whan i kill my car, after about 1-3 seconds it makes a poping sound. not a loud crack but a quiet puuuuh. i think maybe its the 20 yr old cat holding emissions then, when i kill the car either they ignite, or they simply are released

but i dont really know thats a guess and i was wondering if any one could pinpoint the problem.

 

Any mods, or is the car stock?

 

Carb too rich or time to change the plugs and/or wires...