What to do to fix a dieseling vehicle?
You need to take it to a independant mechanic.
I’d try seafoam first. Dump 1/3 of the can into the full gas tank, and 1/3 into the throttle body while the engine is running. Expect an amazing amount of smoke out the tailpipe when you do this. Once you’ve run through the tank with Sea Foam in it, your problem may well be solved, or significantly reduced. If it’s reduced a lot, run the Sea Foam again.
Dieseling can be caused by carbon deposits in the cylinder. They get hot enough to ignite the fuel/air mix themselves without needing a spark plug. the Sea Foam can clean those deposits off.
If that doesn’t work, take it to a mechanic.
That depends whether or not it actually is a diesel under the hood…if so then everything is A-OK… Diesel Away.
Sorry… I couldn’t help that… Hmmm an 05’ should not be dieseling it suggests to me a very lean running condition…which ironically makes the combustion chamber hotter than normal, it also leaves the combustion chamber with an abundance of oxygen as well, which can thus facilitate this condition with the slightest of fuel type substances.
If you are running lean any combustible will then do to produce dieseling and in your case with fuel injection it is most likely not fuel (gasoline)…it could be oil vapor from your PCV system that feeds this monster however. A stuck open injector perhaps? The stuck injector fools the computer into leaning out all other cylinders because it will enrich the air fuel mix and the computer may try to compensate which makes everything worse… Hmmm I need to think about that a bit more…
Take your truck to a shop and look into its fuel air ratio… Methinks you are running lean…which is bad for a multitude of important reasons. Its kind of important you solve this quickly as you can harm the engine fairly successfully if you do not.
It should be very lean after the injectors are shut off, no fuel at all.
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No fuel per say… but the pcv can introduce oil vapor to an already overly hot combustion chamber. That is fuel.
Timing is off due to cam phasers, timing chain wear, etc. Just a guess.
Timing? There shouldn’t be any spark with the ignition off.
Fuel injected? if not remembering tom and ray suggesting squirting water down the carb to clean carbon buildup.
Incorrect timing could cause excessive heat inside the engine, leading to the engine run-on. Maybe? Or maybe incorrect timing could leave a little unburned fuel in the combustion chamber that ignites after the engine is shut down. I’m not sure what the theory behind it is, but I’ve read that incorrect timing can cause dieseling. Plus I have an overwhelming desire to blame all 5.4 engine problems on the variable cam timing setup 
. I should probably seek out counseling for that.
Especially since I jumped to the conclusion that it was a 5.4 in question.