Mechanics

VIBRATION

1990 Cadillac Deville

Engine Mechanical problem
1990 Cadillac Deville V8 Front Wheel Drive Automatic 130, 00 miles

Cadillac DeVille 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. 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.
Avatar
OldeTom
October 1, 2009.



In addition to a compression test, there is also a leakdown test that can be performed to identify the source of low compression. If the leakdown is low, and compression is still low, then one would suspect an induction problem: rocker arm, support, broken valve spring. Something not allowing intake air in. If the leakdown % is high, and compression is low, then the air can be heard leaking out the throttle, exhaust, crankcase, or adjacent cylinder. From that a hypothesis can be made as to the cause. Compression is not typically an intermittent failure. Is this compression gauge reliable, I have had some in the past that I could not trust, were always giving me correct info. I don't know why the dealer could not identify the code. It is possibly you have fuel, or fueling issues. Possibly an injector or two. If you indeed have low compression, it needs to be identified as to why, and a leakdown test will draw you closer to a conclusion.

Factoryjack
Oct 1, 2009.