I haven't personally had any experience with dieseling,
but just had some ideas come into my head so I figured I'd
post 'em in case they are at all helpful in diagnosing the
problem...
If it is dieseling, and it sounds like it is, I guess that
means fuel is getting into the cylinders somehow. (Either
that, or there is enough residual fuel in the cylinders to
ignite after the engine has shut off (though that doesn't
seem very likely...) As others have mentioned, an EFI motor
isn't as likely to diesel as a carbeurated motor since the
injectors are electrically controlled. Although the fuel
pump is electric, there is still pressure in the fuel rail
after it has been shut off. A possibility might be an
injector is sticking open and allowing fuel to keep flowing
until the remaining pressure in the fuel rail is depleted.
What seems strange to me though is that it happens when
the engine is cold. Could moving the truck in the driveway
really build up enough heat in the head to ignite the fuel once
the engine has been shut off? It doesn't seem likely unless
there was some carbon or something in there getting heated
up. (On the spark plugs prehaps - when was the last time
they were checked?) And even if that was the case, why
wouldn't it do it when the engine is hot? One possibility
that comes to mind is perhaps the cold is contributing to
causing an injector to stick open, whereas it works fine
(or acceptably well) when it has been warmed up. ?
Someone suggested running some fuel system cleaner
through it, and that does seem like a somewhat inexpensive
first step. It might just be an injector gumming up...
Is the condition repeatable? If so, you could start doing
some basic diagnostics. For example, someone suggested an
ignition/electrical problem. It doesn't seem likely that
the plugs themselves are actually firing, but you could
eliminate this possibility by putting a spark plug test
light on the coil wire. (Its a wire with a light on it
which flashes when the plug is fired, but it goes in-line
so it will still pass the spark through to fire the plug;
they're only a few dollars usually.) Anyway, do whatever
you have to to cause the dieseling condition and after the
engine is shut off, check to make sure that the plugs
themselves aren't being fired.
You can also do a similar test for the fuel... Checking
the injectors with a multimeter would determine wether the
computer is still firing them after the engine is off.
(Unlikely, but I guess you never know until you check.)
If you have a fuel pressure tester, you may want to try
hooking it up to the test port, turning the key on to turn
the fuel pump on and build the pressure up, then turn it off
and watch to see how fast the fuel pressure bleeds off. If
it bleeds off quickly, this may point to a leaky injector.
It would be interesting to try the test both on a cold and
hot engine to see if there is a difference in the time it
takes for the pressure to bleed off. Unfortunately, I'm not
sure how quickly is "too quickly" :-) IIRC, the fuel pressure
in my '96 Dak stays up for a while. Unless it can bleed back
down past the pump, in theory the pressure should stay where it
is (when you shut off the pump) indefinitely. If you need me
to, let me know and I will run this same test on my Dak and let
you know how long it takes to bleed down (if it does at all)
so you can compare the results.
Oh, one last thing, I am pretty sure dieseling is not real
healthy for an engine, so you will probably want to get it
fixed as soon as possible. I believe dieseling is essentially
a series of misfires - it usually does not follow the correct
firing order. :-) So, its probably rotating the engine and
then stopping it immediately or even trying to rotate it
backwards.
Sorry for the rambling nature of this message. :-) Like I
said, it was just some ideas floating around in my head; when
I read your post I thought of a few things I would try to narrow
down the problem so I just kinda puked 'em up on the screen
as I thought of them. ;-)
Good luck!
---Jon-
.---- Jon Steiger ------ jon@dakota-truck.net or jon@jonsteiger.com ------. | I'm the: AOPA, DoD, EAA, NMA, NRA, SPA, USUA. Rec & UL Pilot - SEL | | 70 Cuda, 90 Dak 'vert, 92 Ram 4x4, 96 Dak, 96 Intruder 1400, 96 FireFly | `------------------------------------------ http://www.jonsteiger.com ----'
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