Overheating randomly
 

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Warmed up with a rough idle, but no high temps. Hit 46c water temp and idle evened out. Took it for a lap around the block and it hit 128c, buzzer going off and got into the driveway and the overflow poured coolant. Lots of “possible” reasons, but ast is fine, hose isn’t leaking or kinked, fans working, coolant hoses not leaking, new engine maybe 5k miles and new hoses. With this info, would the consensus be thermostat? I am just looking for the most likely place to start.

New thermostat when in question is cheap insurance.
Possible air pocket that was never bled out of the system.

Why is the car running rough at idle?

no idea, hadn’t done it until this morning. But everything else was fine temp wise until I took off.
and the idle issue stopped once it hit 46c water temp

Is the pressure cap on the ast good? If the pressure cap doesn't seal or "spring" anymore then you will get this since the system is effectively open to atmosphere.

The ast should really be removed either way

New AST, so I don’t think that is it…I’ve replaced almost everything but the turbos. Secondary seal is blown so they are next.

Premature failure of the thermostat is possible but unlikely. I would start with WHERE the coolant came out from and work backwards. The origin can lead to the reason it happened.

did that, came from the overflow. No leaks anywhere else. Checked the OF hose, no kinks, no separation in the line, no leaks in the line. No leaks anywhere, just overheated, then the OF tank purged once I shut the car off.

I think there is an overflow port on the reservoir tank. If you start with the coolant already overfull, then when the expansion coolant gets sent to the overflow tank, it releases coolant.

Also the seals around the bottom of the neck, as it enters the tanks, rot away. Leak could be there.

Also, something might be preventing the coolant getting sucked back into the motor when things cool down. Hence your coolant low alarm.

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You can get new for about $200 on Buyee

As mentioned already, the spring pressure cap on the AST could be bad. If you have a tester that would be a good place to start. If not, replacement caps are cheap and easy to swap out and test again. After that, I would do the thermostat since you were seeing high coolant temps.

I’ll take a look, but OF tank is new as well, so seals shouldn’t be rotted.
why would it idle at 46c even when I press the gas and rev (to play with the funky idle) wouldn’t it go up in temp. It wasn’t until I drove half a block and it shot from 46-128c. Seems more like the thermostat isn’t working correctly. When I shut it off, I could see the pfc temp drop to about 118c within 5 min or less and continue to drop as it sat.

128C is very hot. Pressure cap is probably working correctly and just venting the excess pressure at that temp which is above the boiling point for 50/50 coolant mix.

Since it overheated very quickly a stuck thermostat is certainly possible IMO.

The ast is new, maybe 5,000 miles on it. