mfendley

I'm getting a bad vibration between 3500 - 4200 rpm in all 4 gears. Vibration goes away when I step on the clutch and let the engine rpm drop below 3500, and it is reproducable when stopped by revving the engine to 3500. I think the symptoms eliminate the drivetrain from the transmission output shaft through the rear end to the rear wheels, but that leaves the fan and fan clutch to the clutch as suspect. The car is a 77 L-82 with a 4-speed. Any common problem areas that I should look at first?

Mike

gerry72

It could be any number of things that are forward of the trans output shaft.

You can isolate the accessory drives from the issue by just removing the belts. If it goes away, you then start diagnosing what the vibration source is. It could be a bad damper (wobble from a slipped inertia ring) a dented or loose pulley, a bad water pump or fan clutch, or bad bearings in one of the other accessories. It could even be just a loose accessory.

If you still have the vibration once you eliminate the front of the engine, life becomes far more difficult. It could be a worn input shaft bearing along with a worn pilot bushing. It could also be an imbalance in the pressure plate or clutch disc.

It can also be a mechanical issue with either the engine's rotating or reciprocating assembly.

You can also pick up a significant vibration just from the exhaust contacting a frame or body member. It could also be bad engine mounts or an ignition problem.

It's not easy tracking these things down and I wish you luck.

pws69

I would be very suspicious of the Pilot bushing / bearing.

mfendley

I installed a new pilot bushing last year when I replaced the clutch, and I don't remember any vibration then, but the vette was in the shop for paint from OCT - MAR, and to tell the truth I can't remember if the vibration was there before or not. I'll undo the accessories off the front of the engine first, since that's much easier - then I'll go back into the clutch area. Maybe now's a good time to research a 5-speed swap!

Mike

Tom Johnson

The engine vibration I had in my 71 was a busted drivers side motor mount.

mfendley

Thanks, Tom - that's another thing I can look before before I pull the tranny again (call you tell I'm not looking forward to that again!)

Mike

Duke94

I had exaxtly the same problem with my 71' SB . I changed everything on the front of the engine (2 different dampner pulleys as well) and nothing solved the problem. I finially pulled the tranny and changed the clutch also had the flywheel machined and balanced. That cured the problem. All the parts I removed were only a couple years old and I couldn't find anything wrong with any of them. Go figure! Good luck, I spent a month on it.

mfendley

I may end up like you suggest. When I bought the car last April, the clutch was slipping, so I pulled the tranny, had the flywheel turned, and installed all new clutch components. Not to say that some of these may turn out to be the source of the vibration - but I'll check the belts, mounts, and other easy-to-check stuff before pulling the tranny again!