solar88

Stock 88 Z51 L98 manual with 74k miles. I get a pretty good vibration around 3000 rpm under load at any speed, it feels like a drive line issue, but not sure. Thinking a Harmonic Balancer ? Is this a common issue with a L98 ? I don’t think it’s a driveshaft problem because it’s smooth in any gear below 3000 rpm at any speed.

ex-x-fire

Check for a binding U joint.

bjankuski

Rev the car in neutral and parked and see if the vibration is present, that will tell you if it is engine or drivetrain, and then go from there.

1993C4LT1

What if it does it on both?

bjankuski

Then it is the engine.

solar88

UPDATE: The vibration does happen when the car is in neutral and I rev the engine up to 3000 rpm, so I’m thinking the harmonic balancer ? It seems straight and true with no wobble when reving it, so not sure.

bjankuski

Just curious is the flywheel stock one that came with the car?

solar88

Good question. I bought this C4 with 74k miles a few months ago and it had been sitting for 12 years prior. It’s all stock in appearance, but there’s definitely been work done. I found a trans to bell-housing bolt loose, and checked the rest of the BH bolts as well and they’re tight. Yeah, I sure hope it isn’t the flywheel.

bjankuski

You better check the flywheel. 1986 and latter SBC with the 1 piece rear main seal have an externally balanced flywheel, that has a weight cast on to it to balance the engine. If a person buys aftermarket flywheel they can come with a bolt on weight since some people will internally balance the engine. If the flywheel in bolted on without the weight it will vibrate as the RPM increases. I have seen this many times when people install aftermarket flywheels or flexplates, they do not always put the weight on.

solar88

That would be my luck. I had a couple bourbons last night and I ordered a harmonic balancer, so looks like I will be replacing that first. Also I think the balancer timing mark might have slipped, because the HEI distributor where the power wire and tach plug into was turned up against the firewall, I checked the timing at 6* and the engine pings under load, so thinking the timing might be way off.

ex-x-fire

If you don't know you'll need an install tool for that balancer. I'm pretty sure the 86 up flywheels have a different bolt pattern.

AmoriFati

Also, Try it with AC on and Off..

solar88

It’s either the HB or the flywheel. Replacing the HB this afternoon and I’m hoping that fixes the issue, there are a few clues that the outer ring might have slipped a bit. The distributor was turned way out of its normal looking setting and the 4 wire connector and the tach plug were turned way up against the firewall, and even though the timing is set at 6*BTC it pings under load. Also I adjusted the valves using the timing mark at TDC and they still seem a little noisy. Will post back later with an update, fingers crossed.

bjankuski

The HB is neutral balanced so I don't think that will be the issue, but you will soon find out. (It will be an interesting exercise, hopefully it works)

solar88

I know, and I think it is a flywheel issue too, but I just trying to stay positive because Ive been wrenching on this car for 6 months and Im almost to the finish line. The flywheel would be a bummer, but I've already priced a new flywheel and clutch set, and I have the abilities and facility to do it. Fingers crossed

solar88

I got the old one off today after work and it was definitely on its way out, the rubber was mushy and coming apart and the ring has slid back a bit but the timing mark looks like it’s still in its right spot. I ran out of daylight before I could install the new one, but even if this doesn’t fix my vibration issue, I still feel like this wasn’t a waste of time and money.

Vets-Vet

Just a thought.
Has the old part detiorated and moved or is the new part improperly made. Just saying, while you have it out maybe find a way to check it before install. I am finding I have to doubt / test everything new. Or, maybe it doesn't make a difference ??

solar88

The old HB was definitely on its way out, and I replaced it with a “genuine GM balancer” that was made in Mexico lol. I know what you mean though, new parts are sometimes worse than the broken one you replaced.

solar88

Well, I believe you are right in your diagnosis. Got the new HB on this morning and though the engine feels smoother, I still have the vibration. I was in denial about the flywheel, but had to at least rule out the possibility, and the HB was going bad, so not a complete waste of time and money.

So now I’m going to have to drop the transmission and see what is going on, will most likely just replace everything while I’m in there. The car is a stock 88 Z51 4+3, do you know if a LuK flywheel and clutch kit would be the way to go, or should I look at a different brand ? Thanks.

bjankuski

I guess I would see what you have, maybe it just needs the weight added on to it to make it work if it is an aftermarket design.

I do not have a preference for the flywheel or clutch, there are many good ones to choose from. I have a stock 4+3 flywheel at my house. If you need that it could be shipped to you.