Engine runs after key is turn off

Hello all
I have a problem I have been trying to solve for some time now.
Sorry if this is in the wrong place or not formatted correctly. I am new to posting.
Important stuff
97 xj 4.0 auto

Sometimes but not always the engine runs for an additional 30 to 45 seconds after the key has been turned off. This time has been been tracked to when the PSI in the fuel rail reached 0 psi.
This condition presents itself during warm to hot months and can be controled by running the Aux fan several minutes before turning the jeep off.

It sounds like a leaky fuel injector but when i use a fuel guage to track Fuel psi the fuel maintains constant pressure (it might lose 1 or 2 psi after key is shut off). Vacum pressure is steady around 18 psi. any ideas would help. i have searched this on but i have not found anything that fits this condition.

Is it "dieseling"--running on like crap--or running like you haven't yet turned the key off?

If it is "dieseling" yo may have carbon built up in the combustion chamber, the hot glowing carbon keeps detonating any available air/fuel mixture. For this problem Mopar Combustion Chamber Cleaner, MCCC, from the dealer.

If it simply isn't shutting off you need to determine why the system is receiving power. Things to check: faulty ignition switch and/or key cylinder; corrosion under the PDC; sticking ASD relay.

After the key is turned off, there should be no more spark AND no more fuel.
When it is running after the key is switched off, does it run normally - if you hit the gas does it rev up - or is it bucking and stumbling?
When I was a kid on the farm, we had an old combine that would run on, up to a minute BUT it would be bucking and stumbling the whole time. (It had a heavy load even at idle, so the combustion chamber would get hot enough to ignite the fuel even with no spark). I can see this happening with a carburetor, not with fuel injection UNLESS you have badly leaking injectors, and your engine is carboned up a lot.
You mentioned that it stops when fuel pressure drops to 0 PSI, but then in the next sentence you say the fuel pressure stays constant. ???

the_big_h
You mentioned that it stops when fuel pressure drops to 0 PSI, but then in the next sentence you say the fuel pressure stays constant. ???

The fuel rail drops to 0 PSI when the runon condition presents itself. The fuel rail maintains constant psi when you turn the engine off and it does not runon

joe_peters

It is like it is deiseling. There is a very slight pause when I turn the key off then it stumbles until the fuel rail is empty. I understand the heat(spark) part of the combustion process(I will try MCCC this week). I am also trying to find out if there is a part in the fuel system that could be introducing the fuel.

Thanks for your replies

It sounds like the fuel supply system's voltage is being cut off as normal, but not the ignition coil. Next time it's sitting there running with the key off unplug the low voltage connector to the ignition coil and see what happens, that should kill it right away.

whats the mileage on the pup?

Is she "knocking" when she stumbling? Need more details.


kinda sounds ignition related

Simple easy check. (no $s and little effort)

Check ASD (auto shutdown) relay in fuse box under the hood. (Its labeled.) Sometimes it gets stuck. A light tap with a screwdriver can unstick the relay. Might try pulling the relay when the engine won't shut off.