[Derkyb]

I have a 87 SUBURBAN 454TH400 that I use as my tow vehicle. Problem is it has a vibration that starts at about 60mph then it goes away as you accelerate, but it has progressively gotten worse(louder). I noticed the other day that it is slightly noticeable at idle if you bring it up to about 2500 RPM. Before this I had tried new tires and rebalanced the driveshaft. I inspected flywheel and it doesnt appear to be cracked or missing any weights. Anyone have any ideas on my problem before I pull the trans?

[1931 steve]

Check for loose bolts at the flex plate to torque converter.

Does the vibration go away if you put the trans in neutral, or shut the engine off altogether (if you try this do not go so far off with the key that the steering wheel locks)?

Steve

[Derkyb]

It only does it when there is a load on the engine at a certain speed. If I put it in neutral or shut the engine off it goes away. Its feels like when an engine isnt runnig on all cylinders, that kinda shudder.

[strummin67]

I'd check your spark and your compression in all cylinders.

Strummin

[Derkyb]

Found the problem! Harmonic balancer was moving almost a half inch at a certain RPM looked like it had been hit with something because it had rubbed all the paint off on one side. Go figure....