overheating 4000q

[Huw Powell]

> I have a puzzling problem with my '85 4000s quattro overheating.  It has
> 156k miles on it.
>
> The overheating only occurs when the outside ambient temperature is over
> 90 degrees Fahrenheit, and only when traveling for 15 minutes or more at
> greater than 70 mph.  When this happens, the water temp needle slowly
> climbs to almost the top of the gauge, and very shortly there after, the
> oil temp climbs until the little red LED at the top of the bar lights
> up.
>
> Again, it only overheats when driving for 15 mins or more, at greater
> than 70mph, when outside temps are 90+.  It does this whether or not the
> A/C is on.
>
> It runs cool as a cucumber when the A/C is on high and I'm stuck in
> traffic, driving in the city, or crusing at only 60mph, even when
> outside temps are 100+ degrees.

Ok, that is totally weird.  [scratches head in frustration]

try this... although at those speeds, your fan should never come on
anyway, when your AC is on high the rad fan is automatically engaged via
other means.  You should test your coolant switch in the bottom of the
radiator i guess.

unplug it, short the wires.  fan run?  yes, pull sender and test.  no,
find fan problem and fix.

test sender with boiling water and ohm meter (it closes around 200 dF).
get new one, install.

But it's not really AC related, just 70 mph+ related... darn.

Maybe your cat is clogged...?  You start working the engine hard in one
of those bricks at 70, and if the exhaust can't get out nicely it starts
warming up the engine...

> I have replaced the water pump, thermostat, coolant expansion tank, all
> coolant hoses, the radiator, radiator fan, water temp sender, and
> flushed the coolant more than a few times.  I'm running a 50/50 mix of
> distilled water and G12, and there are no leaks in the system.  In
> addition, I had the coolant system pressure tested at the local VW/audi
> independant shop, and had them ensure there were no air bubbles stuck in
> the system.
>
> What other components could I possible check???
>
> The engine starts and runs marvelously, revs cleanly and smoothly all
> the way to redline.  it is meticulously maintained, and I've taken
> several recent ~500 mile drives without any problems whatsoever (with
> the exception of the aforementioned overheating under certain conditions
> situation).

--
Huw Powell

http://www.humanspeakers.com/

http://www.humanthoughts.org/