Mystery coolant leak

About a month ago I found a slight coolant leak was apparent. Not the "cursed plug" at the rear of the head because it doesn't exist anymore.
Searched all over for about a month, no smell, no coolant in the cabin, and a compression test was good, i.e. no bubbles in the radiator and good numbers. Turns out it was a pin hole in the "T" coming off the thermostat neck. Got a replacement from Panicmotorsports and started the swap.
What a PITA that was, remove cam cover, front cover, water pump drive pulley, cam gears, plate behind the gears and finally the neck itself
where it now resides in a neighboring county after I hurdle the SOB.
Put everything back together and after 3 1/2 hours and a 100 mile, drive all appears well.
Just thought I would post my find in the event it may save others some grief.

i had a similar situation but my pin hole leak was on the coolant bypass pipe (the one by the header). luckily it was right by the hose so i pushed back the hose to cover it. i'll prolly replace the pipe when it's wp/tb time again

I was hoping to do the same, but mine was almost at the joint. No luck using the hose to cover that.

stuff like that is why I am adamant about changing hoses and seals when doing the TB maintenance.

I too have a mystery coolant leak, no overheating, but coolant resorvior was basically empty, and it appears as if the coolant leaked towards the rear of the engine, and down the back, I recently did the TB/WP and recommended seals, this post was very useful, and I will also check out "T" coming of the thermostat neck, any other ideas considering where the leak is? Thanks

The rear of your engine, the heater hoses, water temp sensor, and the cap they plug into are the main culprits for a coolant leak on the back of the engine. Is it on the left or right side?

Attachment 12055

Ugh...another tool I HAVE to buy. :p

My last miata had a "steam" leak coming out of the heater core hoses. The pipe was just barely dinged (my heater core was second hand) and shaped sort of like a pacman type dent in on the circumference.
It only lost coolant once the car was hot, and even at that it took like 2 weeks for it to lose an appreciable amount....


Well thats not entirely true, the radiator also had a hole in it so I threw a mishimoto in and the reservoir had hairline fractures in it from heat and age.
But the weird steam leak stuck around after.