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Took 91 Cadillac Deville (96,000 miles) with 4.9 into shop two weeks ago for a tune-up as it had just started missing; slightly. Shop ran the diagnostics and everything checks OK and they only changed the plugs as one was fouled; # 5. Drove it 100 miles on highway and it began to miss very badly. Back to shop, they run diagnostics and everything looks OK (to include the valves)but the timing seems to be advanced and causing engine to run rich; thus causing all plugs to fowl but especially # 5. The engine RPM also wants to flucuate as indicated by the mechanic. The only flucuation that I have noticed previously was at idle at a stop light when it would run I would say just about a perfect idle speed then take a slighty lower RPM after about 15 seconds. Just slow enough to make one concerned that it would cut out. Missing very badly now and mechanic is stumped. Mechanic wonders if computer is bad or if it could be a bad cam. If it is a bad computer, how would one know and what would be the repair cost? Replacing the cam is out of question I understand the entire engine would have to be pulled.
Answer -
The TYPICAL answer when someone who lacks the expertise to diagnose a problem is the famous quote:
"IT'S THE COMPUTER".
In this case it is in all probability being 95% that it is NOT the computer as it would NOT cause this problem.
Has the CHECK ENGINE LIGHT ever illuminated on the dash ?.
Any COMPETENT KNOWLEDGEABLE automotive technician can diagnose your problem in less than an hours time by simply performing some basic diagnostic tests of which some tests require special testing equipment and a high knowledgeability of your type of vehicle.
Obviously your mechanic lacks the necessary skills to perform the tests, So your ONLY option is to stay away from this mechanic and find one that is qualified and knowledgeable.
As to put the cause on a SPECIFIC component causing your problem, Further hands on tests need to be performed.
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No idiot lights to include the check engine light have come on. I will begin search for a Caddy mechanic. Could this mechanic be correct that the symtoms could be a bad CAM?
AnswerIt IS possible. Howver a GOOD automotive diagnostic technician can measure the height of the rocker arm lift on the intake and exhaust valve on the cylinder and determine by how much lift there is tell if the cam is worn without pulling the engine apart to physically inspect the cam.
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