[Vwdiesel] pressure was] '81 Rabbit overheating

B & R Decker bdecker001 at centurytel.net
Thu Dec 20 14:51:01 PST 2007

Hi Stephen	
	I hate to write this because I don't want anyone to get burned. BE
CAREFULL. Let your engine idle till it's warm enough that the fan cycles (
on a diesel in the winter time that can be near impossible unless you make a
hard run and then stop the rig and let it idle. Just after the fan cycles if
your Rabbit cooling system is working properly there will be NO pressure in
the system when you remove the cap on the recovery bottle. If there is
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.
	Also it is good to check the pressure cap. They are quite prone to
failure after 20+ years
Brian Decker

-----Original Message-----
From: vwdiesel-bounces at vwfans.com [mailto:vwdiesel-bounces at vwfans.com] On
Behalf Of Stephen Kraus
Sent: Thursday, December 20, 2007 1:28 PM
To: r.c.brown at ieee.org
Cc: vwdiesel at vwfans.com
Subject: Re: [Vwdiesel] '81 Rabbit overheating

What if you have slight pressure in the system?

On Dec 20, 2007 4:23 PM, Roger Brown <r.c.brown at ieee.org> wrote:

> Rolf Pechukas wrote:
> >> I thought I would wait to see if someone else would comment, but
> >> water boils at 212 F (100 C) at sea level. It boils at a lower
> >> temperature as altitude increases & air pressure decreases. As
> >> pressure increases, the boiling temperature increases. This is why
> >> modern cars use pressure radiator caps to increase normal operating
> >> temperature. This is why modern cars often use 14 pound pressure
> >> caps. Then the operating temperature can run much higher. This is
> >> why some temperature gages go up to 265 degrees F. There is a
> >> formula to figure how much the boiling point rises or lowers with a
> >> change in pressure, but I forget what it is.
> >
> > yeah, but my system for whatever reason is not pressurizing
> > hence my concern about approaching 200+°f
> >
> > Rolf in MA
> No pressure in the system is a good thing, as far a negative sign of a
> leaking head
> gasket.  Coolant must exceed the boiling point to cause a pressure rise
> (due to the vapor
> pressure).  With a mix of glycol and water, the BP is probably up in the
> 230-240F range,
> depending on the mix you are running (plain water is 212F at sea level,
> but add antifreeze
> and that rises a fair amount).
> --
>   Roger
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