NEVERL8

I have been chasing this problem for a couple of years now. It is more of an aggravation than a serious problem but I would still like to get it resolved. Here is a little history and background.

The vibration starts getting noticeable at approx 3500 rpms, and amplitude is proportional to the load on the engine/drive train. Under light or heavy acceleration the vibration can be felt through the car but not really through the shifter. The harder the acceleration the more noticeable the vibration and it also gets more noticeable as the rpms climb to the 6K redline.

I did in past have an incorrect clutch fan that caused a severe vibration in the fan/water pump assembly (it was definitely more severe as the rpms increased). So severe in fact that after a couple of full throttle redline blasts it caused a water pump mounting bolt to back out and coolant to leak all over the place. I replaced the fan clutch and all is good. I wonder if I may have bent the water pump shaft or somehow injured it, thus causing the vibration. Since the vibration I am experiencing is somewhat load related I discounted this.

I had the engine rebuilt and spent considerable time and money having all the internal reciprocating parts and assemblies balanced. I also had the flywheel and pressure plate mated and balanced together (I realize some do not like this approach) to a very high precision. I doubt it is engine internals causing the problem.

The harmonic damper was on the car when I bought it. Again, since the vibration is somewhat load related I did not see this as a potential problem source.

The car has AC. I disconnected the belt from the AC compressor, it made no difference. I have not disconnected the alternator or the water pump and fan assembly. Since the vibration happens at higher rpms and is load dependent I would want the engine and oil up to operating temperatures when I hammered on it. Disconnecting the water pump and fan under these conditions would drive the cylinder head temps up way too high for my liking with no coolant flowing in the engine.

If the car is in neutral and I rev the engine I do not notice a vibration. I only blip the throttle and do not hold high engine speeds at no load. I do not think this is good for the engine.

I have considered ignition (have an older Petronix conversion) or something in the transmission on the input side. The plugs look as expected, an even brown color. I also thought about U-joints (because the severity varies with load) but since it is also a function of engine rpms I did not consider this much further.

I can live with the vibration since I do not often visit 4500+ rpms but it still bothers my perfectionist side of me.

midyearvette

have you checked the rear tranny bushing and front drive shaft yoke for wear??

silverslashstreak

One of my Muncie 4 speeds has that type of load vibration. When swapped it for a T5 and got the u joint angles correct, smooth as silk, with the same clutch and engine set up.

Even though you might have perfectly original drive line components and drive line angles they are not always perfect. I have seen times that I could shim the rear trans mount up or down by as little as a 1/4 of an inch or the rear snubber mount 1/4 up or down and totally cure a load vibration.

Good luck
Jeff

magicv8

My load related vibrations have all been cured with a new set of plugs and wires.

Depending on your cam, your vibration seems centered at the top of the torque range, and could be a ujoint either in the driveshaft or the axles.

...those are my likely suspects......

NEVERL8

If I rule out load related ignition issues my intuition tells me you are pointing me in the right direction. Though the vibration is somewhat dependent on engine speed, it does manifest itself when the drivetrain is under significant load. So even though I keep trying to convince myself the primary contributor is engine rpm, most likely the primary contributor is engine torque. This, therefore, points to drivetrain u-joints or transmission output shaft issues. The half shaft u-joints were inspected and two of them replaced not more than 100 miles ago. I really think I need to look at the drivetrain. The rear differential was replaced and a fiberglass mono spring installed before I purchased the car.

Thanks for the replies.

SupremeDeluxe

Bruce,

Check the transmission mount. I had the mount on the big block literally separate where the rubber was vulcanized to the upper plate which bolts to the transmission. The trans had shifted over 1" to the side and the output shaft yoke made contact with the floor.

That balancer on your car was a new replacement from Jim Pace. It is not visually correct for an L-79 but is correct to the application.

Nick

Frankie the Fink

Here is a vibration troubleshooting document I picked up somewhere over the years....haven't used it, but the approach seems logical..

Subfixer

I'm watching this as I have the exact same issue. Just had the differential rebuilt so that rules the pinion bearing out.

DansYellow66

I would definately disconnect all drive belts and check it for vibration. I don't why the bad fan clutch couldn't have damaged your water pump, although you would probably have a leaking seal. I also agree you should check the rear trans bushing and yoke. It just takes a few minutes to reach under there and grab the yoke and try to move it from side to side. If it has more than barely perceptable slack that could be a cause. I wouldn't worry about the half shafts as they don't rotate fast enough or have enough off center mass to cause a problem unless damaged, but a driveshaft can. I would check for missing balance weights on the driveshaft and set up a dial indicator on it to see if it might have excessive runout or be bent. Mine was screwed up from the factory they found when I had it retubed. And an engine miss, as pointed out, sometimes feels like vibration - although if you can pull 6000 that is probably not the problem. Good luck.

silverslashstreak

Check the alternator, I had one that had a bent shaft and it you have the original alternator mount that attaches to the exhaust manifold, a vibration can start there and run right through the exhaust system into the cab.

Jeff

JohnZ

I think you've successfully ruled out the engine, and isolated the issue to the driveline under torque loading. First thing I'd look at is the bushings where the diff nose bracket attaches to the frame crossmember. If they're worn/deteriorated (or have been replaced with poly instead of the OEM rubber), it'll force the nose upward and ground out the bracket to the frame, or cause the U-joint to touch the top of the tunnel.