Causes of engine dieseling or run on.....

[Moparfiend]

I have to kill my car (automatic) in gear lest I get dieseling. Wondering what might be causing this. I know its prevalent in high perf motors and I assume its due to high compression. If it is the high compression what is the mechanism that causes it? Is it just temperature and pressure?

Why does killing it in gear solve the problem? Is it just the load on the inertial force?

 

[khryslerkid]

Yes to your last question.

Idle too high

Excessive gas in the carb. (Float level too high)

Choke not completely open

High carbon build-up in the cylinders.

Sometimes the timing setting

Vacuum leak

 

[68Moparmaniac]

I have to kill my car (automatic) in gear lest I get dieseling. Wondering what might be causing this. I know its prevalent in high perf motors and I assume its due to high compression. If it is the high compression what is the mechanism that causes it? Is it just temperature and pressure?

Why does killing it in gear solve the problem? Is it just the load on the inertial force?

It can be caused by hot carbon deposits in the combustion chamber, pull your plugs and take a look and see if there's some build up.

 

[69a100]

Either one or all of the above will cause your problem. Good Luck

 

[turbine68rt]

From what I understand, contributing factors can be excessively high idle speed, carbon deposits that remain hot enough to ignite after spark is gone, deposits on plugs/too high of a heat range. I used to get dieseling in a 66 Coronet with a modified 383 and for some reason it did it much more frequently when using regular gas vs. premium.

 

[bm02tj]

https://www.summitracing.com/search...OI5z8CKjYRI5F-Nm9oNoloWl9YODFv8waAmC8EALw_wcB

Adjust your idle so it is ready to die then add one of these to adjust idle to what you want
This will stop the run on

 

[Ernie ingling]

Timing
Engine temp up
Low octane gas
Had a dodge van dieseled bad had to put bumper against a tree leave in gear shut off. Took to dealer parked across all lanes I could put it on park shut off took key walked to service mgr. gave him key van still dieseling still. He never asked why i was there. Left in a cab week later called said it was ready picked up drove home and yes had to use tree to stop.
Hope you’re lucker than me
76 van 318 Sold nine yrs later and still dieseled.

 

[khryslerkid]

 

[Don Frelier]

If I remember correctly, this is the high compression engine right like 12:1?

Fuel injection would shut it off.
I know I'm a bad person...

 

[Ranger16]

I had the same problem when I first bought my Road Runner. Had to start to let the clutch out in gear after turning the key off or it would run on forever. I plugged a vacuum line into one of the vacuum ports at the base of the carb and stuck the other end into a glass of water and ran the engine at about 2000 RPM until the glass was empty (doesn't take very long). That cured the problem immediately and it has not dieseled since. Obviously this water injection procedure is for carbon build up in the cylinder. Works like a charm if that is your problem.

P.S.
Be sure the engine is at operating temperature when you do this.

 

[69Bee]

https://www.summitracing.com/search...OI5z8CKjYRI5F-Nm9oNoloWl9YODFv8waAmC8EALw_wcB

Adjust your idle so it is ready to die then add one of these to adjust idle to what you want
This will stop the run on

This is what I did on one of mine. Used the solenoid to set the idle (idle stop screw was removed), then when you remove power to the solenoid, the butterflies close complete and won't allow it to diesel...

 

[Mike67]

Do you have a MSD box, they can contribute to run on even after the key is off.
You can check Holleys site to help t-shoot of you do have one.

 

[warren King]

I have to kill my car (automatic) in gear lest I get dieseling. Wondering what might be causing this. I know its prevalent in high perf motors and I assume its due to high compression. If it is the high compression what is the mechanism that causes it? Is it just temperature and pressure?

Why does killing it in gear solve the problem? Is it just the load on the inertial force?

My first thought was timing and or carburator tune/mixture? To retarded/late timing or to advanced/early time could cause it. To lean of a carb. mixture could also be a factor. Timing thats to retarded and/or to "lean" of a mixture will cause excessive cylinder temps, making flame propogation and "ignition" happen to early (like before spark) to erraticly and "undeveloped"/uncontrolled thru the burn process. Resulting in detonation and pre-ignition, i.e. dieseling. To much advance timing kind of the same thing, especially under load and acceleration. Without knowing much about your engine I read in one of the replies 12:1 compression? Possibly type of fuel, like to low of octane could be a factor? 12:1 might be getting a little much for straight/ regular pump gas? Even the premium? Vacuum leak could also cause a lean mixture condition, raising cylinder temperature leading to the same. Ehh, my 2 cents...

 

[CudaChick1968]

I'd be more inclined to blame our gas, especially if it's ethanol. Low octane and high compression don't play well together. Mix in some 116.

 

[Moparfiend]

Thanks everyone gives me some good ideas. Yes the engine is running 12.5 TRW pistons so it’s possible the actual CR is 12:1 but I am searching the Phoenix area for someone who has a Whistler that would be able to confirm. Also my idle is set high 1200 RPM due to the aggressive cam which I now know is a solid lifter set up. And yes the gas in the Phoenix area sucks but I do have easy access to 100 and 110 octane fuel that I will play with. My timing at idle is around 17 degrees. A perfect storm.......

 

[Moparfiend]

And I have an MSD 6AL set up.....

 

[Moparfiend]

Oh and I can rule out carbon as I looked the tops of the pistons are clean so I will assume the combustion chamber is also. I know its an assumption....

 

[Moparfiend]

Other than Ether whats a good spray for looking for vacuum leaks?

 

[Don Frelier]

A propane torch, but at 1200 RPM speed changes might not be as noticeable.
1,200 RPM? Not really a street car.
This is part of your problem I think (besides the way high compression)
Can't you run it at 750 - 800?

 

[Moparfiend]

A propane torch, but at 1200 RPM speed changes might not be as noticeable.
1,200 RPM? Not really a street car.
This is part of your problem I think (besides the way high compression)
Can't you run it at 750 - 800?

Yes but when I dump it in gear it drops 400-600 RPM....but it will idle well at 900 if left to its own in park.