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

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
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.
(3) Answers
(3) Comments
I had similar misfire issues with 1999 Seville. I replaced all 4 coil packs (ebay S100) and replaced all spark plug wires. Problem solved!
You gave no mileage. I would be curious about the compression on each cylinder and to see how even they are. Also curious about coils output. You said spark plug "boot". Did you mean all boots or just one boot? Is the CEL on again?
The car has 97000 miles. The invoice said spark plug boot kit. What do you mean by 'coils output'. The check engine light (CEL) in not on.

Thanks for your quick response and help!
Boot kit means all, not in original post. I am now still recommending the compression test to make sure you dont have a mechanical prob in any cylinder(s) and coil output is electrical power put out under different load conditions. They could be breaking down and giving weak output. Many things can cause misfire codes.

As backyard tech says could have head gasket probs. That can allow coolant getting in to cylinders during combustion process and caused misfire and compression problems. The recommended compression test could pin this down. If you ask then try doing what we recommend. Even if it costs more money. Or you can guess at it.