Engine sputters when I shut it off

[David Vandercook]

Hello everyone
My 86 W250 with the 318 always sputters and shakes like it wants to stay running if I shut it off after it has warmed up. This lasts five seconds at the most. I never really saw anything like this and it really scared me the first time it happened. Is this a problem? What causes it?
Thanks, David

 

[armyvet25]

The old term called "dieseling" caused by continuing detonation, how this happens is over time carbon builds up on piston tops and valves as engines wear over high mileage worn rings etc, the dieseling effect is caused from the carbon deposits built up glows red hot from the engine running and when you shut the engine off it acts like glow plugs in a diesel hence the name. It causes the engine to continue to run by by firing fuel without the aid of ignition and each fire draws more fuel in continuing the process, if unable to stop this when drastic, turn ignition back on, will run normal, place in gear then turn off ignition before putting in park, this will shut the engine down.
Try pouring water into intake while revving engine, will blast carbon deposits off to some extent, maybe an additive to help remove them, drastic measures may require engine rebuild.
Typical for high mileage older engines. Will only get worse if not helped now.

 

[David Vandercook]

It's got less than 100k miles on it and really doesn't show other signs of high mileage... Is there anything else that could cause dieseling? It spent most of it's life as a farm truck so it never really racked up that many miles.

 

[armyvet25]

only other thing comes to mind, poor ignition components causing carbon build up ie tune up, incorrect timing, also check vacuum mercury, basically properly tuned vehicle. If not original owner a vehicle can carboned up due to lack of oil change maintenance even at lower mileage,

 

[turkeybird56]

No, dieseling was common on those older 318's. The way to stop it when turning off IS turn off in gear, than put in park. Been a long time since I heard that, last 318 was in Mom's Grand Fury III wagon from 1975. Pulling stuff while cleaning helps. I DISAGREE on pouring water down the carb into intake. Get a product made to specifically be put into the carb for cleaning. NOW I can say, back in my "poor" days as a kid, we used to dump GM ATF down the carbs, but that was in a Rochester Quad on a 396 L4 motor, but do not do that. There are many products out there in 2024 made for cleaning and such, like seafoam in gas tank, B-12, other stuff to dump in carbs. Do a lil research and use what U feel comfortable using.

 

[turkeybird56]

only other thing comes to mind, poor ignition components causing carbon build up ie tune up, incorrect timing, also check vacuum mercury, basically properly tuned vehicle. If not original owner a vehicle can carboned up due to lack of oil change maintenance even at lower mileage,

Vacuum may be leaking at advance dashpot, maybe timing too far advanced, low speed idle too high (doubt that tho), timing needs lil retardation, hard to say unless there. Internet guessnosis no better on a 2024 than an old 318, LOL.

When the last time points, rotor, condenser, distributor cap, plugs, wires, air filter and fuel filter been changed? U kinda gotta keep up on that stuff on the older rides. It was almost always automatic to replace most of that stuff every 5-10K. I did mine on my 396, 307, 350 every 5-10 with exception of plugs, those done every 25K, but than again, we talking old school stuff here, (heck I even used to double over matchbook cover to set points/dwell), and just how I learned/was taught way back in the "day".

 

[armyvet25]

Vacuum may be leaking at advance dashpot, maybe timing to far advanced, , low speed idle to high (doubt that tho), timing needs lil retardation, hard to say unless there. Internet guessnosis no better on a 2024 than an old 318, LOL.

When the last time points, rotor, condenser, distributor cap, plugs, wires and fuel filter been changed? U kinda gotta keep up on that stuff on the older rides. It was almost always automatic to replace most of that stuff every 5-10K.

no massive amounts of computers and sensors to add to the mix on an older 318, guessnosis?? he asked for advice, otherwise no one should comment on anything here, just advising on a direction to look.

 

[CanuckRam1313]

Agreed on the Seafoam...

My $0.02..

Dump a couple of cans in the tank and fill with premium.
Dump a half can into the crankcase run for a while (take a good drive and let everything get to operating temps for a while) and then dump and do an oil & filter change.

Run a can through the vacuum line off the brake booster. While someone is holding about 2000rpm.. siphon off the contents of the Seafoam can until empty.

Turn off and let it sit for maybe 10 minutes max... fire her up and drive it spiritedly (you'll fog out everywhere for a bit and kill all the mosquitos everywhere around you).

Then let her cool and replace the spark plugs and check the timing.

Likely a good time to do a thorough carb cleaning, too.

She should run like a champ afterwards.

 

[David Vandercook]

Most of that stuff has been replaced, however, my carburetor adjustment job was a bit amateur and the idle is a bit high. I do currently have sea foam in there but I also have cheap gas. I'll use ethanol free next time though since the tank is so big. I rebuilt the carb a couple months ago when I bought the truck.

 

[David Vandercook]

I turned down the idle speed a little bit and it only coughed weakly twice instead of dieseling for a while. I looked at the spark plugs and they do look a bit carbony already though so I will have to carbon clean it. Thanks for the help.