Q&A: Engine won’t turn off immediately after turning the key?

by Marco Mutzke

Question by Sprinter: Engine won’t turn off immediately after turning the key?
Hi, its a Daihatsu Charade 1.0 petrol, 1985, 220k kms.
The car starts first time every time and runs fine, engine seems to be in good condition as well (doesn’t smoke or burn oil).
But lately I have noticed that when I turn the key to off position, the engine take a while to turn off, around 5 seconds after I have removed the key.
What could possibly be wrong?
Thanks for your answers.

Best answer:

Answer by Mr. KnowItAll
Run-on by dieseling is caused by fuel in engine igniting without spark.

Possible explanations:
a.) Running winter fuel in warm weather.
Winter formulated fuels have a higher vapor pressure, meaning that they evaporate easier. Only vaporized fuel will burn, not the liquid stuff. The cure is to run the fuel tank nearly empty and fill up fresh. The oil companies change the fuel formulation seasonally, and sometimes even monthly.

b.) Using gasoline with alcohol content.
Gasahol gasoline contains about 10% alcohol, usually methanol alcohol. The alcohol vaporizes at a much lower temperature than the rest of the fuel. The result is alcohol vapor in the combustion chamber, and the engine runs on easily. This stuff can also cause vapor lock in a carbureted engine on a hot day, or when you shut off a hot engine and then try to start it five minutes later, after heat soaking the carbs under the hood. This is a nasty problem with my MGA, with only a feed line to the carbs — no return line. A carb with a fuel return line to the fuel tank will help ease this problem. Keeping the fuel circulating back to the tank can keep the fuel and the carb cool. Also carbs located on the same side of the engine with the exhaust manifold are especially susceptible to this problem. My solution is to stay completely away from the alcohol content fuels.

c.) Using low octane gasoline in an engine that should have high octane fuel.
High octane fuel is more resistant to both evaporation and pre-ignition. Check your owners manual (if you still have one) for the recommended octane requirement. For good measure, add one or two points to the recommended number. The method of rating fuel for octane level changed some years ago, and the numbers are now a couple of points higher to achieve the same results.

e.) Hot spots in the combustion chamber.
Any carbureted engine having neither of the above mentioned devices will continue to draw in fuel/air after shut-down. Any hot spot inside the combustion chamber can ignite the fuel causing run-on. Right off hand I can think of at least three sources of hot spots.

1.) Spark plugs of the wrong temperature range can run red hot at the tip. This can also lead to premature failure of the ceramic insulator near the tip of the plug. Check the service books for the correct spark plug number.

2.) Carbon build-up in the combustion chamber. The carbon deposits have a rough and irregular surface with many small cracks and edges exposed at the surface. Carbon deposits are also a poor heat conductor. The combined effect is extremely hot bits of carbon which can ignite the fuel after shut-down. For especially heavy deposits, the only solution is removal of the cylinder head and physically scraping away the deposits.

In milder cases, I have known of limited success with the use of “mouse oil”. This is a generic term applied to anything liquid that costs two cents to bottle and sells for more than a dollar. (STP Gas Treatment and Marvel Mystery Oil may fall into this category). The primary ingredients are often nothing more than kerosene, lacquer thinner, and the like. It is often called “upper cylinder cleaner”. Instructions for use may call for pouring the stuff down the carburetor at fast idle with a hot engine, pouring in the entire container full at a slow trickle over a period of five minutes or so. This usually causes a lot of smoking from the rich mixture, and sometimes pinging from pre-ignition. While the pinging is OK for this short procedure, don’t pour the stuff in so fast as to cause detonation (severely loud pinging), as this can damage the engine.

You may also add this stuff to your gas tank. The results here are not so obvious and definitely not immediate, but in long haul it MIGHT clean you engine inside (no guarantee) or may be intended to help keep it clean. Personally I think this type of application is more effective a cleaning the parts in contact with liquid fuel, such as fuel jets in a carburetor and fuel injector nozzles. Many of these compounds are already included in certain brands of detergent gasoline, so you may be wasting your money here by duplicating the additive.

3.) Sharp edges on metal parts in the combustion chamber. These sharp corners can get red hot while the engine is running because of poor heat dissipation. The hot edges can ignite the fuel after shut-down. These sharp edges also have a few sources.

On an older engine, when valves are severely burned they can develop cracks or sharp edges. If an engine shop does a “cost sensitive” valve job, they may reinstall valves that have been ground too far, exposing sharp edges on the outside of the valve heads. These sharp edges on valves are a definite no-no! Do not attempt this false economy. Replace any valve with a thin edge.

When doing a valve job, it is common to resurface the head (milling or surface grinding) to assure a flat surface for the head gasket to

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