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Discussion Starter · #1 ·
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, timing belt, water pump 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 (mountain view) to SSF, 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?
 

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What is the vintage of the radiator? Is it the original or has it been replaced?

Was the water pump changed on the replacement engine?

You say coolant "should be 50/50," might want to drain and be sure. Actually 50/50 is not necessary in the Bay Area. A lower ratio of glycol to distilled water would be better, say 30/70.

Might want to pressure test the cap and/or the system.

That would be the big stuff I can think of, other than what you have done already.
 

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Discussion Starter · #3 ·
What is the vintage of the radiator? Is it the original or has it been replaced?

Not sure. It looks original, if not original, a replacement stocker

Was the water pump changed on the replacement engine?

Yes. Original post edited to reflect that.

You say coolant "should be 50/50," might want to drain and be sure. Actually 50/50 is not necessary in the Bay Area. A lower ratio of glycol to distilled water would be better, say 30/70.

Will try that next.

Might want to pressure test the cap and/or the system.

Cap was just replaced today. How do you pressure test the system?

That would be the big stuff I can think of, other than what you have done already.
 

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If your system isn't losing coolant, sounds like a flow / pressure problem.

How is the radiator? They can crack around the neck, causing you to boil off coolant and lose proper pressure. Sometimes the plastic end tanks can crack at the seams to the core. It can be hard to see these leaks because once the system is warm, the coolant doesn't necessarily collect anywhere, just boils off.

One way to verify that the pump is working is to depressurize the cooling system while *cold* by cracking the radiator cap, start the engine and *carefully* remove the radiator cap. Since you have no thermostat right now, you should see quite a bit of turbulent flow in the radiator. If not, you may have a pump problem.

I've also found that non-oem thermostats don't work nearly as well as the oem part. Good that you are going back to factory. Many non-oem products don't have the small bypass that the oem part does.

Do you still appear to be losing coolant? Watch this carefully. Might be hard to see over a couple of days. Check all your hoses for signs of leaks where they connect to the rad or heater core / engine.

The other thing I can think of is that the system is plumbed incorrectly, which would be pretty hard to do, but worth checking anyway, at least to rule it out.

Bruce
 
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