Dieseling

[Jolly Green]

Hey guys--been a while since I've posted.

My truck has been dieseling when I shut it down for a few days now. It was real bad for a while--then I backed the timing off and it seemed to help a lot. It doesn't do every time now, and when it does do it's not as bad BUT it still diesels when I shut it off once in a while.

Is there anything else that could cause it besides timing? Should I back the timing off more?

BTW: 351M -Stock Motorcraft carb, stock dizzy--everything stock except a mild cam.

Thanks!

 

[blacksnapon]

Too fast on the rpms at shutdown will do it, thermostat can contribute as well.

 

[Mil1ion]

You could always try a water mister shot into the carb air horn.

Run-on or Dieseling

Ideally, a car's engine should stop running as soon as the driver switches off the ignition switch. There should be no more raw fuel entering the chambers and the spark plugs should no longer ignite it. But things don't always work this smoothly in an internal combustion engine, especially when other sources of ignition and raw fuel still exist. Whenever a car's engine continues to run after the ignition has been shut off, mechanics refer to it as dieseling.

Dieseling occurs whenever fuel is ignited by another heat source other than a spark plug. It could last a few seconds or few minutes, depending on the amount of fuel remaining in the combustion chamber and how long it takes for the ignition source to cool down.

Dieseling refers to the process used in diesel engines to ignite the fuel and keep the motor running. Instead of a series of explosions generated by timed spark plugs, a diesel engine uses a single "glow plug" to burn the fuel. Unlike traditional internal combustion engines, a diesel engine may have to idle in order to keep the ignition process running properly. In a traditional gas-powered engine, however, such a practice of keeping the engine running is not strictly necessary. If a gas-powered engine begins dieseling, there is usually something wrong somewhere.

One possible cause of dieseling is the use of cheaper grade fuel and oil. Whenever an engine burns lower quality fuel, carbon deposits often develop on the valves and pistons moving rapidly within the combustion chambers. If a carbon deposit causes the piston to rub against metal, a hot spot could easily develop. When the ignition is switched off, the spark plugs no longer deliver a spark hot enough to ignite the remaining fuel in the combustion chamber. A red-hot carbon deposit, however, could still set off an explosion if it comes in contact with unburned fuel. This ignition of raw fuel is often the reason a car starts dieseling. It will only stop when the remaining fuel is gone or the carbon deposit has cooled down.

Another possible reason for dieseling could be a faulty link between the carburetor and the engine block. Many older cars have a fuel choking system which is supposed to shut off all delivery of gas to the engine. Sometimes this choking system fails to work properly, allowing gas to flow into the combustion chamber. If that gasoline comes in contact with a hot piece of metal or carbon, it will explode and cause dieseling. More modern engines have a fuel injection system designed to stop introducing fuel into the combustion chamber after the ignition switch is turned off, so dieseling is not as common but still possible if fuel remains in the chamber.

One of the best ways to reduce the chances of dieseling is to keep your car properly supplied with oil and engine coolant. When car engines overheat, hot spots can develop and cause dieseling. You may have to have your pistons and valves examined for the development of sharp edges which could become overheated. Switching to a higher grade fuel or using carbon-removing additives could also help prevent future dieseling.

Some experts suggest that dieseling could be triggered by inferior or improper spark plugs, so you may also ask a mechanic to make sure you have the proper size spark plugs for your car's engine. Many dieseling problems can be addressed during a regular engine tune-up, so tell your mechanic about any irregularities you may have noticed after shutting off the ignition.

 

[Jolly Green]

Thanks for the replies, guys. Great write-up Mil1ion!

I thought about the carbon that this article suggests, but I still have less than 40k on my rebuilt motor. Could there still be carbon build up with so few miles?

I just had my carb rebuilt last year by a mechanic with a real good reputation for his carb work. I know that doesn't mean there couldn't be a problem--but, maybe not as likely.

Just changed the plugs--the proper ones--less than 3,000 miles ago.

I have been running the cheap stuff (87 octane) in my truck, so maybe I should premium in it. What do you guys run?

The water in the air horn--that's to break up carbon, right?

I forgot to mention that sometimes when it does this--I'll turn the key back on, then turn it off again and it quits.
Thanks.

