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Q: why is my car still missing after a tune up? on 2000 Cadillac Seville STS

After a mechanic tune up my 2000 Cadillac STS,it idles like it hasn't been tune up. He said he changed the plugs,the spark plug boot,fuel filter,changed the pcv valve,used a fuel inject cleaning kit and a kit that would help in cleaning the valves.

The check engine light came on prior to the tune up. He did a diagnosis and said the there was a 'misfire' on cylinder 4 and 2. He said the tune up would take car of the rough idling,but it hasn't.

The car runs without hesitation when you step on the gas and runs great without any signs of misfire at low speeds and high speeds.

I am taking the car back to the mechanic and asking him why the rough idling after a tune up. I need to know a little more about what might be causing it to idle rough.

Any advice would be greatly appreciated.
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I had similar misfire issues with 1999 Seville. I replaced all 4 coil packs (ebay S100) and replaced all spark plug wires. Problem solved!
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