update on overheated S4...
 

Local mechanic said fans,thermostat OK.
Wife was in low gear.

Only problem found was the pressure cap not holding pressure. So the theory is as it got hotter and higher pressure, the cap leaked fluid out which caused it to get hotter and higher pressure which caused more to leak etc.

Anyone buy it? :>

scot

A failed cap can cause a car to overheat...yes.

Well, in theory if the system can't hold pressure to 15 psi, yes, you can overheat.
 

However, if the temp spikes too high...the coolant will find it's way out of areas it normally shouldn't. It's not designed to hold much more than 15 psi.

Replacing the cap is cheap, so it can't really hurt. I doubt it'll fix the problem though. How did the mechanic verify the thermostat is working correctly? If it is working correctly, my next guess would be a clogged up radiator. A4 guys seem to run into that around the 7-8 year mark, depending on how well the car was cared for.

Here's the thing...
 

if the cap is left open, the coolant temp will rise and reach a point where the thermostat will open. Once that happens, the radiator should be able to keep the temps in check...it's just that the thermostat will remain open more often that it would if everything was operating normally. I really doubt the car will overheat if the cap is left open, but everything else is functioning normally.

Actually ...
 

not convinced they checked the thermostat - just
the fans/belts, etc., although I did ask
specifically about it. I'll probably just
replace it myself then try a test trip back up
the mountain!

Anyone have a part number?

scot

But, it's a pressurized system. If the temp outside is high enough, and the engine temp is high
 

enough, you're just going to start boiling fluid over. At some point, you're going to not have enough coolant left to properly cool the car.

At atmospheric, the boiling point is 100C, and the thermostat opens at 90C.
 

And that's just with pure water. With a 50/50 coolant mix, you have a little more headroom before it boils over.

The thermostat is deep in the bowels of the car. The test is simple. Start car and wait for warm
 

coolant to be cycled through the upper coolant hose on the driver's side. If the temp is at the middle mark (on the temp gauge) and you don't feel warm coolant flowing through the upper radiator hose, the thermostat is not opening. You may as well do the timing belt job at that point cause it's going to be exposed, anyway.

Ahhh, didn't know that's where the points were. I concur with you, then:p

It might not be that simple...
 

If the thermostat is stuck shut, that works great. If will not open all the way, or the radiator is blocked up a bit, then you've got a situation where the cooling system capacity is hindered. The upper hose will be a little warm, but once you romp on the engine, it will still overheat.

He originally mentioned "wife was in low gear". I'm not sure what to make of that, but I'm thinking a heavy load condition caused the overheat condition...but perhaps the car runs normally otherwise??

I'm rambling a bit...the point is it might not be that cut and dry, in which case it might just be time to swap the thermostat, do all the timing belt work, and while that radiator is off the car, flush some water through it and see if it's plugged up. Also, backflush the exterior fins on the rad and ac condensor to clean out dirt and debris that has accumulated over the years. That will hinder cooling system capacity as well.