1990 Cadillac Deville Vibration

1990 CADILLAC DEVILLE
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OLDETOM
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1990 Cadillac Deville V8 Front Wheel Drive 130000 miles

Cadillac DeVille 1990 130,000 miles scrupulous maintenance 4.5 L code 3 fuel injected
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.
Underwent $500 tune up.
Several days later vibration as above this time with trouble light on and dealer verified vibration this time. Passenger can feel vibration.
Dealer could not identify a code. Suspect warning light intermittent. Checked more compressions. several low.
No antecedent symptoms; no oil usage. No blue exhaust smoke. 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.
Oct 1, 2009 at 2:50 PM
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Why did the dealer perform maintenence when there was already a mechanical problem with the first cylinder having low compression?
Oct 1, 2009 at 6:27 PM
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OLDETOM
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[quote:6c7ae5db54="Service Writer"]Why did the dealer perform maintenence when there was already a mechanical problem with the first cylinder having low compression?[/quote:6c7ae5db54]

Hoped it would help; was not sure.
In airplane engines low cylinder pressures common and do not cause trouble like this.
Oct 1, 2009 at 7:32 PM
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Maybe "low" is relative. Low...out of spec...means probably misfire and rough idle. I'd love to know the logic of a tune up bringing up the compresion. You need a second opinion. I would not trust that one.

Maybe you do need an engine, but the tune up was a bad recommendation.
Oct 1, 2009 at 8:05 PM
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