vibration, engine or clutch?

Hey Guys,
I picked up a clean 90 Pete 377 10 wheel dump with a 3406C(mech. pump). After running it a few weeks I took it to the local Cat. dealer because it seemed a bit down on power and has a vibration which matches engine speed and occurs even when I'm not moving, just running the rpm's up. They put it on the chassis dyno and it produced 365 HP at the wheels with the fan on. They said this was very good for an engine rated 425 HP at the flywheel. They believe that the vibration is an imbalance either in the engine or the clutch assembly. They recommended that I unbolt the pressure plate and draw the clutch and pressure plate away from the flywheel and then run the engine to see if this eliminates the vibration. This seems sensible enough and I think I can do it myself. Anybody run up against something like this before? It really is a nice truck..starts quick..no smoke...no leaks...shifts good...goes down the road as nice as anything I've driven except for this vibration. Any thoughts. Thanks nvce

 

Thanks for your replies. I tried a lot of different things to try and chase down this vibration, coasting down in gear..out of gear..clutch in..clutch out etc. etc. The key seems to be that sitting still, in gear with the clutch disengaged the vibration persists. The clutch plate, input shaft, and transmission should all be at rest in this arrangement, only the engine, flywheel, pressure plate, and dampener should be moving. The Cat tech told me there were plenty of problems with dampeners on these engines, so many, that an upgraded viscous dampener was produced and is the standard replacement when a dampener fails. This truck already has the upgraded part, not that that's a guarantee the dampener's OK. When I get the truck home I guess I'll try backing the plate off and running the engine. I'll put some marks on the parts so I can try re-indexing them during reassembly. Might get lucky, who knows?
I hate to think the problem's in side the engine. Has anybody come across one that had something wrong inside but still ran well and without any noise or junk in the oil? Thanks again. Sean

 

I'm picking up the truck in the morning so by Sunday I should know what's what. Wish me luck. Sean

 

Well, spent a few hours today with the 377, pulling the pressure plate back and trying to isolate the vibration. I found that the shake changes noticeably with the plate stopped but I can't say it's 100% smooth. It seems the shake is only reduced. Then I tried bolting the assembly back together 90 degrees off and rerunning it. The shake changes again with the worst vibration now occurring at a different engine speed. Turned the plate another 90 degrees and the shake shifts to another engine speed. I left it there and road tested it. Better but, far from perfect. I did notice that the pressure plate is a pretty new reman unit and the flywheel has been resurfaced. The saga continues.