1996 Mustang 3.8L - Mystery Overheating

SYMPTOMS

- Car gauge appears to show a pending overheat when driving, with the needle moving from the O in NORMAL to beyond the L. This occurs sometimes in city driving, sometimes on highway with no real set pattern.
- Coolant expands to the overflow tank when hot as expected, but only sometimes is drawn back into the radiator when the car cools. Other times, most of the coolant remains in overflow tank and as a result, the coolant level in the radiator is low.
- Even when coolant is sucked back into the radiator after the engine cools, removing the radiator cap and checking the fluid level in the radiator shows it to be lower than completely full.

WORK DONE / TESTS PERFORMED

- New radiator, coolant, radiator hoses/clamps, and stant thermostat installed. The thermostat was tested, and was observed to open at the correct temperature of 195.
- Cooling system "burped" to remove air after filling with coolant, including opening the bleed screw AND filling the radiator using a Lisle noleak funnel to ensure radiator is full.
- No coolant observed in engine oil or transmission fluid; no sign of white exhaust smoke when the car is running.
- No coolant observed leaking inside the passenger compartment.
- Lisle combustion leak tester kit shows no combustion gases present at radiator (i.E. Blue fluid remains blue)
- Mityvac radiator pressure test kit used to perform a static pressure test (engine off) to 16 psi, and after 30 minutes the pressure was only down slightly to 15 psi.
- Mityvac radiator pressure test kit used to perform a dynamic pressure test (engine on); no rapid pressure build up was observed when the engine was started (indicating no blown head gasket), and pressurizing the system to 16 psi with the mityvac pump showed no vibrating of the gauge need (indicating no damaged block or head).
- Cylinder compression test performed, and all 6 cylinders report at roughly 180 psi
- Cooling fan observed to come on, and always turns and stays on when a/c is active
- No leaks observed at the water pump weep hole (or anywhere else); no coolant observed under car ever.