[V6-12v] Thermostat lifespan

Tom Christiansen tomchr at gmail.com
Mon Jun 19 22:21:28 EDT 2006

Austin (and others),

Here's the situation: The engine takes its sweet old time coming up to
temp. On a cold-ish morning (temps in the 50-60'ies (10-15C)) it
likely won't come up to the usual 1/3 fat mark up on the thermometer.
The temp will stay around or just above the minimum mark on the gauge.
The heat is pretty limited and if I pop the hood, the engine is
clearly pretty cold even after half an hour of freeway driving.

In stop & go traffic, the temp will come up to the second fat mark
just past the half-way point on the thermometer. The front rad fans
turn on. Once out of stop & go traffic, the temp comes back down. So
the thermometer and associated sensor seems to work.

I bought the car in April of 2003 with 107k on it. Replaced the
thermostat at 110k with one I got from The Parts Connection. I forget
which brand it was. The old thermostat was broken and, thus, stuck
open. I had the timing belt replaced at 123k but elected not to do the
thermostat as it only had 13k on it... The expansion tank cap is less
than three years old. I replaced it with the OEM blue cap because the
previous owner had used some random replacement cap that didn't have
the pressure releif valve in it. I have not checked if it rattles when
shaken, but I don't suspect a defective cap. I'm not sure I understand
why overpressure in the system would cause the thermostat to stick
open. If the entire system is overpressurized, the pressure
differential across the thermostat should be pretty close to zero
(maybe except for the pressure from the water pump which would be
there even in normal operating conditions). But it's a pretty easy
thing to check. I'll do that and report back.

The coolant level in the expansion tank is about mid way between min
and max when the engine is cold. It's just below max when the engine
is at normal operating temp.

I think what brought on the failure of the heater core was that I
replaced the heater hoses last fall. The heater core was cracked at
the top where the pipes enter the plastic top of the core, so I
suspect I was a little too violent getting the hoses off. So much for
preventive maintenance... :-|



On 6/19/06, Austin Franklin <austin.franklin at charter.net> wrote:
> Hi Tom,
> I did mine last year at 60k, and it was fine...just 10 years old, so I
> replaced the timing belt, water pump and thermostat.  It's a bit of a PITA
> job just for a thermostat though!
> BUT...I wouldn't rule out that there is some other problem.  Check the
> expansion tank cap...see if it shakes (after you take it off).  If there is
> excessive pressure in the system, it will hold the thermostat open and cause
> the exact same problem.
> Regards,
> Austin
> > -----Original Message-----
> > From: v6-12v-bounces at audifans.com [mailto:v6-12v-bounces at audifans.com]On
> > Behalf Of Tom Christiansen
> > Sent: Monday, June 19, 2006 12:06 AM
> > To: v6-12v at audifans.com
> > Subject: [V6-12v] Thermostat lifespan
> >
> >
> > Folks,
> >
> > It obviously figures that immediately after I replace my heater core,
> > the thermostat is now stuck open prompting me to spend even more hard
> > earned cash on that liquid Audi gold coolant. Dammit!!
> >
> > Anyway. It got me thinking: What's the average lifespan of a
> > thermostat. Mine has a couple of years (2-3) and 30k miles on it. Is
> > it normal for a thermostat to fail this early? I'd think it was
> > designed to last at least 60k so it could be replaced along with the
> > timing belt.
> >
> > At least it's stuck open. Not shut...
> >
> > Tom
> > _______________________________________________
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> > V6-12v at audifans.com
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