[V6-12v] Car not getting up to temp in winter...thermostat not the problem

Poncho ponchojuan66 at yahoo.com
Wed Jul 6 13:10:46 EDT 2005

Tom, Jim,

An expansion tank top?  Boy that's a new one for me. 
I would be skeptical, but who kknows.  This Audi has
always had cooling system quirks.

It appears that Audi uses a typical designed cap. 
Basically it is a pressure relief valve that opens
between 17 psi and 22 psi to relieve excess pressure
due to over heating.  It is not a "vent" per se,
except in the sense of releasing pressure.  The
typical symptom is leakage; overflow because the cap
won't hold the 17 psi of pressure.  Normally a water
pump will produce about 30 psi flow in the system.
This can cause a thermo to open late if under
pressurized, but this would raise the temp even

I suppose a system could run a little cooler if it was
under pressurized, but theorectically this would only
happen above 212 degrees or the boiling point of the
fluid ( actually water; coolant is higher). 

My experience over the years is that the 12V engine
thermos often stick "open".  This causes the engine to
run below temp when the ambient temperature is below
around 65 degree. Below about 50 degrees it will throw
an engine light reporting a bad sensor.  There is a
tech bulletin on this issue..

You can run from May until October here in the
northeast and you won't even know you have a problem.

See how the temp holds on your car when it gets
colder.  If you have the same problem, let us know. 
IF its gone away, my bet is that you had an air
pocket; the 12V can be a stickler to purge after
refilling, sometimes.

I reported a problem a year ago to NAPA and Thompson
concerning their thermos for the 12V.  They tend to
stick open because the backing plate is too large
causing the symptoms you describe.  If you have to
replace the thermo, use a good OEM from Wahler or
whatever the Worldpak dealer provides.  And by the
way, you dont need to pull eveything apart.  The
thermo can be weaseled out of there without removing
the timing belt.  A pain, but not major.

Good luck



--- Tom Christiansen <tomchr at ee.washington.edu> wrote:

> You can also use a resistor:
> This page is worth looking at for general
> substitutes for the VW electrical 
> tools as well:
> Tom
> At 04:12 AM 7/6/2005, James Whitehouse wrote:
> >Yeah, the other thing that's always worth checking
> before replacing a
> >thermostat is that the multifunction sensor which
> feeds the dashboard temp
> >gauge (not the other coolant temp sensor which only
> feeds the ECU) is
> >working to spec. If not it can read low/high and
> you end up misdiagnosing a
> >faulty thermostat. You can do it with a volt meter
> before you go to the
> >trouble of doing a thermostat change. Glad it was a
> simple prob in the end
> >though.
> >
> >James
> >
> >
> >On 6/7/05 3:51 am, "Austin Franklin"
> <austin at darkroom.com> wrote:
> >
> > > Hi,
> > >
> > > 96 A4Q 12V, was not getting up to temp in
> winter, so I just put cardboard
> > > over the radiator, and it worked great for the
> cold weather (I got 
> > heat, car
> > > was at temp), and removed in the warm, works
> great as well.  I thought it
> > > was the thermostat...and given that it's major
> surgery, I decided to do the
> > > timing belt (car has 70k miles, and is 10 years
> old), water pump, pulleys
> > > etc. as well as the thermostat.  All went well
> (KTR in Ayer, MA did all 
> > that
> > > work for $300 and I highly recommend them, plus
> parts were $200 from
> > > Shokan...they let me buy the parts) but the
> thermostat wasn't bad!
> > >
> > > It ended up being the expansion tank cap wasn't
> venting.  It's supposed to
> > > rattle when you take it off and shake it (like a
> PCV valve) to allow
> > > pressure to escape...as the pressure will keep
> the thermostat open...so a
> > > new $8 cap fixed the problem.  I needed to do
> the timing belt etc. anyway,
> > > but I wish I had figured out the cap in the
> winter.
> > >
> > > But, all is well now with the cooling.  On to
> the mirror and other minor
> > > issues.
> > >
> > > Regards,
> > >
> > > Austin
> > >
> > >
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> > >
> >
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