My black 1994 miata has an overheating problem that I cannot diagnose. Most of my driving is on the highway, where the car does not overheat. But if I get caught in stop and go traffic or city red lights after driving across town on the highway, the needle begins to rise above the halfway mark towards H. Turning off the A/C and blasting the heat lessens the problem. Resuming highway speeds completely returns the engine to normal temp (the needle is just left of the halfway mark).
When I stop the car I can hear the coolant boiling. Under the hood I can see that I've been losing coolant directly out of the overflow tank (the area around the left headlight is wet with coolant directly below the hose that vents from the overflow tank into the engine bay). When I check the radiator afterwards (after I let the car cool down). The coolant level is low by about a quarter gallon even though ample coolant is left in the overflow container.
Mysteriously, the car does not always overheat. It happens about 50% of the time when I drive in the conditions mentioned above. It does seem to happen more often on hot days. I live in Florida and want to fix the problem before the summer arrives.
Since this problem began, I have replaced the following parts:
Water Pump - New
Radiator - New (but installed under questionable circumstances - I took the car in for a leaking water pump, the shop said the pump was fine and I needed a radiator. After they installed the radiator, they "noticed" that my pump was leaking (like I had told them) and needed replaced. Beware of the Daytona area Mazda dealership. I feel they unethically inflate their bills).
Thermostat - New - replaced twice (once without the dual valve (my mistake) once with)
Radiator cap - New - The one that came with my new radiator didn't hold pressure (grumble, grumble)
I'd greatly appreciate any further suggestions or speculation as to what the problem may be. The car has 118,000 miles, but runs fine otherwise. Does anyone know how I can check if the fan is functioning properly? How about the fan relay switch?