I recently picked up a 94 miata with 107000 miles. Car runs great...except on long hiway trips. I can run 80mph all day long, but when I slow down the car starts to run HOT. Guage will move about 2 o'clock position then slowly climb, if i keep speed around 50 it will stay at 2 o'clock but will slowly ( 10 min. ) cool, but will move to H if I go slower. no visible overheat signs. lots of bubbles in overflow and overflow full, as I would expect. After about 5 minutes I can pop radiator cap with out any explosive results, overflow nevr does siphon back into rad. Rad. is down equivelent of overflow, maybe a little more.
No pinging, no sign of steam or coolant at tailpipe, or any where for that matter. No loss of performance. Can drive in traffic ( atlanta ) with no indication of symptons until maybe 40 min. to 1 hour, then revving brings it right down.
Ideas?

 

I had checked the plugs earlier. I started up the engine cold and noticed it was pushing air into the overflow already ( hadn't done that before ) #2 has coolant in it when I checked the bores this time......merry christmas lol

 

when was the last time you replaced the water pump?

the weep hole does not always leak when it is bad. i had a very similar problem on a 93 protege. no leaks or anything. decided to change the water pump, mainly because it was time to change the timing belt. when we got the water pump out, it had been completely "eaten". it had one measly little fin left.

 

It was a blown head gasket