborated on a couple of things, his conclusion was "inconclusive"....

I do hear some throw out bearing noise with the clutch out, so I know that will need attention.

The car starts right up, idles smooth when cold or hot, runs strong throughout the rpm range. I have no problem changing gears & the clutch pedal feels "right".

I suspect the flywheel or clutch assembly itself....it feels like an unbalanced tire would feel. Bill said maybe the pilot bushing. I need to add that sometimes when downshifting from 5th to 4th or 5th to 3rd, the pilot bushing will make a sound like rubbing your finger on wet glass.....its very brief & happens exactly when the clutch releases.

Could the vibration be the pilot bushing? Aside from the 900-950 rpm, its as smooth as silk....any ideas or experiences are welcomed.

Sorry for the long post....TIA
John

[petefias]

I wouldn't think that the pilot bushing would cause the vibration only in the 900-950 RPM range.

You didn't mention any work being done to the drivetrain other than brakes, b/u light switch and cleaning that could have triggered symptoms like that.

If I was a betting man I'd say injectors... (they don't flow nice as they are opening)

[32valvZ]

Thank you for the input petefias

More clues from todays drive:
I definately notice the severity of the vibration diminishes a great deal after the car is at operating temperature...

[Sgreg]

Do you have an aftermarket light weight aluminum flywheel ? They are a little noisy at idle. Quiet down at speed.

[32valvZ]

Sgreg
I'm almost 99% sure the clutch is stock.....its not so much a noise as it is a vibration.
thanks, John

[LGNDLT5]

The pilot bushing. I need to add that sometimes when downshifting from 5th to 4th or 5th to 3rd, the pilot bushing will make a sound like rubbing your finger on wet glass.....its very brief & happens exactly when the clutch releases.

Could the vibration be the pilot bushing? Aside from the 900-950 rpm, its as smooth as silk....any ideas or experiences are welcomed.

Sorry for the long post....TIA
John[/quote]

Hi John. I can't help with the vibration but the noise you describe is almost certainly the pilot bushing. I have had this since my clutch was replaced 30K+ ago. The noise in my case subsides and returns but Never has caused a vibration. Only the noise when the bushing spins in the crank. It is getting replaced with the delicate but smoother needle bearing pilot bushing when the clutch gets done again. I have heard of one instance where the harmonic balancer rubber failed and that can cause a vibration. One 'old school' diagnosis method was to remove the accessory belt and eliminate them and run it for a very short period. A few seconds to see if the vibration changes. (No water pump or charging system with the belt off. I am not sure if this recommended for the LT5. ) Good luck. John

[Sgreg]

One other possibility could be the heat shields which surround the header/cats. If loose they can set up a low RPM vibration/noise. Just another voice in the wilderness !!!