Re[2]: running on

[Toby Erkson <Toby_Erkson@ccm.intel.com>]

Running-on, or "dieseling", is caused by leftover fuel igniting by itself in the
combustion chamber after the ignition system has been turned off.  With a 
carburetor some fuel can still be sucked from it and drawn into the combustion 
chamber since a carb supplies fuel on piston demand.  When you turn off the 
ignition system all you are doing is turning off the spark -- if the engine is 
turning then the [mechanical] fuel pump is still pumping fuel to the carb and 
the carb will still flow fuel until the engine stops producing power.  A fuel 
injected engine normally doesn't have this problem because all of the power to 
the system is turned off -- fuel pump stops pumping; injectors stop pulsing; 
ignition has no power, and; computer has no electricity.

If an engine has carbon deposits in the combustion chamber, particularly 
deposits that have an edge or sharp point, these can get very hot and ignite the
entering fuel.  A very hot spark plug can do this as well.  These nasties also 
contribute to pre-ignition (the fuel mixture igniting before it's supposed to).

Stopping the engine by putting it in gear is NOT a good idea as this causes the 
engine to stop SUDDENLY and not gradually like it normally should.

How do you keep an engine from dieseling?  Use a good gas!  Most of the higher 
octanes by the major manufacturers contain cleaners that help keep the tank, 
lines, carb jets/fuel injectors, valves and chamber from getting dirty.  And a 
higher octane helps reduce detonation/pinging.  If you insist on using a cheap 
gas then at least every once and a while fill up with a tank of good gas 
(inexpensive?, yes, cheap?, no!).  Or you can add a fuel cleaner.  This is 
available at all auto part stores (read the directions!) and I would venture to 
say use it once a month if cheap gas is the cars main meal.

You may want to pull the spark plugs and inspect them or have an experienced 
air-cooled VW mechanic do this.  When done make sure they're properly gapped and
cleaned.

    Toby Erkson
    air_cooled_nut@pobox.com  <-- Please use this address for email responses
    '72 VW Squareback 1.6L bored and stroked to 2.0L
    '75 Porsche 914 1.8L, ORPCA member
    Portland, Oregon, http://www.geocities.com/MotorCity/8501/



______________________________ Reply Separator _________________________________
Subject: Re: running on
Author:  type-3-errors@umich.edu at SMTPGATE_MIME
Date:    2/2/98 9:15 AM

MrRebel666,
     If you have carbs, this recommendation comes from  John Muir's
book "How to Keep Your Volkswagen Alive". Under the topic 'Electromagnetic 
Cut-Off Jet' Mr. Muir writes,  30 PICT carburetor & all carbs since: This is 
another one of the engineering dreams gone wrong. They changed the carburetor to
provide a little more fuel at the jets & found that with the high temperature  
in the engine, the ---- thing would run without ignition until the gas in the 
jet was run out_ after the ignition was shut off. This was embarrassing, but 
instead of going back ...