Dieseling

[Bohdan]

Got my new engine all fired up and runs great. Just diesels after the key is off. Some say it is air idle mixture. On a 40 IDF if the screw is all the way in, is it allowing air and only gas or is it the other way around. Mighty obliged.

[Semper_Dad]

I always though that diesling was a timing issue
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Walküre Restoration Thread: https://www.thesamba.com/vw/forum/viewtopic.php?t=737492

[tallman206]

Dieseling cannot be a timing issue, because the engine continues to run AFTER the igntion system is turned off!

Dieseling can be caused by very hot cylinder head temperatures, high compression, along with an adequate supply of fuel/air mixture, perhaps with a very low octane rating. The fuel/air mixture has to be heated to the point that it self-ignites, causing the diesel effect.

Look for very high head temperatures, heavy carbon buildup in the heads, unusually high compression (by improper engine rebuilding or by heavy carbon buildup). Having high compression will cause the fuel/air mixture to compress greater, causing a higher fuel/air temperature, causing self-ignition by compression (der Diesel compression ignition). Lower octane fuel will aggravate the problem, because it will self-ignite quicker than a higher octane fuel.

You are not running a stock carburetor, which may also be exacerbating the problem. The electromagnetic idle air cutoff on the side of the Solex carb is meant to also keep this from happening by cutting off the idle air circuit when the ignition switch is turned off.

Also, there are rare cases where there may be a leak-current power supply to the ignition coil when the key is turned off (caused by improper wiring of the generator/alternator back to the ignition circuit. This can cuase a small amount of voltage from the generator/alternator to feed back to the coil, powering it for a short time it when the ignition is turned off and before the engine stops turning. Never heard of it on a VW, but alternator equipped US cars have had this phenomenon.

I would venture to say that it is probably very rare for a VW engine to diesel, so I would think something is dangerously wrong with the engine, its setup, or perhaps the wiring (least likely).

tallman206

[Bohdan]

Thanks. I will have to I guess look at my fuel mixtures. Are you saying perhaps my mixtures are not right? Too lean or too rich? Do you know if the mixture screw on the carbureator if it is running lean is the screw all the way in? or out? Mighty obliged.

[mikeyj007]

My Thing will diesel when I tweak the distributor cap too far.

[tallman206]

Sorry Mikey, but what you are describing is not dieseling.

Dieseling or diesel engines will run by themselves in the absence of an outside ignition source, such as the spark plugs and Kettering Ignition System. They run by 'compression ignition', which is accomplished by causing the fuel/air mixture to burn due to the heat generated by the compression process alone, or with the assistance of somthing very hot within the cylinder (carbon, wrong plugs, etc.).

What you appear to be describing is knocking, presumably caused by major advancing of the ignition timing, to the point where the ignition point is well before TDC, causing much stress on the engine as it attempts to fire the cylinders well before they reach TDC.

Dieseling is great for a diesel engine, because the fuel is injected into the cylinder at the precise moment/angle it is required to produce efficient power. Dieselling in a gasoline engine can be very destructive, because the timing point is not controlled, it just happens whenever the fuel/air mixture gets hot enough to self ignite.

[Bruce]

[pointhwerp]

every once in a while after a long drive mine will kick over 1 or 2 cyls after i key off, i know its not dieseling but its not normal, would running too rich cause this?

[Bruce]

More likely running too hot is what's causing it. If you were just running it hard on the freeway then exited and quickly shut it off, there's the cause. But if you were able to drive it for a couple of miles at lower speed, it will have a chance to cool off.
How is your cooling system? Missing bits? Gaps? Even a longer than stock belt will cause it to run hot.
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[Wildthings]

Dieseling can be caused by many factors, timing and heat are certainly ones. Timing that is too far advance or retarded can cause extra heat and cause dieseling. Mixture can as well, in particular an idle mixture close to stocheometric than is apt to not burn smoothly enough. Any thing that causes the combustion chamber to build up deposits can cause Dieseling. One big cause of Dieseling is too much air at idle, sometimes due to design as in the case of a VW engine with a DVDA distributor or sometimes because the engine is ratty and needs more air than it should to idle. Automatics can be worse than manual transmission engines since the base idle is often a bit higher for automatics.

With a manual the solutions is just to let the clutch out a bit before killing the ignition, with an auto leaving it in drive may (or may not) help to lessen Dieseling.

[Bruce]

[Wildthings]

[Bohdan]

Changed the jets one size down. No more dieseling. Hmmmm...

[icekoffee]