Let's see some suggestions on how to STOP an engine from "dieseling" during shutdown.

[Kern Dog]

Try the best gas you can get with an additive?

That is an interesting point. Maybe I have it wrong but I thought that if the engine wasn't knocking, the gas is good enough. Is it possible that the cheaper brands of gas I've used don't lead to knocking but can cause run-on?

Yup.

Try Lucas fuel additive. About 1oz per 6 gallons.

I'd appreciate a little help in understanding how "better fuel" or treated fuel would make a difference if the engine does not detonate and runs strong.
Please understand that I am not disputing you guys, I'm trying to understand this.
I currently have about 1/4 tank of ARCO 91 octane gas in it now, I'll have to drive it to use up the fuel or pump it out. I can then run some 91 from a local Shell or Chevron (With "Techron")
I don't remember the car dieseling much, if at all a few years ago with the same engine combination but using 91 octane exclusively. When on road trips, I stopped at what was available. Many times it was a Chevron, the most expensive gas in this state. I can't call that a strict scientific comparison since my records are not complete or accurate on this issue.

 

[Billccm]

Well the Lucas additive would be to reduce carbon in the combustion chamber, which would happen over several applications of the product. I believe to see any improvement in changing gas octane you should probably wait until the second tank, or so to see a difference.

 

[Kern Dog]

My curiosity about the "better gas....fuel additive" suggestion is based on the fact that my heads are as clean as new, the pistons are as well.
The heads were off in February/March when I swapped in this new cam. There are no carbon deposits in play here.
See, I could accept the possibility of fuel dripping into the chambers as the engine slows down to a stop BUT something would have to ignite the fuel. If there are no carbon deposits, could the spark plugs be retaining enough heat to cause this?
I still can see if the curb idle screw and throttle blades have enough adjustment to close up more. I can back the screw off completely and hold the engine at the 800 RPM idle speed and lift off the gas on shutdown to see if the suggestion about a throttle stop would help.

 

[Wietse]

When the engine is shut down there is no spark anymore, there is still fuel and air present inside the cylinders.
While the crank is slowing down it is still able to compress any air/fuel mixture which could detonate only by heat itself.
Carbon deposits are suspected by many, but in my opinion this will be only happening under high(er) load to make any deposits that hot they will become an ignition source. (or like right after a race or dyno pull and instantly shut it down)
Something to try is to fatten up the idle mixture to reduce cylinder temperatures at idle?

 

[68-500]

It maybe sharp edges on the pistons around the valve reliefs that are a hot spot. Not carbon.

 

[Strato58]

If you're testing octane Id go to 93 if its available to you. If it doesn't help then move on to something else...89 Octane: Has lower resistance to auto-ignition under heat and pressure. In a hot 440 combustion chamber, 89 octane ignites easily off residual heat or hot spots as the engine coast-down creates compression.

91 Octane: Provides a noticeable improvement in thermal stability over 89. For many stock or moderate big-block builds, 91 raises the auto-ignition threshold enough to stop mild dieseling if chamber temperatures and throttle blade angles are under control.

93 Octane: Offers the highest auto-ignition resistance available at standard pump pumps. The jump from 89 to 93 gives the maximum chemical safeguard against spontaneous ignition from hot chamber surfaces or glowing spark plug tips.

 

[Wietse]

I am using European RON 98 fuel, which will be a 93-95 octane comparison with US standards and have the same issue as Kern Dog describes.
Mine hasn't been apart and is more likely contaminated with carbon, but again, glowing carbon or sharp edges are more likely present at high combustion room temperatures like under load etc. and it seems hard to believe that that is still occurring with an engine just idling.

 

[Rich H.]

Something to try is to fatten up the idle mixture to reduce cylinder temperatures at idle?

I always shut the GTX off in gear, it tried to run-on after shutdown one time and I never allowed it after that. Agreed it seems like a crutch and wish I didn't feel I needed it. Because of cam specs mainly I have gone richer than stoich for the strongest idle quality, but didn't experiment to see if that helped or hurt hot shutdown behavior.

maybe related somewhat ? My little 16HP gas tractor has manual choke, and has acted up a few times after hot shutdown. On it, to kill the engine when hot, it probably sounds funny but I pull the choke and kill the ignition at the same time. Fuel as coolant/some of the air is cut off/ignition is cut off and....it stopped freaking out on me.

 

[Kern Dog]

91 octane is the highest that is widely available here. 93 is not an option.

 

[Don Frelier]

I always shut the GTX off in gear, it tried to run-on after shutdown one time and I never allowed it after that. Agreed it seems like a crutch and wish I didn't feel I needed it. Because of cam specs mainly I have gone richer than stoich for the strongest idle quality, but didn't experiment to see if that helped or hurt hot shutdown behavior.

maybe related somewhat ? My little 16HP gas tractor has manual choke, and has acted up a few times after hot shutdown. On it, to kill the engine when hot, it probably sounds funny but I pull the choke and kill the ignition at the same time. Fuel as coolant/some of the air is cut off/ignition is cut off and....it stopped freaking out on me.

I know you're trying to solve a problem.
But throwing more fuel in as you shut it off is going to wash away your oil and generate wear.

