patsmx5

iTrader: (16)

Sorry to ask a dumb question, but what kind of temps are you seeing at idle? At cruise?

Pull the radiator cap at idle, and see if there is substantial water flow when it overheats. You should have a large amount of flow at 2,000-3000 RPM when the thermostat is open. This easy test would help determine if the overheating problem is water-flow, or airflow, related.

LetItSnow

The temps I'm seeing are the ones right before I shut the car down. I was at 210 and climbing at idle with the nose in the air, hood open, fan blowing in the bumper opening, and both radiator fans spinning.

Opening the cap at idle sounds like a great way to get a burn! How am I observing the flow in your test? Like I said, with the t-stat open, revving the engine doesn't change the pressure in the upper hose or swing the level of fluid in the funnel. I'd sure be happy to find out that it's just the water pump.

patsmx5

iTrader: (16)

Open cap, then cold start the car from a cold soak.

To observe flow, just look into the top tank of the radiator where the cap was before you removed it.

Sounds like no water flow to me. Is the air coming off the fans 210*F when the motor is indicating 210*F? If it's not, the radiator is not getting hot water.

NBoost

OE, as in OEM Mazda? I saw you said you replaced it, just thought you might have had a defective one, or two. On that note, where did you source the water pump?

Definitely sounds like you have a blockage somewhere.. Pat's cap open flow test will tell you for sure. And please for the love of god do it from a cold start.

LetItSnow

The radiator is definitely getting hot coolant; it's just a question of how much. Both thermostats came from the Mazda dealer a couple of miles from home. I got the water pump with the typical timing belt, idlers, seals and such collection from Rosenthal/Arlington (RIP). I did those when I got the car about three years ago. I only put about 4k miles on it each year, cos our road salt would consume it in a matter of minutes and I have to drive the Forester once in a while during the summer to save it from rusted rotors and a dead battery.

Believe it - I won't open a hot cap. I know some folks need that laid out clearly for them!

shuiend

iTrader: (24)

In the OEM thermostat there is a small hole. Do you have that hole pointed up when you install the thermostat?

NBoost

I was going to ask if its possible to install the thermo in the wrong/backwards direction, just couldn't remember if it even fits any way other the the correct way.

sonofthehill

iTrader: (1)

No one is actually suggesting you open the cap after the car warms up.
All of us without overheating problems, have started the car cold with no cap and a magic funnel. When the thermostat opens it looks like a river flowing from the upper radiator hose. Some bubbles come out, I usually touch the throttle a tiny bit but I don't rev the thing very high.
Then let it happen again, twice, all with the cap off. Once it is stabilized, plug the funnel and the level will be perfect.

I had a problem with my original thermostat that had rubber in it's construction. Some of the rubber had deteriorated and got stuck in the little right angle tube that comes off the water pump outlet.

LetItSnow

Yep, I have the "jiggle stick" at the top, spring side in.

I'll finally have a chance to get in the garage tonight. Details as they come in...

LetItSnow

Well, here's what's up.

We pulled the plugs and tested the cylinders out. No bubbles, no waves, nothin'. Plugs look good, fwiw.

Then we drained the radiator to just above the core and fired it up. When the thermostat opened, the radiator got plenty hot, but there was no river. Rev rev, no river.

While it's pretty unlikely that the last thermostat gave up proper operation over time and that the new one failed out of the box, I'm thinking it will be mighty quick, simple and cheap to pull the thermostat and run the car without it just long enough to see whether I'll get a radiator river. No river, the t-stat goes back in and I've pretty much got to blame the water pump, right? What else is there?

patsmx5

iTrader: (16)

I'd pull the thermostat and retest as you mentioned, and I would not pre-drain the radiator any, just have it at normal level when you do the test. At least that's what I've always done. Sounds like sticking thermostat to me.

LetItSnow

My challenges regarding that diagnosis:
* That would be two sticking Mazda brand thermostats, including one brand new out of the box.
* The top hose and top tank of the radiator definitely get hot, and it starts at the right temperature per TS. If they're both sticking, they're both simply sticking not open very much.

patsmx5

iTrader: (16)

If pulling the thermostat results in a river of water and a car that won't warm up, you know it was the thermstat restrictive coolant flow.

If you have a way to measure the air temp exiting the cooling fans, that would be telling as well. If the air exiting the radiator fans is very close to the engine coolant temp, then the air is saturated with heat and the fans are not moving enough air. Stock fans are plenty strong enough to cool a miata at idle, but you're not running stock fans and it's very very VERY likely the fans you have now move less air than the stock fans.

Easy way to diagnose this problem is to measure things. Temperature of coolant in/out of radiator, and air temp out of radiator will literally show you the problem.

Water exiting radiator is cool, and motor overheats? Waterflow problem.

Air exiting radiator is withing 1-2*F of water exit temp and motor overheats? Airflow problem.

patsmx5

iTrader: (16)

Also FWIW I've had 3 bad thermostats before. Two bad Stant thermostats, one bad autozone fancy expensive one. Went back to using my old cheap autozone thermostat that's worked great for years.

LetItSnow

I do appreciate your help, but we did this. Stock radiator and stock fans at idle also overheated. Stock radiator and stock fans on the highway (where money's best fans shouldn't matter) also overheated. The fans are irrelevant in the overheating that I'm seeing right now.

LetItSnow

I pulled the thermostat, and there's good flow now. The river test passes, the hose squeeze test passes. The pump isn't failed.

I filled the system and put some extra in magicfunnel. The temps cycle up to 200 (primary fan on) and back down to 190 (primary fan off). I let it do this a few times.

So this has got to be a coolant flow issue, cos the air is doing its trick. Is it possible that the water pump is working under expected efficiency, to the degree that zero resistance at the thermostat housing qualifies but the resistance of the smaller opening a thermostat gives won't give proper circulation?

18psi

iTrader: (76)

it's a simple impeller . it usually either works or breaks/falls apart.

patsmx5

iTrader: (16)

You had two bad thermostats, or they were installed wrong.

LetItSnow

I had them both installed spring-end in, jiggle stick at 12 o'clock. It's pretty hard to botch a thermostat. Both thermostats were the same, part number and all. It's a pisser that one would come out of the box as a dud at the cost of the things.

I'm gonna pick up a third party thermostat tomorrow, shove it in there and see what happens. Unfortunately, the Stant regular that was all the rage two years ago appears to have fallen off the radar. Is there an "it" piece these days?

LetItSnow

For lack of a better option, I picked up a 195* Stant today. I'm amazed at how oppositely polarized m.net and mt.net are regarding the OEM thermostat and any aftermarket ones, regarding which is the only choice and which is outright garbage. Alas...

All temperatures are based on TunerStudio dashboard readings:

At idle, the car now works its way to 200 and will rest there. If I hold it at 2500RPM or so, the temperature will go down to around 195. Highway cruise is around 210, certainly higher than it used to be. Romping on it a bit (wastegate pressure is 5psi), I got it up to 220, and I stopped messing with it around there, having felt that I'd made zero actual progress.

I'm looking at the datalog I grabbed to see if anything stands out.