[Vwdiesel] pressure was] '81 Rabbit overheating

[Val Christian]

> pressure something is wrong. Also the cooling system on a Rabbit has not
> only a thermostat but also a heat activated thermo-switch in the radiator to
> turn on the fan. If you change to a different temperature thermostat you
> should also change the fan switch in the radiator. They come paired. If you
> change to a hot thermostat and keep a lower Thermo-switch in the radiator
> you will probably have excessive cycling of the fan. The opposite is to put
> in a colder thermostat and not change to the colder thermo-switch in the
> radiator and the fan may not cycle till it's too late.

	This is not necessary, nor have I done it.  In general, a higher
	temp thermostat works fine on a VW diesel.    Peak heat waste is
	at highway power settings, in the upper half of waste heat
	generation profile of the engine.  At highway speeds, the radiator
	has sufficient airflow to not need the fan.

	Most users do not run 30+ hp out of their engines without
	substantial airflow over the radiator.

	[Side note: I've seen quite a few industrial installations of the
	VW diesel, and several of them had horizontal radiators, relying
	on convective upflow through the radiator, augmented by a thermo-
	statically activated fan, only when needed.  As I recall, after 
	about 35% steady state power, the fan started cycling.  Lower
	than that they were not necessary.  The installations I'm thinking
	of were pumping applications at amusement parks.]

	Exceptions are extreme heat, and running an airconditioner,
	and possibly towing (should anyone do this).

> 	Also it is good to check the pressure cap. They are quite prone to
> failure after 20+ years

	The first cooling system failure I had was on a 1978 VW Rabbit.
	The car was living through it's first winter, and it was 
	a learning experience.  Fuel suppliers had wax problems
	(cloud point) and water problems (leading to waxing).

	That winter we had a couple of weeks of cold, with temps hitting
	-20F many mornings.  Bottom line is that the overflow bottle 
	never got warm enough, and condensate on the pressure cap
	froze over.  This lead to the potential of a burst cooling 
	system.  VW quickly modified the design to have flow through
	the overflow bottle, to heat that area better.