 

[73F100Shortbed]

Sorry to bring up an old thread, but Steve did you ever find a solution? I'm having the same problem as you.

 

[Jolly Green]

Sorry to bring up an old thread, but Steve did you ever find a solution? I'm having the same problem as you.

Yep, I did.

The final fix, as far as I can tell, came with running premium fuel.

Adjusting the timing helped--but did not fully eliminate the problem.

With a tank of 93 octane the problem stopped. The next time I filled up, I got 87 and the run-on came back. Switched back to 93 and it stopped again and has not done it since. If you run the cheep stuff now, I would suggest switching.

I think I got the idea to try this from the write-up Dennis posted in this thread.

 

[blackhat620]

Yep, I did.

The final fix, as far as I can tell, came with running premium fuel.

Adjusting the timing helped--but did not fully eliminate the problem.

With a tank of 93 octane the problem stopped. The next time I filled up, I got 87 and the run-on came back. Switched back to 93 and it stopped again and has not done it since. If you run the cheep stuff now, I would suggest switching.

I think I got the idea to try this from the write-up Dennis posted in this thread.

If higher octane fuel stopped the dieseling then, the underlying cause is either timing, carbon deposits, incorrect spark plugs, idle rpm or a combination of these. Fix the underlying cause and you will not need higher octane fuel.
The higher the octane rating of fuel the more difficult it is for the fuel to ignite. Higher octane fuel is needed for high performance engines do to higher compression ratios and more advanced timing settings. Your truck was designed to run on standard grade fuel (ie low octane), switching to a higher octane fuel is only a bandaid to mask the underlying problem. As soon as the underlying cause of the problem gets worse your dieseling will re-occur. Lower octane fuel is Not an inferior product compared to higher octane fuel, low octane fuel just ignites at a lower pressure & temperature than higher octane fuels.

 

[73F100Shortbed]

Thanks for the response. I'm going to lower my idle a few hundred rpms since it is a little high and try 89 octane gas.

 

[blackhat620]

Thanks for the response. I'm going to lower my idle a few hundred rpms since it is a little high and try 89 octane gas.

Just a tip, make sure your truck is at normal operating temperature before setting the idle speed and then check your timing after resetting the idle speed.

 

[Jolly Green]

blackhat:

Thanks for the info. I have an RV cam, which I figured might have changed things enough to warrant a different octane, since I checked everything else I could think of. What are your thoughts on that?

 

[LEB Ben]

blackhat:

Thanks for the info. I have an RV cam, which I figured might have changed things enough to warrant a different octane, since I checked everything else I could think of. What are your thoughts on that?

A new cam can/will alter stock timing specs, and can/will alter octane useage if there's no head work...so you were probably well within your means.

 

[blackhat620]

blackhat:

Thanks for the info. I have an RV cam, which I figured might have changed things enough to warrant a different octane, since I checked everything else I could think of. What are your thoughts on that?

Most RV cams are fairly mild and generally should not effect it. That being said was the cam properly timed when it was installed and what shape is the timing chain in. An improperly timed cam or sloppy timing chain can cause dieseling and other problems.

 

[Jolly Green]

Most RV cams are fairly mild and generally should not effect it. That being said was the cam properly timed when it was installed and what shape is the timing chain in. An improperly timed cam or sloppy timing chain can cause dieseling and other problems.

It's all sound. 40k on a complete rebuild...so I hope I'm not developing something like that yet, but anything is possible.

 

[Jolly Green]

A new cam can/will alter stock timing specs, and can/will alter octane useage if there's no head work...so you were probably well within your means.

Thanks for that insight, Ben. That's what thought--but it's always good to look at all possibilities.

 

[blackhat620]

It's all sound. 40k on a complete rebuild...so I hope I'm not developing something like that yet, but anything is possible.

Well if you have 40k on your rebuild and you installed the cam at rebuild but are just now getting dieseling, then the cam would only cause it if the cam timing has changed. I doubt this has happened in 40k miles (though anything is possible). More than likely your dieseling is being caused by one or more of the other problems i mentioned earlier in post # 7 (ie ignition timing, carbon deposits, incorrect/bad spark plugs, idle rpm or a combination of these).