Miata's have great little engines but they don't take over heating well at all. I had a secondary radiator hose cut at the clamp and split and lose fluid. I didn't catch it till it was pegged on hot, big bad on my part.My thoughts are that I now have a blown head gasket but It doesn't act like a bad gasket. No water in the oil, no oil in the water, I see no leaks or steam at the engine or tail pipe, heater core is dry yet it will take a pint of fluid about every 75 miles and will gradually sooner or later heat up if I don't add fluid. Every blown head gasket I have been involved with has super heated the water in a few minutes and no way could I drive a hundred miles before it would show hot. But then again the water is going some where and the head gasket is my first guess.
Plugs were just changed a week or so before this happened, they look new and all the same because they are very new.No amber color at the tip, no steam clean look. I don't have a bore scope or I would take a look at the piston tops for the steam clean appearance. What else could be done that would pin point this issue I'm out of ideas..... Tedd

 

How is your other hoses? Odds are they didn't go bad at the same time, but I had something on my daily driver that was making the reservoir go dry. One day I squeezed the hose in just right place and saw a tiny stream squirt out of a pinhole. Never left a puddle on the ground, but under pressure it was likely venting through this pin hole.

 

Pressure test? Maybe a small external leak at temp and while driving that burns off without evidence. Bore scopes are not so expensive anymore.

 

You can get one pretty cheap from Harbor Freight.

 

I have a complete set of hoses ( I think 9 in all) that are setting in a box on the shelf with the radiator and cap to match waiting for me to change them out, I'm a little late, bad timing on my part.

I might swing by Harbor Freight tomorrow and see whats up there. Buying tools gives me a up lift I can use anyway.... Tedd

 

is there a high point in the system that may have some air bubbles in it. Thus the system is not filled all the way?

 

Yea,I know about burping Miatas sometimes they are hard to get full. Most times if you jack or lift the front a foot or so that troublesome air bubble will escape and the radiator will take a quart or so extra water. But I have something different going on here I believe.. More to follow when something different happens... Thanks all for your ideas and help.... Tedd

 

These things work great. You can also pull a vacuum on the cooling system with it and see if it bleeds up indicating a leak.
http://www.tooldiscounter.com/ItemDi...CABEgKgxvD_BwE

 

Just a quick after thought, I noticed that the return overfull tank never seems to change (stays the same level) no matter how much liquid is in the radiator. Wouldn't that be a sign than the system has a leak somewhere?.... Still could be a blown head gasket though that is defiantly a leak.No air bubbles at the radiator cap. Typing out loud here.... Strange issue... Tedd

Ps I'm off to Harbor Freight.

 

Depending on how the head gasket failed, a compression test may reveal something.
In the past, Harbor Freight's inspection camera's probe was too big to go through a 14mm spark plug opening.

 

Fastest check...change the rad cap!!!