Hi everyone, i have a 2005 neon, mtx, 80k miles. Recently when i shut off the engine it will shake and then you hear a clunk sound, but starts and runs just fine. gas mileage is kinda bad though, around 20mpg. I'm guessing its trying to diesel, or run on. It doesn't run hot, and it doesn't idle above 1k rpms, usually idles around 700. so, what else could be causing this? carbon build up?

 

See if you have any vacuum leaks

 

Dieseling is usually caused by an idle that's too high. Since you say it usually idles at 700RPM but not above 1K, it seems like there is some fluctuation going on? A vacuum leak could do it.