Mazda Repair: overheating, plastic grocery bags, motor brushes

Question
i recently purchased a 1990 miata. IT overheated and blew a hole in the radiator. We fixed that then within an hour of driving it it overheated again and there was a hole in a hose.
Fixed that and within an hour it overheated again. All three times it has drained the radiator of all fluids but takes nothing from the resevoir. We changed the thermastat but still is trying to overheat. The previous owner said she changed the water pump so I'm at a lose of what could be wrong.

Answer
Top tank failures are common with overheats and especially on Miatas 12 years+, the plastic gets brittle.  Two main things to look for.  Coolant flow and air flow.

Top tanks on Miata radiators are pre-stressed plastic for a reason and when replaced you should also use plastic not an all metal radiator.  That plastic acts as a fuse in case of an overheat.  Better to take out a radiator than a cylinder head which is the next thing in line.  If you used a cheap all metal radiator keep this in mind, you no longer have the protection of the plastic tank design, engine damage will occur if you overheat and do not catch it in time.

If you run the engine to temperature with the cap off you should see coolant flowing while you rev the engine lightly.  It may even burp some out so be careful it will be hot.  The main thing, is the coolant flowing?.

Next, air flow.  Both fans should come on when the AC is on.  The main fan (driver side) however, will cycle on and off with the temperature of the engine.  If this fan is not working the engine will overheat. A fan can appear as though it is working by spinning, if the motor brushes are worn it may spin and not pull power.  Check to see it you feel a good flow of air from the back side of the fan.

Also, Miatas are know for sucking up plastic grocery bags and junk into the mouth.  These items can restrict air flow also causing overheats.  Look inside the mouth.

Before running the car to an overheat point a good pressure test of the cooling system should be done each time a hose or fitting is opened.  It should hold 10 psi for 10 minutes with a 1-2psi drop maximum.  Pressure testers can be bought or even loaned from some parts stores.