Engine Runs After Shut Off

[Richard Madison]

What causes an engine to continue to fire for a few seconds after the key is turned off?

Sounded like a crackling, backfire sound...and once, a puff of burned oil came out of the air cleaner.  A peek into the exhaust manifold showed some oil streaks getting past the closed butterfly.

Richard M, NYC

[ja17]

Hello Richard,

Often times carbon build up in cylinders will cause this kind of "dieseling". A good "Italian Tune Up" (high rpm run) will help burn out the carbon.

Make sure the car is in good tune, high compression, higher engine temperatures, improper timing settings and low octane fuel will also incourage dieseling.

I used to take care of a 18,000 mi 190sl owned by a little old lady. the car never saw over 2,500 rpm. The car would "diesel" terribly when she brought it to me every couple of years. It would diesel even after standard tune up items were taken care of. A few spirited laps around the block always sent the car home happy with no dieseling.

[hauser]

I have seen this happen when low octane fuel is used.

[graphic66]

At every oil change I decarbonize the engine with aerosol decarbonizer. You get it at marine supply stores. OMC engine tuner is one, Yamaha combustion chamber cleaner is another. Most outboard motor manufacturers sell the stuff. It is needed because the dispersants  that were added to fuel years ago were eliminated. At that time the TCW-3 outboard 2 stroke oil with dispersants was introduced to help stop the carbon buildup and a fuel additive is also made along with the aerosols. The 2 strokes built up carbon on the top rings and caused "ring Jacking", the rings hit the carbon and the carbon hit the head and you broke a ring. I blew up a 200hp Johnson right at the time all this went down. I now run the additive to fuel, the TCW-3 oil and clean my 300HP Johnson V-8 with the aerosol, no problems, except once with oxygenated fuel. I also use the aerosol in all my engines at oil changes. Follow the directions and let it set in the engine overnight if you can. The stuff melts carbon right off. It is great at cleaning heads and other carbon encrusted parts also. It strips the varnish buildup in carbs when you are cleaning them great also. I always keep it around.