Engine and Transmission Tech

Hello,

I have recently aquired a TR-6 with a 4.3 Chevy V-6 transplant.

I have been having trouble with vibration and had been thinking it was driveline related. I took the drive shaft out and had it balanced, put in new u-joints all around new differential mounting bushings and new tires.

Got it all back together and still have the same problem.

Starting from scratch I sat in the car and started the engine with the clutch in. When I revved it up, what do you know vibration. It seems that the engine is the culprit. I cannot say exactly what rpm it comes in on as my tach is not working but there is a band where it is a problem and then it will smooth out as the revs build up.

Not sure where to start in to try to solve this issue. Anyone have a good plan of attack for me to follow?

Thanks for the help.

Craig

The 4.3 V6 is a 90 degree V6 which is inherently unbalanced. After about 1992, GM added a balance shaft that solved the problem. Early versions have a front cover identical to that of a 350 V8. Maybe you have the early engine.

Even so, you may be able to minimize the problem. There is a vibration problem with my Ford 302 around 2000 rpm. I was able to minimize it by putting flex joints in the exhaust after the H pipe. The rigid exhaust system acted as a lever to amplify the engine vibrations, and the flex joints prevented this.

Other areas to look at are if the exhaust is touching any part of the body or chassis, and if the engine or transmission mounts are excessively stiff. If possible, use the standard GM mounts if they will fit.

Another thing to look into, does the engine use the same crankshaft flange as any other engine? For instance, if it uses the generic SBC flange it could have a SBC flywheel. Unless the engine is internally balanced, which to me seems doubtful for a 90 degree V6, that could cause vibrations.

JB

The 4.3s have the same pattern as a v8 and the flywheels that come for the Package cars have a counterweight cast into it and are externally balanced to a certain degree, the damper is however from what I can remember without a counterweight. of all the ones that roll through I have only seen one with the balance bar in it.
Usually if it has a vibration the pilot bushing is shot and the clutch disk is off center.

Some good ideas, thank you for the help.

If it is a pilot bushing issue I guess I would have to yank the motor and trans to get that rectified?

It is a pretty tight fit in there and I think the exhaust manifolds may be very close to the frane rails if not actually resting on them.

I believe the engine is 1996 vintage but I will have to check the numbers to confirm, so seemingly it would have the balance shafts fitted.

I get an odd noise when motoring along in 5th gear (T-5 trans) and it seems to be coming from the trans. Might lend creedence to the bad pilot bushing and some sort of damage being done to the trans.

Craig

Craig, All I know is the somewhat comparable, Buick 90deg. V6's. Up to about 77' were odd-fire, the complaint with themwas "gallup" more than vibation. I would check for having exactly the correct dampener/flywheel balance. I would run the car, securely on a lift, with rear suspension loaded. With the dust cover removed, perhaps you could observe clutch in/out, drive train run-out etc. Just because the driveshaft is balanced, doest that also mean it is correctly "phased" ? Good Luck, roverman.

Thanks Roverman a good idea to run it safely up in the air and see if I can pinpoint the trouble spot or spots to focus my efforts.

Craig

Hi Craig,
Good call on pushing the clutch in to isolate the driveline from the engine.
As the vibration is still there that removes the pilot bearing, clutch disc, trans, driveshaft etc. from the equation.
All 4.3 engines are internally balanced so there should be no balance weights on the flywheel or balancer.
None of the engines are what you would call smooth running. Even the balance shaft engines shake some.
They go through a harmonic period around 2500 rpm and again around 5000 rpm.
Having said that, the vibration isn't uncomfortable just annoying.
Larry is right on the money with the exhaust flex joints. A solid exhaust system will make all sorts of odd noises and vibrations.
I generally mount the flex couplings as close to the manifolds as possible.
Use couplings that are lined. If you can see the corrugations inside then they aren't lined and will make a bizarre "rushing" noise when you are on the gas.
Motor mounts are also critical with these engines.
Urethane ones will shake you out of the car. Use the most flexible rubber ones that you can find. That includes the trans mount as well.
Another thought just hit me. How's the driveshaft length? It should have some room to move.
If it was bottomed out in the trans that would explain a lot.
Hope that's helpful
Cheers
Fred

P.S. Craig. I'm not too far away from you.If you're ever up in Abbotsford I'd be happy to have a peek at the car.

Hi Fred,

I was not sure how the pilot bearing fit into the whole equation but I found an article about isolating vibrations and the first step was to rev the motor with the clutch disengaged. I expereienced the problem right then and there.

I am guessing the vibration starts around 2500 RPM and perhaps again at 5000 RPM as you indicate. When driving the vibration builds and then dissapears as you speed up. It is more than a little annoying as the car doesn't have any door weatherstips in it and there is a lot of rattling of one thing or another.

The exhaust is certainly a good call as it is a rigid system and as I think I indicated elsewhere the exhaust manifolds are very close to if not touching the frame rails.

I can see that the home made looking transmission mounts is made of urethane not sure on the motor mounts but suspect the same.

The slip yolk still has some room to go, perhaps an inch or so before it would hit bottom so sounds like that might be okay.

There is no harmonic balancer on the front of the crank just a pulley. The standard cooling fan had been elimnated and a thermostatically controlled electric one put in it's place.

I will give you a shout if and when I feel bold enough to cross the border with this hybrid. Maybe I can take it to the ABFM next year at Van Dusen? I am sure I would be scorned by the purists and perhaps denied entry!

Thanks,

Craig