Car is a 92 1.6 5spd Miata w 315k miles on it.
When it was purchased the clutch needed to be replaced, so figured might as well swap out the motor.
Had a JDM motor installed with all seals, gaskets, and hoses replaced with Mazda parts, fuel filter, engine mounts, clutch, flywheel, all replaced as well.
Replaced thermostat with aftermarket (the shop chose it).
Belly pan is in place, no front plate to block radiator. Car is bone stock less suspension.
Coolant should be 50/50 mix.

Car looses coolant; shop damaged heater core, car overheats. shop replaced heater core and seems to be fine.
Drove car home from shop 30 miles, and the temp stayed rock solid at a tick under 12 o clock.
Two days later the car starts overheating (about 3/4) while on the freeway or on the street. Turning on heater and keeping revs down maintains more normal heat.

Doubt it's the headgasket, no oil in coolant and no coolant in oil, no excessive white smoke when driving.

So we replaced the thermoswitch (coolant temp switch to turn fan on on the radiator), removed thermostat, and replaced radiator cap. New Mazda themostat is on its way from trussville, should get here on monday.
Car still overheats.

What could be causing the problem? Read Miata.net until my eyes started hurting. So now I ask the big boys. What would you try/troubleshoot next?

 

are the fans going on?did you flush the radiator, it might be clogged?

 

There's a few things I can think of too.... check your rad... make sure it looks to be in good shape.... Fans working? If you jumper the FEN and grd (I think that's it, check the manual for the jumper places)... do the fans come on with the ignition?

Check the coolant for air pockets. Maybe they didn't "burp" the system properly.

Have you checked if the motor "feels" hotter than usual? maybe the wiring for the gauge got damaged when the dash got ripped appart?

Also, when does the temp gauge go up? Right after getting on the highway? Stop and go? Can you describe when it happens?

Dave,

 

Ever replaced your radiator cap?

The radiator cap can be old, it will not hold pressure so the coolent gets hotter....

 

Mine had this problam because of where the PO had the license plate.