350 putters after shut off

madJeepJ10

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Aug 5, 2009
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Marion
i have a 78 gmc with a 350. some times when i cut it off it will putter and spit before it dies not all the time and its like the first time i shut if off if its been siting for a min then i go and drive down the road. the timeing is set right and the idel is alittle high but it sits at about 1000 rpm i just did spark plugs but thats it. please any tips on where to start would be great thank you
 
Commonly called dieseling.

Two most likely reasons are it's running way hot or has a ton of carbon build up from never being ran hard .

Make sure your cooling system is in order, then give it an Italian tune up.
 
Not sure about on your motor, but the 350 in our cheyenne has a solenoid that sets the idle, when the key is off, it lets the throttle plates close all the way. It hung open and the truck would run for a few secs after turning off the switch.
 
You said that you changed the plugs, then this started. Did you swap the same type/heat range plug in? My inclination is that you put a hotter plug in, causing the cylinder temps to go up quite a bit. I'd make sure you have a same or a comparable plug. I had a 383 that would diesel when running on pump gas because the compression was too high (which caused combustion temps to go way up).

What @Ron said about the cooling system is a good place to look, too. If you've got some plugged up cooling passages in the head, they can be getting hot causing the same thing.
 
sorry for the misunderstanding it was doing it befor the plugs but when i changed them there was a lot of build up in them kinda hard like a rock.
also if i have a msd on there i was told to gap the plugs at 50 is this right the reason for the question is because you said the temp in the cylinder might be to hot
 
madJeepJ10 said:
sorry for the misunderstanding it was doing it befor the plugs but when i changed them there was a lot of build up in them kinda hard like a rock.
also if i have a msd on there i was told to gap the plugs at 50 is this right the reason for the question is because you said the temp in the cylinder might be to hot
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holy gap, batman...back that down to .035. And run some Seafoam through it.

Pull the vacuum hose off of your brake booster and put it in the bottle of sea foam. It'll smoke like a freight train out the exhaust, but it'll help clear the carbon buildup out of the cylinders.
 
Did you change the plugs for the same plugs?
Build up would indicate too cold a plug. Usually dieseling is caused by too hot a plug causing cylinder temps to stay high. Of course the build up could be an indicatiom of carbon.

Do you drive it only short distances repeatedly?
Do you ever get on it etc.
 
On the gap, are you using and "t" or an "ts" plug .
 
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I used to slowly pour a little water down the carburetor at a very high idle,maybe 1500 rpm to remove the carbon in a high mileage Oldsmobile. Lasted for several months before treating again
 
What octane do you run? 87? I've seen a switch from 87 to 91 cure dieseling and other problems in carbureted vehicles. The higher volatility of 87 is more prone to run-on if there's hot carbon in the cylinders. Helps with pre-ignition during warmer months, too.
 
I would lower the idle speed. Try 600 -700 in neutral. Get the carbon out of the combustion chambers. If you haven’t changed the valve seals it’s needs done unless the rings are shot. Leaking intake gaskets allows oil to be sucked out of lifter valley.
 
Scooter402 said:
My inclination is that you put a hotter plug in, causing the cylinder temps to go up quite a bit.
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i would disagree with this statement. hot vs cold plugs have to do with how well they are insulated, with how well they take heat out of the cylinder.
 
I got ac delco plugs and i noticed this morning there is oil coming out the plugs and they are tight i will try the 91 gas next time i fill up and will adjust the idle after work today thank you all for the input
 
Check timing again (with a gun, not by ear...ear can sound good but run like shit)...too far advanced and it'll diesel, this is where 'running hot' usually derives from. No idea off the top of my head where factory specs are for a 350, but most v8 stuff from the 70's are in the range of 8-10* for manuals and 10-12* for autos. Whether the distributor wiggled loose, chain stretched or jumped a tooth, that's what would cause it. Check float levels, too much residual fuel will cause dieseling, the truck just tries to keep running. 1000rpm idle is too much, if you need it to sit there, that would bring me back to too much fuel or too much timing. You either have vacuum leaks or floats too high/stuck or both. I'd start spraying down vacuum lines (all rubber hoses, booster, intake/etc) with starting fluid, if the rpm picks up, you found your leak. Other than that, pull the sight plugs in the carb...at idle on flat ground, fuel should be just at the bottom of the hole. Plug heat range and carbon build up I'd be inclined to agree with if it did it all the time, and not just at shut down.
 
ckruzer said:
i would disagree with this statement. hot vs cold plugs have to do with how well they are insulated, with how well they take heat out of the cylinder.
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You caught me, I had poor wording. You're right with how they transfer heat from the plug to the head. But the "hotter" plug will keep the heat in the plug longer, potentially causing some preignition and unfortunately his dieseling problem.
 
ok i retimed it was a little off i also took the idel down to 6 lets see what happens when i go to work in the morning
 
It seems to do a lot better i still have oil coming out spark pluf hole i think you can get seals go go on you spark plugs to stop that am i right
 
Is it oil or is it carbon?
Does it smell heavily like gas.

I will ask again how often do you full throttle it
 
how built is this 350? I'd drop gap a little on carb 350. I run acdelco r44t at .35 with stock hei, If ya don't have extra air/fuel going in there no need for bigger gap imo... most the time I've seen dieseling in 70-80 small block is when engine cylinders to hot often it's wrong plugs/timing related! From my own personal experience with 70s 400s it's "almost" always timing related but never had dieseling for more than 2-3 seconds after turn off!
 
i just got home and i ran it hard up spruce pine mtn then at the top pulled it over and turnd it off and nothing it has a lot more power and its not so hot in the cab as for the oil it dont realy have a smell to it. and i dont floor it. it has a rv came and is bord out about 40 over and the heads was worked a little. i love it now and it gets a lot better gas milge to. thank you to all who helped me on this i guess the bolt was not tight enough on the dis
 
i did the test and this is what i got 1/125/2/122/3/150/4/120/5///90///6/120/7///80///8/119/. what yall think
 
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