Hey guys, I've got a severe vibration issue that I can't identify. The whole car will vibrate heavily in 4 - 6th gear under load. It doesn't vibrate when no throttle is added, and the vibration will get worse the more throttle you add. The vibration is also most apparent in lower RPMS in high gears. I've already checked the drivelines for U-joint play, and pulled the trans to look for any issues. I'm not an expert in transmissions and clutches, I'm more of a driveway mechanic. Let me give some backstory of the car to help diagnosis.

My dad bought this car sometime in the 2000's, then bought another black C4 as a parts car to fix it. At this time it had some kind of stroked motor in it. 2 and a half years ago that motor's crankshaft split down the middle and my dad bought a crate motor. This crate motor is a long block LT1 that makes around 500 horsepower on a good day. While this motor was on it's way, my dad passed away so my brother and I continued with the car. We bought a RAM clutch kit to replace the old clutch, specifically the 75-2175N kit. My brother did all the measurements and installed the clutch kit, and we bought a used transmission to fit into the car. Our first issue with the complete car was the flywheel, it was externally balanced while the engine was internally balanced on both sides. We got the flywheel surfaced and rebalanced, but the next issue was the C-Beam coming loose. After fixing that with some new bolts, we come to where I am now.

My guess is that the vibration is coming from the clutch kit or the transmission. The transmission makes a chattering noise too when power is added, but multiple sources say that the noise is normal. RAM Clutches called it "gear noise" in their video.
Any guesses? Whatever you might say is appreciated.

 

RPM MPH gear when vibration is at its worst. Where do you feel it? Seat, Floor. Tunne,l steering wheel.....

 

1500, 55, 6th gear. I feel it through the whole car at that point, it's more of a shake. Most of the time I feel it in the back through the seat, and the floor.
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A reed tach or EVA ( electronic vibration analyzer) would be very handy. It would pickup the frequency and amplitude of vibration, leading to where to look.
Zf6 has a rattle to it particularly in neutral. It's because of the clearance between the gears. It shouldn't rattle with any load. Engine and clutch assembly will vibrate with RPM. 1500 regardless or gear or speed. 55 is within a popular speed for driveshaft concerns...dampner and run out. Is dampner present and has it moved? Measure driveshaft run out front middle rear. Less than .010. If not flip it see if it changes.

 

Which dampener are you talking about? To measure the run-out I'd have to get a read indicator of some sort, though I think I know a shop that would measure it for me.
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Dampner on driveshaft yoke. Yes, dial indicator required. Driveshaft shop could check it for: run out, balance, and U joints.

 

This kind of dumb. But, it did happen to me. Motor mount? Just a thought. My car would shake I checked out the front end. Everything was fine. Lifted the hood. Put the car in gear with the brakes on. And the motor lifted up out of the mount. Just a thought.

 

This is my guess, ad engine miss at high load low RPM.

 

I'll pop the driveshaft out and take it out to a shop. I'm also going to take a picture of the dampener you're talking about, I'm not even sure that I have it.

 

Would a misfire throw a code in the old OBD computers? Now that you mention it, it does sound very very likely given a new engine and all.

 

I'll look at those too. Do I just lift the hood and rev it in gear?

 

Just a quick update, I was finally able to get the Corvette lifted and the tail shaft bushing is bad. There's like more than .050 inches of play in the front driveshaft U-Joint. Bill from ZFDoc suggested I also recheck the Flywheel for neutral balance. I'm going to be replacing the bushing, installing the beam plates, replacing the yoke, installing 2 new U-joints and installing a thicker washer to eliminate the gear rattle noise. I'll return with an update to see if it fixed the vibrations.

 

Just to make sure, you did check the u-joints in the halfshafts too, right? Your symptoms sound to me exactly like a bad u-joint, and those are the likely culprits in my experience.

 

Those are the U-joints in the rear suspension, right? I gave them all a firm tug and none of them moved or had any play. I don't think its those.

 

Yes, they are the u-joints in the two lateral driveshafts that run from the differential out to each wheel hub; and they also serve as the upper control arms for the rear suspension. There are two joints in each one, and I bet one of them is bad. Not sure what you mean by "firm tug." You need to jack up each rear corner so the tire's off the ground, with the parking brake off and the car in gear. Try to rotate each rear tire and watch each u-joint. You shouldn't be able to get any significant rotation, but if one of the joints is bad you'll probably get at least 1/2" of rotation at the tire circumference. You'll probably also see the bad u-joint get noticeably off axis one direction or the other.

 

Heh, that's proof that all of my U-joint knowledge is from a Chrisfix video. Can I do this on the lift with the other rear tire up? I did so and there's very little play, maybe 1/16 of an inch. Thank you for the info.

 

Here's some progress pictures, tomorrrow the tailshaft bearing and seal comes in.

Old Tailshaft Bearing


Old slip yoke

I apologize if the pictures are larger than life. You can feel the scoring in the slip yoke and the bearing too. Maybe the lines are normal? I know the scoring in the yoke isn't normal.

 

Yeah those look hammered.

 

Sorry to revive this old thread but I want to be sure that I post as much info as I can for other members later on. Life got in the way of me but I'm back to trying to figure this out and get it working right.
I've got some recorded videos of the vibration I'm going to link.
At this point I've done the tailshaft bushing, replaced all the driveline U-joints, checked the lateral driveshaft U-joints, but I was too dumb to pull the clutch bellhousing off because I didn't want to replace any clutch bolts or anything. In hindsight I should have. I also installed the shim kit for the countershaft in the transmission, though the noise wasn't really helped by this.
When I get the lift back I'm pulling the bellhousing off and having it professionally balanced by a machine shop.
As far as the issue being the engine, I really can't see that. The engine we got was a long block with new injectors, and we bought new wires and plugs and a new optispark. I also really don't know what I'm doing so I could be wrong.

Thanks for the help so far, hopefully the issue gets fixed.

 

Did you ever solve this?