Overheating/losing coolant issue
 

I recently purchased a '99 Miata from an auction, and I drove it home 2 hours with no problems at all. Before setting off I checked the oil and coolant and everything seemed in order. After I get home I take the car out for a relatively spirited drive, but nothing too crazy. As I'm getting back to my house, less than a block away the coolant temp gauge shoots all the way up, and the engine starts idling funny. I pulled into my driveway and turned the car off, and could hear the coolant boiling. After a few hours I go to start the car, and see that the coolant overflow tank is completely empty. I filled it up, and the car started up just fine. White smoke was coming out of the exhaust, along with some coolant. I left the car alone for a few hours and when I came back, the overflow tank was completely empty, so I filled it up again. It now sometimes has trouble idling, especially when the engine is warmed up and I rev it. I haven't taken it out on the street since it overheated. Today I did a compression test and got cylinder #1: ~180/240 wet. #2: ~150/210 wet #3: ~150/210 wet #4: ~150/200 wet. These numbers, although not ideal, seem a bit high for it to be a blown head gasket. The information I got with the car claimed that the radiator was replaced 20k miles ago, and well as the coolant being flushed at the same time. The car has 123k miles. I also pressure tested the cooling system, and it went from about 15 psi to just over 14psi in 10 minutes, which seemed ok to me. Any ideas on where all my coolant is going? how is some of it ending up in the exhaust? why is there white smoke coming out of the exhaust (I think its because of the coolant burning)? Any help is greatly appreciated. If there is any other info that is needed or any other tests that I need to do to determine what is wrong I would love to hear about them.
Coolant coming out of the exhaust.
https://cimg8.ibsrv.net/gimg/www.mia...e0ad1bb8d6.jpg
The only leak I could find, this little hose that leads to nothing underneath the car on the passenger side, right by the transmission. Is this supposed to be plugged into anything?
https://cimg9.ibsrv.net/gimg/www.mia...ccb7f08e19.jpg
The puddle of liquid that was made by that hose that leads to nothing. Seems like a lot:
https://cimg2.ibsrv.net/gimg/www.mia...915c622618.jpg

Try replacing the radiator cap.

That hose is just a drain.

Do a leak down test.

Learn to differentiate between smoke and steam.

Pressure test at 20psi.

Compression test each cylinder and report back.

That's condensation. All three photos. Sounds like you need to do some research on your car.

You're on the right track with the thermostat. Look into that, fan (and blade) condition, look for clogged ac condenser, radiator, check for ducting, etc. aka: basic maintenance for a 20 year old car. Judging from your posts I'd hand it off to a qualified mechanic before doing the head gasket. Despite how easy some write ups on the internet say it is, there are a LOT of simple mistakes than can ruin the job.

What is the oil condition? Clean, dirty, or milkshake?

there are only so many ways coolant can get into the exhaust. If it’s burning coolant now, a thermostat or water pump ain’t gonna fix it. The radiator and flush was probably the previous owner’s last attempt to fix it before giving up.

if oil is milky, I would not even bother with pulling the head. Coolant in oil will destroy the bearings fast.. a rebuild or used long block would be the way to go in that case.

Sounds like a bad head gasket to me. When I got my car it was doing similar things to yours, losing coolant in the overflow and when driving "spirited" I would actually shoot coolant out of the overflow. I wasnt burning coolant but I had a head gasket leak that pressurized the coolant system. Are you burning coolant or is it coming from the overflow tank? Either way it sounds like a Head Gasket issue to me. My compression was 180/145/150/180 and the HG leak was in between #2-3.

You can try a rediator cap to see if that helps and a thermostat, I did both those things but to no avail. I ended up boosting the old motor for shits and giggles and blowing it up real nice then building a motor. Most fun indeed.

pressure tested at 20 psi. dropped to about 18.5 psi after 10 mins.

That's what it seems like to me. I'm going to replace the cap and thermostat this weekend, and if that doesn't help then I will do the water pump and head gasket.
I checked everything else that was suggested and it all seemed to be in working order.

So I replaced the thermostat, that was the problem. The old one was stuck closed (tested it on the stove). Going to do the head gasket in a couple weekends, planing on using this kit since its only $100 and seems to have everything I would need. Any reason to use oem parts instead? Anything else I should replace while I'm in there already? Thanks. Head gasket Kit

You don't need all that stuff. You should:
get your head resurfaced
replace head gasket
Replace the water pump and try to get one with cast blades. It isually comes with gaskets for the block and the mixing manifold, and also make sure to replace the o-ring for the hard line that goes from the heater core to the mixing manifold.
Replace the timing belt
Replace the tensioner and idler pulleys
replace the cam seals
Replace the crank seal
Replace the woodruff key
Replace the valve cover gasket

Thanks. I decided to go with the cheap water pump because it had decent reviews and seems to work fine. Otherwise I think I got everything you mentioned.
Cam Seals
Crank Seal
RockAuto Stuff
Woodruff Key

Why would you cheap out on an important part that will essentially last for 60-100k miles on a motor?

I bet your head is cracked and filling cy1 with coolant

remove cyl1 spark plug and start the engine. you'll probably shoot coolant out of it

hmm ok well that's good. because the symptoms sure do sound like it's got a crack

Don't use a cheap head gasket. Use OEM or Fel-Pro