Mazda Repair: miata overheating, combustion ignition, volt motors

Question
I have a 90 mita with 123 k miles on the chasis and about 100 on th engine. Its new. When I drive on the highway like 70 mph the temp goes up almost near the very hot H. But doesn't get all the way there. But once I get to normal driving it goes back to halfway. I don't know why this happens. I have anti free 40 and water 60. I just got a new engine. Is my themostat off or something? Or is it overheating? And what can I do to avoid or make this better? Thanks

Answer
First thing, never run it past the mid line on the temp gauge it will damage the engine.  They are tough but, heat can kill.

There's no great mystery to a cooling system.  For it to work think simple.  You must have;
The proper coolant mixture (which you have)
The correct amount (trapped air removed)
and
Fluid flow.  (while it's cold take the radiator cap off and look inside while the engine warms, once hot you should be able to throttle and see the brisk movement of coolant).  (clogged lower radiator crossovers if you had old neglected coolant or sediment, stuck or failing thermostat)
also,
An unobstructed path for air to flow
and
good functioning fan motors. (12 volt motors must show they have power a 12 volt motor can be bad and still spin so be sure they are pulling air)

Basics.

After that.  What causes heat.  Combustion and friction.

Friction, be sure the oil level is good and your running a proper weight.  Again basic.

Combustion.  Ignition timing that is set too high (OEM setting is 10 degrees BTDC), lean fuel mixture and/or a vacuum leak (manifold should read 17-21 Hg at idle and steady).  Improper exhaust out flow (does it labor to accelerate, hiss out the tailpipe or hesitate).  These can cause overheating at RPM.  Read your spark plugs are they black, brown, white/grey these can also lead you to where to look.

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