Misfiring on a 97 Chevy Lumina

when first starting up car and the car is cold, car runs fine for hours but the minute you shut the car off it starts misfiring, then again after car cools off for a couple of hours it then starts up and runs fine. changed mass air flow sensor, checked ignition module control-it is ok, So what could it be…PLEASE HELP!!!

Did you replace the module?

This sounds like a heat issue though.

You need to get the EGR circuit looked at. It is probably gunked up. The EGR system will lower the exhaust temperatures to reduce the formation of NOx and SOx. The added benefit is lower valve temperatures that prevent the run-on and dieseling. If the system is clogged, you lose this benefit.

So it is a hot restart problem, correct?

I would suspect fuel rather than spark.

  1. Leaking fuel injector that floods a cylinder. After the car has set a long time, the excess fuel evaporates.
  2. Cold start circuit with a bad temperature sensor that thinks the car is cold whenever it starts so it enriches the mixture. When the car is cool, no problem, but when the car is warm, this is too much fuel.