Rebuild Vibrates

[Crowbob]

I did a top end 3.0 rebuild (i.e., no changes to the rods or crank) with all new bearings, seals, rings, chains, etc. Runs great. However, when the engine is at operating temp., it vibrates like it's out of balance thru the 2000-3000 RPM range. It does not do this when it's cold or at any other engine speed. I'm planning on dropping the engine to adjust the valves, chase down a minor leak and do whatever else I see needs to be done. I've done a search on 'vibration' but haven't found a thread describing this problem. Can someone give me some advice about this or direct me to any prior discussions?

[jpnovak]

Missing in a cylinder? COuld be injection or ingntion related.

Did you replace the PC set? Did you balance them?

[Crowbob]

Thanks, Jamie.

I did not replace the PC's; and reassembled in the same locations as prior to the tear-down. New rings, though. New plug wires, rotor and cap, I checked for good connections. The only things left that I can think of are bad injection to one cylinder (won't the plug tell the story?) or possibly the cam timing is off. Why would it only do this at operating temp? Wouldn't it be something is not balanced (but I can't imagine what)?

[stevepaa]

Sounds like a mechanical resonance. Is the vibration like a cylinder is missing or does it build and then decay? How about all the bolts for your exhaust system and hangers? All nice and tight?

[Crowbob]

Thanks, Steve.

I haven't checked the exhaust bolts. The vibration begins a little before 2000 rpms, gets progressively worse as the rpms increase until a little over 3000 rpms then disappears at higher revs. I put 964 cams if that helps. The vibration is not terrible but is worisome. Only when hot. Could the crank have deformed with overheating at some point in the past so that only when it's hot it bends then when it cools it goes back to spec? I don't know what would indicate a cylinder miss. It has good power on acceleration, decel is smooth. I'm stumped.

[911mot]

A long shot but could it be a worn/loose CV joint?

[Crowbob]

Thanks mot,

[Crowbob]

Oops!

Mot, the car could be stationary or in motion.

[goose2]

CV joint would correlate with car speed, not rpm. Other things to check: motor mounts, alternator/fan bearings, clutch/flywheel. ........does the vibration occur when free revving or just when driving?

[stevepaa]

I don't think any heat or forces even under the conditions for a rod failure could really deform that crank. It is rugged.

Sounds very much like mechanical resonance.

[1974carrera]

Maybe this bonafide PORSCHE tech tip will help? Sorry about the poor images

[Crowbob]

Thanks Bob,

I never thought of misallignement between engine and trans! I 'll do all the great suggestions above.

Thank you all.