Ok, so since everyone is talking about radiators and cooling, this would be a good time to ask for advice about my cooling/heating problem. I had these problems before, so I did the following to no avail:

- New thermostat, installed with the little hole pointing up.
- Changed/flushed coolant.
- New heater valve.

Now, here are my symptoms:

1. Starting the car when fully cold standing still, it will warm up and then hover just below the red mark at idle. Also, heater set to full hot blows cold air.
2. Still standing still, rev the car above ~2.5k RPM. Coolant temperature drops to between the two white lines and hot air blows out the vents.
3. Dropping back to idle does the 1) symptoms again.
4. Driving around, as long as RPM is above around 2.5k RPM, coolant temp is fine and hot air blows out the vents.
5. Driving on a specific stretch of highway at ~80mph where there is a slope downwards, if I put the clutch in and idle down, I see the same symptoms as 1) after about 30 seconds. This means it's NOT a fan related issue.
   

These symptoms lead me to believe that I have a coolant flow problem.
Questions:

- Does anyone have any clever tricks to measure coolant flow in different areas?
- Could this be caused by air bubbles in the system?
- Does anyone in the Boston area have a used Radiator that I could temporarily borrow to see if mine has a blockage? I don't care if it leaks.

TIA,

Dan
'86 928S 5-spd w/LSD

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My guess is a bad water pump impeller ..... guess only but that is consistent with the symptoms . The new pumps used a plastic impellor molded to a metal ring which is pressed onto the shaft . The molding to the metal run can delaminate and strange circulation issues arise. Older pumps use a metal impellor which can corrode . On rare occasions the impellor (metal) can get loose and slide toward the engine block and begin machining away the block which eventually makes the pump less efficient .

Jim Bailey