scoobysnax83

When I get a little wild with my car lately, it will vibrate badly if I push in the clutch and let off the gas at like 5400rpms or so. I just would like a few opinions on what it could be. Something seems as though it is unbalanced. The driveshaft seems fine even at HIGH speeds, but it could be some sort of play in the system when I push in the clutch. Any ideas would be much appreciated. Thanks....

Dave C. '97 Z28

Possibly your pilot bushing is bad. This is the bushing in the crank where the transmission input shaft fits into.

It's a $5.00 part...but you have to pull the clutch off to get to it.

-Dave C. '97 Z28

Kraest

Don't drive it like that.

It could be something much more serious.

When my car was doing that with a new T-56 conversion done by some guys I knew, it wound up being the crank out of balance from having the wrong flywheel on the car and the pilot bushing was inserted incorrectly.
It broke the oil-pump driveshaft and put the car at 0 oil pressure.

After replacing those parts, everything was fine.

Mike

Dave C. '97 Z28

How does a unbalanced crankshaft break an oil pump driveshaft when the two are not in contact with each other? The oil pump runs off the cam, not the crank. Also, it's hard to run the wrong flywheel and have the clutch still work since the LT-1 has a specific flywheel for the way the clutch is set up (spring packs on the flywheel side rather than the pressure plate side like 'normal' setups), except the for the '93 version.

I think guys who did your conversion just forgot to put the pilot bushing in there, or put it in backwards so that they damaged it pounding it in.

-Dave C. '97 Z28

Kraest

The flywheel was an LT1 flywheel, but was neutrally balanced, when it should have been balanced to the harmonic balancer.

Vibration broke the oil pump shaft. Don't ask me how. It mush have been so bad at 7000 rpm that it resonated through the motor into that piece... I still don't know how till this day.

Mike

scoobysnax83

Guys, this is not a new clutch and nothing has been changed internally in this car, ever. It just recently started doing it and only does it at high speeds. Does the pilot bearing stay engaged when you're in neutral coasting? I thought it separated and spun at engine speeds, not drivetrain speeds?

Dave C. '97 Z28

Wow... well, I hope you got it all back together.

Actually, the flywheel weights counter balance the rear crank throw by making up for the 'missing' flange that was on the 2-piece seal SBC. The harmonic Damper up front is neutral on an LT-1, but has holes for small counterweights for fine tuning.


Scoobysnax83:
You're thinking of the throwout bearing. The pilot bushing that we're talking about is pressed into the crankshaft and the transmission input shaft slides into it to keep it aligned properly with the engine. If it wears out, then the input shaft flops around when the clutch is disengaged and can cause vibration.

You can test this by checking to see if it's worse in gear than in neutral. When you get the car to the point where it will vibrate, try putting the clutch in while it's still in gear (6th or whatever), and then switch to neutral and see if the vibration starts to go away. If so, then it's probably the pilot bushing (or something else in the transmission).

-Dave C. '97 Z28

scoobysnax83

Thanks for the help, but it is exactly the opposite. When in 6th at like 65-70, if I push in the clutch it will start, with the car in gear it is like normal. I'm going to go with my u-joints and see what happens. Thanks...

Dave C. '97 Z28

I'm saying that you push the clutch in and leave in gear and see how it is....then move the shifter to neutral and see if it starts going away.

Having the clutch in and having it in gear are two different things.

And if it starts when you push the clutch in when it's still in gear, and then stops when you move the shifter to neutral, then it's most likely the pilot bushing, or the input shaft.

-Dave C. '97 Z28

scoobysnax83

gotcha, I'll check that today and let you all know. Thanks...

FormulaFirebird

I think i'm having the same problem. Coming from pretty much any speed, any gear, if i hold in the clutch(while still in gear) for more than a couple seconds i get this really scary fast vibration. Fast enough to make a buzzing sound. And with the clutch still in, i pull it out of gear and it stops. Also, it doesn't do it all the time... it just started last night and its freakin me out. If the pilot bushing was bad.. why wouldn't it do it everytime i pushed in the clutch. and could this be a warped clutch, bad pilot bearing, or even the slave cylinder? Help!

Pretty much stock 94 with exhaust, ligen. ds., 410's, and ws6 wheels, suspension, and ram air.

scoobysnax83

Its odd, because my problem has gone away on its own, but I do know that I am in need of a new clutch. I feel that my issue is a clutch disk issue, but I have not pulled anything apart yet. Waiting on the new engine to be built in January.