i sound's like the problem might b your air/fuel mixture is not operatting properly or the timing belt or chain might have jumper/ stretched, also there might be water in the tank.
the seat on the exhaust valve is dirty so the valve does not seal it.... not sure how to clean it
i think the computer need to be reprogram
if it only backfires when shutting off, usually this means its getting to much gas!! if it backfires will running adjust carburater.
Most O2 sensors are in the Exhaust pipe running from the back of the manifold. It is silver with a single black wire running to it. It threads into the exhaust pipe like a spark plug and can usually be accessed from under the hood. Happy Wrenching.
If it runs, it will miss slightly and RPM gauge will be lower than the 800 RPM normal. It could exhibit some "thumping" sounds at the exhaust which is exhaust being pulled back because valves are not properly closing - test this by holding a piece of paper at exhaust and see if it pulls back into pipe instead of constantly blows away. If it backfires while starting or running.
If any car backfires it is usually due to retarded timing, running lean or a problem in the valve train.
running lean can cause your 400ex to backfire ..if it backfires on deceleration its lean ...if it pops on acceleration its running rich.====carb issues
No, I recommend go to a mechanic, chances are your firing order is out or running bit to much fuel
Needs a tune up? Bad timing?
Day time running lights?
You can expect your engine to vibrate some. Other than the visible vibration, is the engine running smoothly? No missing or backfires or anything like that? Excessive vibration might be due to faulty motor mounts or missing hardware at the motor mounts.
have a pontiac 6000 2.5 motor and it wont stay running