engine vibration issues
engine vibration issues What would make an engine vibrate in waves of in and out like a bad universal joint? It comes and goes within a second or two and is enough to vibrate the seats, floor board, and door mirrors. The car has been dyno tuned since the head work and new cam and it did not change any. Its ike 10-20 fast vibrations then smooths out then its back again all within sec or two between smooth and vibs. I have been told someone might have changed a rod and piston from another ls engine and that may be where its out of balance, but IDK and this was a guess by someone with no previous knowledge of the car. Front acc have been bypassed no change, new balancer no change. Gonna try a new flexplate but the old one looks fine. Any ideas? 
Factory rubber were worn, I replaced them with poly ones and it did not help whatsoever. Trans mount is stock and so is torque arm mount. I know these wont make a diff when sitting still just thought Id mention it. I have had 3 diff balancers on it with no diff. Swaped injectors and coil packs. Plugs n wires with no change. When it was tuned on dyno with hp tuners they were made aware of it and said they couldnt see any problems when scanning. It throws no codes either. Pulls strong to 6200 where it shifts flawlessly with no kr and dyno graph was smooth and linear. Im stumped. I will listen to any ideas, bring em, im getting ready to buy a new flexplate from gm and install it today. My stock one has no balance weights on it. I was told by gm that this is correct but this one is the only ls flexplate I have seen so I cant confirm it.
Found it!!!!! Either water pump or ps pump!!! removed the main belt and its gone. I tried this before a while ago and thought I still felt the vibration. Must have been another issue that was ultimately worked out through parts replacement. Now which one would be more likely to cause a vibration with harmonics? Water pump spins freely and is not leaking at weep hole. Ps pump cant spin fast enough by hand to tell but does make a engaging sound with a lil slack when turning back n fourth. Im so damned exited at this point Id buy 5 ps pumps and 5 water pumps if that would help!!!! I was seriously thinking the engine had an imbalance issue in the rotating assy.
Good to know, I think the 6.2 in my Denali is an L98. I just replaced my LS1 alternator about 2 weeks ago because it just quit charging but it didn't change the issue. Im gonna try the ps pump first as its roughly $45 vs $120 or so for the wp.
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Well no luck with the ps pump and the water pump checks out good by turning it. It feels like a new one. I honestly didn't think a wp could cause a vib great enough to transmit it all the way through the engine and into the floor of the car. I wonder if maybe the end of the crank is warped a lil and a slight bend on the snout would cause this. Looking at the crank pulley it barely moves enough to notice it if you look close. Maybe the front main is worn and when the accy belt puts strain on it its slightly bending the crank at the front main bearing? The front seal dosent leak so Im very doubtful but Im grabbing for straws at this point. Any more ideas?
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Verdad Gallardo Took it by an engine shop and they velt the vibrations while we reved it up under the hood and said its either a piston skirt broken off or a ring land that's broken and allowing it to have major blow by when running. So Im gonna start a new thread based on an engine Im picking up tomorrow. Thanks for the help Guys.
