Why does a gasoline engine continue to run dieseling after ignition is turned off?

Answer 1

Answer
Because you have carbon buildup. By federal law only 95% of gasoline has to ignite, or burn. So every time you shut off your engine, when those injectors give one last burst of fuel after the key was turned off, that little 5% in that hot engine ends up turning into carbon build up, which sticks to your internal fuel system components. (top of pistons, fuel chamber, ect.) In time, (Usually a LONG time, MANY miles) that carbon build up actually prevents the piston from compressing the way the manufacturer intended it too, which causes the "dieseling effect". You can have your vehicle serviced at your local dealership, ask them for a "major fuel service". This will clean out all that acumulated carbon and allow your engine to run as it was intended. OR, you can try using the next higher grade of gasoline next time you gas up. But this will only prolong the inevitable. This is all assuming of course that we are dealing with a newer computer controled vehicle that does not require manual timing adjustment. Hope this helps.

(Note 1 )---- The above answer is very vague in its explanation of how carbon prevents the engine from compressing the way it should. Does carbon increase the compression ratio to the point that pre ignition is occurring during run time compounding the problem of carbon build up and causing the engine to diesel after ignition shut off due to the raised compression ratio which then causes the fuel to ignite by compression only ( noting that pressure and temperature are constant factors according to the laws of physics ) . Or is the carbon then just left glowing hot when the last bit of fuel enters the combustion chamber and is compressed as the piston comes to top dead center and then ignites of the red hot carbon after shut off. Also the only way to remove carbon from an engine in the short term is to remove the head and forcibly scrape it out. It is a waist of money otherwise to go to a dealer. In order to run the engine hot enough to remove the carbon would damage the engine possibly Even fatally

(Note 2 ) "Dieseling" is caused by incomplete occlusion of the air/fuel inlet. Admission of fuel after shutdown is corrected by properly adjusting linkage.

( Note 3 ) - - - Dieseling is caused by more than incomplete occlusion ( NICE WORD ) -- (ie) A leaking or worn out carburetor in which case adjusting the linkage wont do anything. Further it probably requires some ignition source other than compression only and since you just shut of the ignition what is the source?

( note 4 ) There's no difference between the problem of incomplete occlusion or a leaky carb. As for the ignition source, YOU should know it's the high cylinder temperature. When an engine is shut down after reaching normal operating temperature, and there is sufficent fuel/air mixture, "dieseling" occurs.

( note 5) The original answer to this question was inadequate , incomplete in nature and suggestions. My first response was to bring it to the attention of the original author You are partially correct in you answers but there are more factors involved. There is a difference in a leaky carb and adjusting the linkage to correct a problem that has more than one solution or mechanical problem. - - OK - I just got of the phone with my kid who is a Ford mechanic. ( ta da )- If the temperature of the engine was hot enough to cause dieseling with out a carbon build up then why does an internal combustion engine immediately begin to miss when you pull a plug wire and why stop there. Just pull the coil wire and see if the engine keeps running. ( YOU ) should know that it will not run after pulling the coil wire because the engine is not hot enough and the compression isn't great enough to keep it running. Also please be courteous enough to delineate your note from the previous paragraph so the reader can know when one train of thought ends and another begins. Thank you and Its been fun. D