Space387

So I went to mexico today with my corvette for the first time and had a disturbing discovery. When I spin out the engine I feel a vibration come on thats not present at lower speeds. To be exact its between 3500 and 4k I feel it come in progressivly and by 4500 its a solid feel like an out of balance drive shaft. If I take it out of gear or up shift the vibration cuts out immediately. Im not sure what would cause such an event and would like suggestions on where to start.

I'm working with a 1990 L98 coupled to a ZF 6 speed ( no reverse lock out). I have about 1k miles on it since I bought it and have changed the following:
Fuel tank, pump, sock X2, filter x2, FPR, Injectors, spark plugs, checked wires and cap both looked good

I am trying to locate a coolant leak that I can not find in case this could be associated. Oil has not mixed with coolant.

Over the next few days I am going to get a compression tester, check wires and cap for damage wear again, and get the tester to check for hydrocarbons in the coolant. If there is any other suggestions I would greatly appreciate them.

cv67

It does not do it in neutral?

Agree on pressure testing it..pull the plugs first. My HG leak was temp dependent, by the time it was obvious it was too late.

Not sure about 90s but 89s had some dampner bonded to the driveshaft that can separate over time causing what you describe.

GREGGPENN

Damper bonded to the DS? I don't remember that?

Does it vibration in those RPMs sitting still in your driveway? When you kick it into neutral, I'm going to assume the rpms drop.

If it vibrates at a certain SPEED, it's likely behind the trans. (u-joint?)

If it vibrates at certain rpms, it may be a trans issue (DMF)?

If it vibrates -- sitting still in your driveway, it's something in...or on the motor. (like the balancer, accessory, etc...)


Oh yeah....Does it rattle like a can of rocks sitting at a light?

Have you looked UNDER the PS carpet for the water?

Space387

Yes it vibrates when stationary at the same RPM and is independent of the the vehicle speed not quite as severe in my driveway, I figured this was related the the effort required to get to these RPM's under load.. When I was in Mexico I put it in neutral at speed and the vibration went away immediately. And yea at a light with the car in neutral and not on the clutch there is a sound that to me sounds like gear slack. I figured this was a worn clutch disk until I learned we use the DMF on these corvettes. At this time I have not pulled the carpet on the corvette as the coolant leak is only when I'm driving it and I don't smell or notice any coolant or heat from the foot well.

Kevova

You could use dye for coolant leak.
vibration belt off? Harmonic balancer either wobble or timing mark moved? If flywheel has a problem it seems it would be noisey or was noisy.

QCVette

Is it the stock flywheel? (if not sure, you can see the back side of it by dropping the inspection plate.)

I had a bad aftermarket flywheel that was out of balance by about 20 grams. It would be fine at low rpm, but at about 3000 rpm (and up) it would start shaking.

I replaced the flywheel and it is good now.

I have read a lot of other accounts that people have had problems with balance of aftermarket flywheels.

Good luck.

Space387

I am not sure if the flywheel stock or aftermarket as the prior owners had the transmission replaced with a new ZF and it is not clear if they changed it then or with the clutch change. I did find something though that leads me to think the engine needs work.
From Cylinder #7
From Cyl #5

Compression is about 150psi in all cylinders and most the plugs look like Cylinder 5 or a tad darker from being e little more rich. From what I can figure its time to start planning a head gasket.

billschroeder5842

IMHO... a head gasket may be pre mature.

150 pounds in all cylinders is ok. I'd be concerned with wild swings.

Id start with a good old fashion basic tune up and make sure you have good gasoline.

Drive it for a while and see what happens. Your vibration at higher RPMs coudl very well be due to mis firnings. Your number seven is rachet.

GREGGPENN

In random order....

I get why you are suspicious of the whitish appearance of that plug. I agree #7 has a "coolant" look to it AND (IIRC), #7 is the most likely point of failure.

Also...I'm thinking a pressure test of the coolant system can help identify if you have head gasket failure.

If it turns out to be a head gasket AND depending on your car's value, I'd be inclined to try a "block seal" product. I don't typically feel "additives" are worth much except radiator and head gasket sealers have a fairly high success rate. OK...power steering additives have worked for me too.

Check to see if you have an antifreeze smell at the tailpipe. Check cold AND warm. Also, make sure you don't have an injector issue...where that's actually LEAN. Measure ohms to make sure they are within 25% of each other (16ohms?). You might also run injector cleaner (if they ohm OK) at double strength.

Space387

I will get the coolant pressure tester on my next few days off. I would pull the motor to rebuild before adding any of the snake oil magic fixes to this car, I have seen it damage too many cars by blocking coolant passages instead of the leak.

There is no antifreeze smell cold or warm have been checking. For the fuel system I replaced the entire fuel system and these plugs roughly 1k miles ago and have been chasing a coolant leak since I bought the car. Per the prior owner this leak is why he parked it over 5 years ago.

bjankuski

That plug looks like oil burning to me. Check the viration in your driveway and on the road. If it is rpm dependant I would suspct that someone changed to an aftermarket flywheel and forgot to attach the weight. All 1986 and latter sbc are externally balanced and the flywheels come both ways with a weight to be added on. It is possible the weight is not installed.

GREGGPENN

Guess we should have asked....You've driven/owned it 1k miles. Do you consider this vibration "new" or could it have been present since the PO owned it?

Something has been changed with the trans (if just the shifter). The absence of reverse lock-out didn't happen until 93? Do you have a short-throw shifter? Do you know if it's been converted to a single-mass FW?

Tom400CFI

Possible...but I've made that mistake before. By 2000 RPM, you KNOW that something's gone way wrong.

Space387

The vibration may have been for as long as I have owned the car but I was restraining from working the engine until I had some miles on it to be sure there where no gremlins waiting for me. Records from the shop the last owner used show a transmission "overhaul" but the owner said it was replaced. I would venture to say that when done in 2004 it was a reman of a blue tag unit to the tune of $1680 for just the trans. The same invoice mentions replacing the clutch and pressure plate but makes no mention of touching the flywheel. Because this couple had everything done by a shop I believe the flywheel to be original.

GREGGPENN

Hmmmmm.....A vibration for [possibly] 14 yrs? IDK.

Do you have a descreened MAF? How bad does it shake? Have you verified if the timing mark (on the damper) is pointing to the correct location (with dizzy on #1)?

Another method for finding if a single cylinder is at fault is to systematically unplug spark plug and/or injector 1 cylinder at a time and observe the results. If one cylinder barely changes -- while the others do -- that helps to focus your attention.

Space387

No MAF on a 1990 and at idle and lower rpm there is no decernable vibration over the inherent vibrations of a SBC