3.9 engine vibration

[francis higgins]

Hello. I have an 87 Dakota with a 3.9 V-6 and a 5-speed with 175k miles. At certain engine speeds (not mph) I have a harsh vibration. The truck does this in every gear. Does this engine have a balanced flywheel, or is this taken care of with the harmonic balancer? The truck has had the clutch replaced in the past. Is it possible for the balancer to go bad? If so, how can you tell? I appreciate any help.

[GS -]

Francis, check the driveshaft and see if you've thrown a balancing weight. There may
be a shiny square spot on the shaft were it came off. And yes I believe the 3.9 has a harmonic balancer on the crank. Hope thats it!

[francis higgins]

The driveshaft has been replaced. The vibration is the same before and after the replacement. This vibration is also a function of engine speed, not vehicle speed. This tells me it must not be the driveshaft, but something on the engine. I'm just not sure what. The harmonic balancer looks fine, but I don't know if you can diagnose it by looking at it. I was thinking that maybe something was done wrong when the clutch was replaced. I do know that the clutch is bad, and the throw-out (spelling?) bearing is bad as well. I don't know if the pilot bearing is bad, or even if it has one. I also don't know if any of these parts could cause the vibration. Thanks again for your help!