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Old 08-15-2007, 09:03 PM   #1
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dieseling

Here's another question, sorry for so many. My moho after I idle it for a few minutes and shut her down stays running for up to what seems like 30 seconds or more and sputters along and then finally stops. There is a strong smell of fuel afterward. My dad told me that this is called dieseling. What causes this and how do I fix this issue?
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Old 08-15-2007, 09:11 PM   #2
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Quote:
Originally Posted by jagcb750
Here's another question, sorry for so many. My moho after I idle it for a few minutes and shut her down stays running for up to what seems like 30 seconds or more and sputters along and then finally stops. There is a strong smell of fuel afterward. My dad told me that this is called dieseling. What causes this and how do I fix this issue?
Several things can contribute to run-on, or dieseling. It is an effect of carbon deposits in the combustion chamber that remain hot enough to ignite fuel after you turn off the key. Solutions include, in no particular order:
1- lower base idle speed.
2- run higher-octane fuel. Higher octane fuel burns more slowly than low octane, so it will not ignite quickly enough to diesel, or at least not as much. Also, higher- octane fuel contains detergents and additives to clean the carbon out of the combustion chamber.
3-shut down the engine with it in "drive" or "reverse", then shift into "park" after the engine stops.
4-feather the throttle after turning off the key 9last resort, it will flood the engine, and wash the oil off the cylinder walls).
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Old 08-15-2007, 09:17 PM   #3
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Thanks Terry. I believe I read in the manual that you are supposed to use at least 89 Octane fuel in it. So I will fill it up with better grade fuel and maybe get some octane boost to see if that helps. As for the idle speed, I will have to obtain more advice on how to adjust it. Thanks for the help!
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Old 08-15-2007, 09:21 PM   #4
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Along with the above , check the timing , it may be too far advanced.
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Old 08-15-2007, 09:27 PM   #5
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Along with the above , check the timing , it may be too far advanced.
Generally that will not contribute to the issue, as when you switch off the ignition, there is no more timing, and no spark.
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Old 08-15-2007, 09:42 PM   #6
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To adjust the base idle, different from curb idle, you unplug the idle stop solenoid, and adjust the idle screw at the throttle stop until the engine just barely idles with no accessories on. Plug the idle stop solenoid back in, start the engine, and adjust the idle at the solenoid to 650-750 rpm. When you shut off the ignition, the idle stop solenoid will disengage, and allow the engine to stop.
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Old 08-15-2007, 09:50 PM   #7
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Check to see if your throttle plates have an elecrically operated idle stop. It was a long time ago, but I vaguely remeber that this was a common problem at the beginning of the smog control era. One of the "solutions" was to equip the engines with an idle stop on the carbueretor that retracted when the ignition was turned off. This allowed the throttle plates to close completely, "suffocating " the engine to a stop. I would hope that there would have been more sophisticated approaches by 1986, but you never know.

The problem stems from hot spots in the combustion chamber(s) which ignite the fuel air mixture without the spark from the spark plug. The hot spots are sharp projections, usually carbon deposits, but any other thing that can get red hot and act as an ignition source. When the engine does this, it will shake badly as it is usually firing on one cylinder, and early at that. It quits when the hot spot has cooled off.

One approach is to pull the heads and scrape the carbon, a horrible thought. Another is a tank of premium and a 3-4 hour cruise on an interstate, wihch sounds like a lot more fun.
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Old 08-15-2007, 10:18 PM   #8
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I have also experienced a run on condition when the engine temps are high. If I shut down above 210 and the coach has been sitting a while it will run on for a few revolutions (carbon build up from start | idle | shut down engine excerise combined with really hot temps).

Try a good run around the block. Heck, come visit me. That'll blow out all that carbon!!!
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Old 08-15-2007, 10:19 PM   #9
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Quote:
Originally Posted by overlander63
Generally that will not contribute to the issue, as when you switch off the ignition, there is no more timing, and no spark.
As a matter of fact Terry, I had a '70 GTO that started dieseling, sputtering for up to 4 or 5 seconds, after turning off the ignition key. The distributor hold down bolt had worked loose and the timing changed. Resetting the timing and tightening the bolt took care of my problem. I'm not saying that will solve their problem but it might be worth checking the timing after removing the vacuum source to the distributor.
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Old 08-15-2007, 11:40 PM   #10
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"D" all of the above.

Hi, the biggest cause is carbon in the combustion chambers. And a combination of one or more: Carbon, Timing, Idle speed, Vacuum leak, Worn timing chain, Engine temperature, Leaking carburetor, Low octane, and Cheap fuel.

To de-carbonize your engine: I have five methods. The first three are, and can be, risky, but I have done them all with great success. (1.) I call it "the bonsai run" Go to the freeway on ramp and floor the throttle through first and second gear and hold the transmission in second gear at high RPMs [about 1000 RPM less than redline] until the brown smoke behind you quits. (2.) With the engine running at a very high idle, [1500 to 2000 RPM] with air cleaner removed, very slowly pour a 16 oz. Coke bottle full of automatic transmission fluid down carburetor. Run until smoke stops. [or until neighbors call the cops] (3.) With the engine running at a very high idle, [1500 to 2000 RPM] with air cleaner removed, very slowly pour a 16 oz. Coke bottle full of water down carburetor. Run until steam stops. Note: on steps #2 and #3 I really mean pour slowly because too fast will hydrolock and destroy your engine. (4.) Use a de-carbonizing chemical as directed. Tapped into fuel line, spray in carburator, or pour into fuel tank. Note this chemical as it comes out of the exhaust system is very hazardous; Do not breath it!!!!!!!! (5.) Remove the heads to brush and scrape to excessive carbon off of the heads in the valve and combustion chamber area and off of the tops of the pistons.
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Old 08-16-2007, 06:36 AM   #11
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Quote:
Originally Posted by davidz71
As a matter of fact Terry, I had a '70 GTO that started dieseling, sputtering for up to 4 or 5 seconds, after turning off the ignition key. The distributor hold down bolt had worked loose and the timing changed. Resetting the timing and tightening the bolt took care of my problem. I'm not saying that will solve their problem but it might be worth checking the timing after removing the vacuum source to the distributor.
Craig, I've never seen that. I wonder if it was because the timing was advanced, it allowed the idle speed to be higher than it should have been.
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Old 08-16-2007, 07:03 AM   #12
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I remember my high school days working in a gas station and trying to tune up a Pontiac Catalina. I had the timing so goofed up the engine would not shut off, it ran for two hours before the fuel finally emptied out. I then got to see my first valve job. And you ask why I became a chaplain.
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Old 08-16-2007, 07:16 AM   #13
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Chaplain - and from what I hear your engine diesels all the time
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Old 08-16-2007, 09:29 PM   #14
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Quote:
Originally Posted by overlander63
Craig, I've never seen that. I wonder if it was because the timing was advanced, it allowed the idle speed to be higher than it should have been.
I don't know Terry. It seemed funny at the time so I called a friend who was a Navy mechanic. He said to check the timing so I connected my Sun timing light (still use it today), pulled the vac tube to the dist., started the already warm engine and checked the timing. It was off a few degrees but don't remember which way. I used the distributor wrench and found the bolt loose. I adjusted the timing, tightened the bolt down and then plugged the vac advance line back up. I then shut the engine off and it was back to normal.
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