Engine vibration

1994 Buick Roadmaster wagon, 258,000 miles, original engine, on it's 4th transmission. So back in June the car started to get a slight vibration at higher engine speeds (3,500 rpm's or so), very soon after it started losing engine braking while down shifting the trans. After talking to a few peeps about the issue I came to the "educated" guess that the trans was having issues.... Cracked piston, or sun shell, and burned up over run clutch pack. I've nursed the transmission along for the last few months all while the problem got worse. The vibration now started at about 2,500-2,800 rpm's, and gets worse with higher engine speeds. The vibration is accompanied by a humming sound.

So I got a known good trans from a fellow forum member back in late July (WF NY). I swapped transmissions last weekend (Sept. 26th.) and drove it around gingerly for a few days. The engine braking while down shifting is working great, but the vibration is still there.... Argh. So... I've got an issue with the trans, or the engine is what's doing the vibrating. The car runs fine, still has plenty of power, and the oil looks good. I again felt the vibration last night while driving more aggressively than I had been. Ran the engine up harder again this am, and the vibration is about as bad as before.

I'm thinking it's an engine issue... Seems unlikely that the "new" trans, has the exact same vibration. Maybe a bad harmonic balancer? I'm going to mark the outer ring, and inner hub of the balancer tonight, drive home and see if it all lines up still. Anyone with any other thoughts?

Mike

Mike:

Check both the motor mounts and tranny mounts. Chased the same issue on the Caddy for over a year. Replaced the passenger side (actually replaced both) engine mount and vibration disappeared. The old mounts looked good but had softened badly.

Bill

Trans mount I know is Okay, and the motor mounts looked good, but I'll give them a better look see.
Thanks.

Mike I can tell from your post that you feel that the vibe is engine/trans related. I had a 95 Caprice with similar issues to what your describing. I did plugs, wires, Opti, engine mounts and trans mount...followed by U joints and axle bearings. Had the rims balanced, changed tires...then changed the rims and tires with a set from a vibration free car. It ended up being a worn pinion bearing! Changed that and it was as smooth as a new Caddy.

The vibration is engine speed related though. A pinion bearing would be vehicle speed related.

I would agree...not having felt the vibration myself I can only offer up what has caused a similar vibe on one of my crates in the past. With 258 on the clock I would bet money on the pinion or even axle bearings over a bad harmonic. (Yes I have had axle bearings go bad without making any irregular noises). If you really think its engine related...then I would take a good look at the engine mounts. One time while chasing an annoying vibration I had "competent" mechanics tell me mine were fine...I replaced them and the problem went away. (They looked fine but had been softened excessively by oil contamination).

As you noted in your earlier post...the likely hood of two bad transmissions is highly improbable. I assume you used the converter from the "new" transmission and did not re use your old one?

Yeah, I used the converter that came with the trans I got from a really fine fellow.

bad ignition on one cyl

Wild ass guess - driveshaft?

So.... Harmonic balancer is fine, the lines I painted on the balacncer are still lined up after driving it a few days. I think I've got it though. I removed the serp belt, and went for a drive. No noise. I'm thinking a/c compressor.

Confirmed.... It is the a/c compressor.