straight six slight diesel

[mehoff400]

so not all the time or a lot but when i go to turn off my truck it sometimes will diesel a little. couple seconds maybe ad will die out. i have the timing at about 10-12 degree advance and it has HEI on it. do i need to add more timing to it? with the stock timing tab i have ran out of room to be accurate

[65ny]

Dieseling can be happen when the timing is too advanced. Does it ping under acceleration? Is it difficult to hot-start? If so, it probably has too much advance. If not, check to make sure your throttle blades close completely at idle and consider doing the seafoam steam-out. (or even water) https://www.youtube.com/watch?v=KFtp_jmLF3k

Carbon build up can cause hot spots, which can lead to dieseling

[cg285]

timing does not cause this. when the key is switched off there is no ignition. it needs fuel to diesel. lower the idle speed. some things could contribute to having a higher idle setting (retarded timing, vac leak, carbon etc) but the root cause is the idle too high

[TJ's Chevy]

I believe this engine was just rebuilt to those who answered....I know the OP had head work done at least.
X2 with what has been said. My GMC 305 V6 dieseled once...and that was from shutting it on and off a lot in a short time. Engine was nice and hot and dieseled a bit...there is some carbon on the valves which could have been a cause, but it hasn't done it since. I have the timing about 4 degrees advanced above factory specs so I don't think that has much to do with it. Could though! If you were to let it idle for a bit before shutting down would it not do it?

[mehoff400]

correct engine was just rebuilt about 33K ago. took the valve cover of and looked very clean. the high idle might be it i'm not sure what the idle is running at as i dont have a tach yet. as when it does it or not does it i haven't paid attention to that. thinking about it seems to happen when i just shut if off real quick and not let it idle. shes a little hard to start when cold but pretty easy to start when warm. couple pumps with the throttle it usually fires first try if not for sure on second try.

[68gmsee]

As mentioned above, once you shut the key there is no ignition source so the dieseling has to be caused by fuel getting to the cylinder and being ignited by either super heated valves, carbon or something else in the cylinder.

In some cases -as in my 68 GMC- it's carbon build. It stays hot enough (almost like bbq briquets) to ignite fuel being sucked in from the partially open throttle in the carb. That's why the lowest idle you can set will help some.

If it's carbon buildup you can usually get rid of it with fuel additives but it may take more than one tank full. On my GMC, I had to double up on the additive and it took couple of tanks full. That was few years ago and hasn't done it since.

[TJ's Chevy]

Originally Posted by 68gmsee

As mentioned above, once you shut the key there is no ignition source so the dieseling has to be caused by fuel getting to the cylinder and being ignited by either super heated valves, carbon or something else in the cylinder.

In some cases -as in my 68 GMC- it's carbon build. It stays hot enough (almost like bbq briquets) to ignite fuel being sucked in from the partially open throttle in the carb. That's why the lowest idle you can set will help some.

If it's carbon buildup you can usually get rid of it with fuel additives but it may take more than one tank full. On my GMC, I had to double up on the additive and it took couple of tanks full. That was few years ago and hasn't done it since.

Did you use Seafoam? I use it once in awhile when the trucks sit. Seems to work well as a fuel stabilizer. Does work for cleaning carbon as well.

[68gmsee]

Originally Posted by TJ's Chevy

Did you use Seafoam? I use it once in awhile when the trucks sit. Seems to work well as a fuel stabilizer. Does work for cleaning carbon as well.

The first time I used Chevron but I know use Seafoam. I also use it in the gas tanks of all my vehicles every few months. Not sure if other additives would be just as good, but so far no problems using Seafoam.

[storm9c1]

As mentioned, lower the idle at the carb. You can usually lower it a lot if you connect the vacuum advance to full manifold vacuum source. The full manifold vacuum advance should give you a bump in RPM at idle. Unfortunately this may cause other tuning to be necessary, but it's worth a try.

[RustyBucket]

I had a 250 in a van a long time ago that dieseled TERRIBLY.. couldn't hardly shut it off. Left it in a parking lot one time for about 2 HOURS and it was still rattlin' away when I came back. They make a distinctive smell when they diesel and mine stunk up the whole neighborhood. Turned the key to on and it was running... and the temp. gauge was pegged. Ran for awhile and cooled down, seemed to be ok. I never thought it ran smoothly but it was the only I-6 i'd ever had so I didn't know any better. Found an intake manifold leak right before I sold it. Tightened the int. bolts and it ran much better and no more dieseling.
I've seen anti-dieseling solenoids that let the throttle butterflies(flys)? close when the key is off... they've got to get air to keep dieseling.