Originally Posted by DrLou

Dr. Brad:

Have been reading this thread today, and thought I might add to the 'diagnostic algorithm'...

Don't think enough attention has been paid to the possibility of a dying water pump. Had a situation very similar to yours a few years back (no, no mountains in New York City, but a few very hot days with nasty stop-and-go traffic). Symptoms: water pump was gradually weakening, with the gradual loss of coolant at the pump's 'weep hole' as others have noted here. But it still worked for a heck of a long time - (months!), with the car driven virtually every day. Assumption?: water flow was at least adequate for most situations. It boiled over once when pressed, fully loaded, on a very hot day. Clearly, your mountain climb is this stressor.

By the way, when healthy, this car never gets to 90 degrees under any driving conditions. Radiator was replaced about ten years ago, but because of a leak, not for overheating. Also, though apocryphal: I think the car runs cooler since I started using full-synthetic oil (many years ago).

On the coolant: Ethylene Glycol is Ethylene Glycol, whether expensive or cheap, and whether USA or German-manufatured... Yes, chemically, there is a coolant-water ratio which is the optimal one for cooling, but this optimization is marginal - it wouldn't account for 20 degree difference in running temp.

Auxiliary Pump: Very important, but mostly to handle the augmented thermal load - which is huge! - produced by the A/C condenser. Its primary purpose is to make this recondensation of the A/C coolant more efficient, thereby lowering the strain on - and increasing lifespan of - the entire A/C system. I am in the process of rebuilding mine myself, saving a several hundred dollar replacement with about $10 in parts. Point is, with the A/C off, the aux fan should not even enter the calculus re engine cooling.

In fact, even with our 100-degree weather these days, with the A/C running, and with the aux fan out of the car while I fix it, the car is still running cool.

Granted, my beast is a diesel, so runs cooler anyway, but still...

Differential Diagnosis: R/O impending H20 pump failure.
The Patient's Care Plan: Normal activity, including daily stress test. Monitor fluid output carefully!

Lou

PS - Have given up my practice so I'll have more time to work on my car!