Cadillac Repair: car shakink at idle, oxygen sensor, gas tank

Question
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Followup To
Question - I am sory Bill, the engine sise is 4.5 not 4.2 is the answer still the same?
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Followup To
Question -
Make of car 1998 Cadillac 4.2 v8 (I am sorry the car is 1989)
Changed plugs, wires ditr.cap rotor. And still is shaking. I noticed that when the car is cold it does not shake.
Thank you. Emil
Answer -
Hi Emil, You really have me confused here. Cadillac doesn't make a 4.2 liter engine in 1998 nor do they use a distributor cap anymore. What are you driveing and what year and engine size is it? Let me know and I can give you some things to check. Bill
Answer -
Hi Emil, What you are describing is a classic case of fuel injectors shorting out. 1 may be bad or I have had to replace as many as 5 bad ones. Testing them with an ohmeter cold and then hot should determine which are shorting out. There is a special tester that puts a current through the injector and then you measure the voltage. That is the most accurate way to test but only a shop or tech would have those tools because they are rather expensive. If you have them tested at a shop then you can replace the bad ones if that is something you feel capable of doing. Hope that helps. Bill

Answer
Hi Emil, With the 1989 the diagnosis is differant. Starting in 1990 the injectors act up. Sounds like you will need to take the car in to a shop and have them put it on the oscilloscope and check the computor readings because a bad map sensor or oxygen sensor could cause a rough idle hot. Also it might be worth buying a bottle of Valvoline injector cleaner high strength to put in the gas tank as carbon build-up inside the head or cylinder area could also cause rough running. Bill