Pontiac69

I bought a 93 Coupe 5.0 a few weeks ago and am learning more about it everyday. I do have a vibration in the engine/drivetrain. The vibration is there when sitting still with or without the clutch pushed in. If I rev to the rpm I feel the vibration and then smooths out after the 4500. Normal? Since it does it standing still with or without the clutch pushed in we can eliminate the clutch, trans, and driveshaft. I looked and tugged on the harmonic balance and it seems tight. Since it's easier to swap out that than the fly wheel should I start there? What else could it be? Ignition, timing, tune up, etc

Derf00

Motor/Tranny mounts? 4-4.5K might be just the right harmonic frequency for the engine to vibrate in the mounts when they are going out.

Spent a month chasing what I thought was fuel/ignition issue causing what felt like a misfire or stumble on a 2006 ford escape at idle and reving..turned out a motor mount was causing the issue.

Pontiac69

Hmm. Didn't even cross my mind about motor mount. I wound up buying a stock harmonic balancer that I am going to swap out. I have to replace the timing cover gasket around the water pump as it's leaking coolant. I figure if I pull off that I will just swap the balancer. If that doesn't fix it I will look at the motor mounts. Thinking the only other thing is the flywheel.

Buck Sergeant

Pontiac69, I saw where you checked the drive shaft, but did you check the universal joint? With the car sitting at ride hight get under the car, with a big ugly stick/crowbar, and forcefully stress that rear universal joint. Just shaking the drive shaft by hand, or foot, won't reproduce the stress of 4500 RPM. If you have already done this I apologize for jumping the gun.

Derf00

Would that be relevant if the car is vibrating while sitting still though (driveshaft isn't turning)?

Pontiac69

Buck the vibration is there with the car standing still and just reving. we can rule out the driveshaft.

Buck Sergeant

Pontiac and DERF00, you both are right. I was so focused on the 4500 RPM, I wasn't thinking about the idle issue. Yup, I would agree with you guys about the mounts, flywheel/flexplate, maybe a loose starter motor, clutch, tranny mounts, all mounting brackets associated with the engine and drivetrain. This is really grabbing for straws, but how about your radiator fan/fans? Maybe a blade got dinged or came loose? Does this vibration occur when the car is moving, if so, at what RPM. Also, can you narrow this down to a engine, or drivetrain vibration? Hang in there, we will figure it out.

Derf00

Check the easy stuff first. Motor mounts. Even if they are ok, are they Poly type? Poly mounts just naturally convey more NHV than Rubber mounts. Since you aren't having your teeth rattled out at all RPM's I think we can rule out solid type mounts .

If they're rubber, they may need changing. If they are Poly they you're probably just having the natural experience with them. Only way to get rid of the vibration it would be to go back to rubber.

Pontiac69

I may have to start replacing the easy things. Such as harmonic balancer, and motor mounts. I still have the stock rubber ones. I looked at my harmonic balancer and visually it looks fine. I grabbed it and doesn't seem loose nor is the rubber pulling out. As for my fan or radiator supports. I have an alum rad and electric fan. Everything seems tight.

Buck Sergeant

Pontiac 69, this is old school, but give this a try, unless you already have.
When engine mounts go bad, in addition to the flopping and banging when applying torque, you will notice an increase in vibration at idle. When you do, it’s a good idea to test the mounts in your car by holding the brakes on and applying throttle. You can observe if the engine is moving around a lot, usually more than an inch of movement is an indication to start inspecting mounts. Bad engine mounts will look torn, broken, or separated. We are not beat yet,my crew chief has some crazy idea that it could be bad spark plugs or a dirty throttle body causing the problem. I think he is crazy, but he is a good trouble shooter. Try the "holding the brakes" trick first.