 

[Rich H.]

I know you're trying to solve a problem.
But throwing more fuel in as you shut it off is going to wash away your oil and generate wear.

You know, that has been a concern and I do appreciate you mentioning it.
Will try to come up with a better plan.

 

[Kern Dog]

Here is a thought…
The suggestions related to trying a higher quality gasoline or using an additive: is this somehow related to improve the quality of the fuel itself or to reduce deposits?
I ask because this condition existed before. It has been sporadic over time. It was more prevalent when compression was higher and I had hotter heat range plugs but it has also occurred with lower compression and colder plugs. Currently, there are almost no deposits inside to be a factor yet it diesels about the same now as before I reconditioned the heads.
Does a better fuel somehow react differently when the ignition is shut off?

 

[Rich H.]

You know, that has been a concern and I do appreciate you mentioning it.
Will try to come up with a better plan.

I know you're trying to solve a problem.
But throwing more fuel in as you shut it off is going to wash away your oil and generate wear.

Not to get too far into the weeds (tractor: no pun intended) but the issue was a backfire in the muffler, for which the forums suggest 1. let it idle longer before shutdown (will try that) and 2. verify the function of a fuel shutoff solenoid built into the carburetor.

The solenoid isn't working and is also discontinued (a great combination), so something creative or another plan to deal with it will be needed. Looks like I'll try to let it idle longer.....or I will just do what I do with the snowblower (manually shut off the fuel and run it until it stalls).

Apologies for the highjack.....

 

[Rich H.]

Here is a thought…
The suggestions related to trying a higher quality gasoline or using an additive: is this somehow related to improve the quality of the fuel itself or to reduce deposits?
I ask because this condition existed before. It has been sporadic over time. It was more prevalent when compression was higher and I had hotter heat range plugs but it has also occurred with lower compression and colder plugs. Currently, there are almost no deposits inside to be a factor yet it diesels about the same now as before I reconditioned the heads.
Does a better fuel somehow react differently when the ignition is shut off?

I'd love to know.

Intuitively I'd think if the fuel is so volatile it can fire off of the spark plug threads in the head or something/possibly some kind of hot sharp edge and doesn't require a spark to fire, then it would also fire earlier than desired while running and would detonate. We aren't seeing that, so maybe that's the wrong area to look.

But it could be one of those many things that doesn't follow intuition.

 

[Strato58]

Its to improve the quality of the fuel the detergent additive package varies by brand . Your motor is clean to begin with. The detergent package will keep it that way. Search all the top tier fuel brands and their additive packages. They vary considerably. This is when the additive package is added to the gas "The Top Tier™ additive package is injected at the bulk fuel terminal (the "rack") right as the delivery tanker truck is being filled—not at the refinery, and not inside the tanker while driving."

 

[Kern Dog]

Sorry Strato, that does not clear up the confusion for me.
To repeat:
This condition occurred before I refreshed the heads and pistons and after I reassembled it.
If the deposits in the hangers cased this, the condition would not continue after cleaning.
This is where I’m doubting the assertion that the fuel is the issue. Sure, cleaner gasoline will prevent carbon buildup over time but that isn’t the issue at hand.
It never diesels when cold or even when up to temperature sitting in the shop. It does do it after driving where the whole car is warm.
Now when you have so many variables in play, it may be hard to pinpoint the cause. This is where I look for what exists every time it occurs.

 

[pnora]

Sorry Strato, that does not clear up the confusion for me.
To repeat:
This condition occurred before I refreshed the heads and pistons and after I reassembled it.
If the deposits in the hangers cased this, the condition would not continue after cleaning.
This is where I’m doubting the assertion that the fuel is the issue. Sure, cleaner gasoline will prevent carbon buildup over time but that isn’t the issue at hand.
It never diesels when cold or even when up to temperature sitting in the shop. It does do it after driving where the whole car is warm.
Now when you have so many variables in play, it may be hard to pinpoint the cause. This is where I look for what exists every time it occurs.

Kiss theory applies. Idle solenoid. They put them on for a reason.

 

[Kern Dog]

That is what it may come down to. I’m not sure how easy it will be to fit a Holley based Demon carburetor.
I need to drive the car to get it up to temp where it usually would want to run-on but then back the curb idle screw out all the way to see if it makes the difference.

 

[pnora]

That is what it may come down to. I’m not sure how easy it will be to fit a Holley based Demon carburetor.
I need to drive the car to get it up to temp where it usually would want to run-on but then back the curb idle screw out all the way to see if it makes the difference.

That's what I was going to suggest. Getting it just able to barely idle and see how it shuts down. Easy test.

 

[Strato58]

Kern Dog You previously asked "The suggestions related to trying a higher quality gasoline or using an additive: is this somehow related to improve the quality of the fuel itself or to reduce deposits? I was stating that its to improve fuel quality the added detergent packages are a bonus to keep your engine clean. I understand your motor is currently clean. Maybe a 93 or higher octane will cure the issue but you don't have that option available without an octane boost. I can say I used VP octanium and it did cure my run on issue and I was using 93 to begin with.