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Engine Vibration

1990 130,000 miles scrupulous maintenance

Engine vibrates; frequency proportional to speed and accelerator depression; coast smooth.

Two episodes. First no trouble light. resolved spontaneously. GM dealer tuned engine … smooth. Checked compression one cylinder lo.

Several days later vibration as above this time with trouble light on and dealer verified vibration this time.

Dealer could not identify a code. Suspect warning light intermittent. Checked more compressions. several low.

No antecedent symptoms; no oil usage. Pristine outside and interior.

GM dealer says we need new engine.

Hard to believe because sudden onset; it has come and gone and now come back. Interim smooth as silk.

Two perhaps unrelated observations:

Two weeks earlier someone removed gas tank cap … purchased a new one promptly; could something have been added to tank?

In earlier years trouble light meant a new oxygen sensor was required … although auto was running smoothly … light only symptom.

Like the car.

Desperate for advice.

If you truly have low compression on multiple cylinders your engine is close to giving up the ghost. The only fixes are to rebuild or replace the engine. If this is the case, drive the car as long as possible and put up with the vibration, your engine is toast anyway.

a 1990 Deville with 130K miles? That would be the old 4.5L pushrod engine, correct? or did Devilles get a Chevy 350…If it’s the 4.5 motor, it might be time for a set of injectors. Is the problem vibration or misfire??

At this point, I think the biggest issue is the low compression that the OP reported in several cylinders.
New injectors will not cure that situation!

If you really want to see if you can salvage the engine you might have a leak down test performed to try to locate the source of the compression problem. If the compression problems seems to be valve related a valve job is probably not too unreasonable. If the problem is in the pistons/cylinders then you’re looking at a total teardown.

Am not a car person but it feels like the engine is shaking. The shake rhythm is steady and varies linearly with acceleration that is it is not irregular. The vibration is transmitted to the car itself. I can also feel it if someone is driving.
By the way no smoking exhaust.
The engine is a 4.5 L code 3 and has fuel injectors.