Date:         Sun, 20 Sep 1998 19:43:34 -0400
Reply-To:     Martin Jagersand <jag@CS.ROCHESTER.EDU>
Sender:       Vanagon Mailing List <Vanagon@vanagon.com>
From:         Martin Jagersand <jag@CS.ROCHESTER.EDU>
Subject:      Re: HELP: I'm Over heating after turning off for 2 minutes
Comments: To: Vanagon@vanagon.com, vanagon@VOLKSWAGEN.ORG

This may sound counter intuitive, but the minutes after you just
shut off your engine is when the cooling system is getting the
hottest.

When you run the engine at a steady speed the amount of heat generated
will exactly equal the heat dissipated. Heat
energy is flowing from the source, (mostly the cylinders and heads,)
to the "sink" (radiator). When there is a heat flow there is also a
temperature gradient. Say that the rad is getting 80C water from the
engine. Then the temp at the thermostat may be near 90C, and the
head itself maybe 100C (and hotter in some "hot spots").

When the engine is turned off the circuit is broken between the
rad and the engine because no water is pumped. Excess stored heat will
still flow from the engine and heads to the coolant in the engine,
but will not be carried to the rad, thus the coolant in the
engine is now getting hotter, and the pressure higher than before.

In a well designed cooling system this temp and pressure peak
will not cause the coolant to boil. If this happens some possible
reasons are:

1/ Air in system: This will cause the cooling system to be less
efficient when driving, and more heat is stored in the engine when
it is turned off.

2/ (Small) leaks: Will cause air in the system after a while. See 1/

3/ Water only or otherwise incorect antifreeze mix. Water and
antifreeze has a higher boiling point that water alone.

4/ Not enough circulation. Some Audis had an electric coolant
pump that will continue pumping for a while after the engine
is turned off.

Another approach to reducing the problem would be to lower
the engine temp before shut off. A lower thermostat would
do that, but would loewr the temp all the time. Idling the
engine a while after hard driving will even out temps.
Defroster on max heat might also help.

Good luck,
Martin
--
Martin Jagersand                 email: jag@cs.yale.edu
Computer Science Department             jag@cs.rochester.edu
Yale University

        Slow down and visit the VW diesel Westy page:
          WWW: http://www.cs.rochester.edu/u/jag/vw
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