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Q:

Idles rough in gear on 1997 Kia Sephia

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I've replaced the spark plugs, wires, distributor cap, rotor. Shop replaced timing belt and water pump. Today, the catalytic converter was replaced (P00420 was being displayed). Car still idles very rough. I know that whatever the root problem is probably took the cat. converter out and want to fix before it trashes the new one. Shop looked for problem and could not find anything wrong. Car seems to run fine when driving and if I shift into neutral, idle drops and it isn't so rough.
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Sure enough, a motor mount was busted. The check engine had been coming on every 30 miles with a P0300 (random/multiple misfire) and hasn't come on now in over 300 miles and idles much smoother...its like having a whole new car. Thank you very much.