The Stovebolt Site Forums The Shop Area Engine Shop Dieseling?

My engine, once it's warm and has run awhile, will diesel after I shut it off. What causes this and how do I fix it?

And for all you comedians out there, we're talking a gas burner, not the Dodgebolt (which, having a Cummins, is SUPPOSED to diesel...) :p

Thanks,
John

The first thing most commonly checked will be the timing. I have found over the years that setting it to "spec" doesn't always work due to wear. I usually play around and try advance or retard until I found what works best and still not hesitate or balk out on the road. I'm sure others will suggest bad fuel, incorrectly adjusted valves, actual engine temp, even the type of plugs and the list goes on. Usually the culprit will be in the timing! I know we have a scholar or two here which will tell us all about pre-ignition so lets have it folks

The only time that this has hapened to me, it was a vacuum leak at the carb.

Try richening up the curb idle a little so the combustion doesn't run so hot, and try a step colder spark plug.

In my experience, the two most common causes are high idle speed and high water temp.

I would first lower the idle to the recommended speed or even 50rpm lower.

It,s usually caused by fast idle speed &/or
Vacuum leak. Be shure an auto matic choke is
opening all the way 0r a fast idle cam sticking.

You didn't mention WHICH engine you're working on, but almost all dieseling is a function of too-high idle speed. One sneaky trick the manufacturers tried in the mid-70's when they raised idle speeds to lower emissions was to put a solenoid-type throttle stop on the carburetor linkage. "Curb idle" was set with the solenoid turned on, which was attached to ignition switch voltage, and "Slow idle" was set with the regular throttle stop screw, with the solenoid disconnected. They were about 100 RPM different. When the ignition was shut off, the throttle position dropped to the lower speed setting, effectively strangling the engine. You can also shut the engine off with the auto trans in gear, or leave a standard trans vehicle in gear, and let the clutch out as you kill the ignition. Either one will create enough load on the engine to retard the tendency to diesel.
Jerry

I concur with the high idle being the culpret. And as HotRod has stated you can play with your idle settings if you prefer a higher idle. My old 79 Monte had an A/C idle up solenoid that mounted to the carb and "bumped" the idle when the air was on. You could adapt one of these to set your base idle and when the key goes off it would lower the idle too low to diesel on. You probably need a 12 volt system though,
Scott

Lowering the idle speed is sometimes effective in masking the problem, but idle speed is not the cause. Something hot in the combustion chamber is causing the mixture to ignite in the absence of spark. Look for a lean mixture (vacuum leak or carb settings), marginal cooling system, carbon deposits, or too hot a plug. We run old 6 cylinder gas gensets that shut off very politely from 1800 RPM by simply killing the ignition.

pre-ignition or over run.Is this ride driven everyday??.Sounds like carbon build up on the heads or pistons from a sunday driver.Take her out on the highway for a real good runnnnn.Pedal to the metal.If you don't want to run her hard.This is from the old school.Remove the air cleaner with the engine running,reving.pour about a quart of water slowly into the carb, reving the carb.Don't let it stall.This will remove the carbon from the heads.If you look at the tailpipe you'll see pieces of carbon on the ground.Check timming,adjust carb mixture and chock ,idle.Just remember tap water is cheep

JEEZO-PEEZO !

Don't _EVER_ ' pour ' water into an intake as it doesn't compress and can easily crack pistons , bend rods and crack heads too ~ use a trigger sprayer and carefully mist the water in as it only takes a tiny bit to blast the built up carbon off the combustion chamber .

You must have the engine fully warmed up and revved as fast as you can stand it whilst doing this , be aware black hot carbon bits _will_ come flying out the exhaust so don't be backed up to the white garage door when you do this .

when i have had this problem I usually take off the vacum hose from the distributor and put it in a bottle of water while the engine is running at about 2000 rpm. Using the vacum hose gives you control of the amount of water put in the engine so you dont damage anything. I have tried this on a lot of engines and never had any problems.