Porsche Repair: 1984 944 Water Pump Replacement, water pump replacement, radiator fans

Question
I recently had the water pump replaced in my '84 944.  The new pump was what the mechanic called an "updated" pump.  It went about 6,000 mi. then failed. I got a warranty replacement and it seems to be working OK (no leaks), but when I drive the car at highway speeds the temperature gauge shows the car running hotter than it was before the replacement.  Before the replacement the needle usually hovered around the mid hash mark on the gauge, and the fan would kick on any time it went slightly above.  Now, during highway driving, it hovers near the three-quarters hash mark (and the radiator fans do not kick on).

Oddly, when I slow down to about 30-40 mph the needle drops down to the mid hash mark area, and the fan will kick on.  If I am just driving in city traffic the needle stays near the mid hash mark and the fans kick on when the temperature goes slightly above.  

What this all suggested to me was that not enough coolant was being circulated when the engine was operating at highway speeds.  I asked the mechanic about this and was told it was "normal".  I have tried to bleed the system for air a number of times, but the problem remains.  I suspect it might be some trapped air or perhaps the thermostat is defective or was not properly seated when the replacement was done.

Any thoughts would be greatly appreciated.  

Thanks  

Answer
Hi Mike,

Are you trying to trick the expert?  Is this one of those puzzles where two trains leave Chicago at the same time but one is mysteriously lost in the Rockies and the other is....  It would appear no amount of logic will solve the train problem... or your cooling problem.

I haven't had enough experience with the beautiful 944's (meaning my wife won't let me have another toy) to see this kind of thing, but here's another factor to toss into the mix...just to confuse things a bit further.  The electric fan(s) is triggered by a temperature sensor on the radiator.  The temperature gauge you're looking at in the dash cluster is triggered by a sending unit in the engine block.  So the good news is that it's taken a long time for these two to become out of sync.  Just joking...I couldn't help myself.

So... if you assume the engine block water temperature will rise at highway speeds (unless cooled by the radiator), the temperature gauge in the dash is probably telling the right story.   And as far as the electric fan(s) at the radiator is concerned, it's running as it should, whenever it's temperature sender tells it to.

So...what would cause this new problem of an "overheated" engine block and a radiator who's fan(s) believes the water is only infrequently too hot?

Well,  you are right, a defective thermostat might, a new thermostat with a higher temperature range definitely would (let's say 195 vs 180 degrees):

This thermostat (or your defective thermostat) prevents engine block water transferring to the radiator till the water reaches 195 degrees.  At highway speeds the radiator's not getting hot enough to have the fans come on because of all the ambient (cool) air rushing in,  When you slow to 30-40 in town there's much less rushing air, so the radiator water could heat up which would eventually turn the fan on.

So, after all that, I suspect you have a higher temperature rating thermostat.  Or you might also have that defective themostat...but the defect has to be in the erroneously high temperature setting the thermostat works at.

Another posibility I considered was this:  if the engine block water is leaking past the new thermostat (an original 180 degree one) at the same relative speed as the engine is turning, then the hot water getting into the radiator could be sufficiently cooled by inrushing highway air but when you're driving at 30-40 the radiator needs extra fan help.  This assumes the leaking is occuring at all temperatures.  But it can't explain why the in-dash temperature gauge is reading higher.  No, I doubt if there's an internal leak.

There are resistors in both sending units that could be bad...but the sequence of your repair events suggests it's the thermostat.  Find a way to temporarily hook up a temperature gauge at the hole for the sending unit.  If you plan it right, with a tee fitting you should be able to have the new gauge and the dash gauge work simultaneously, so you can see if they are reporting similar values...then you need to decide if running the engine at higher temperatures is something you want to do.

Dave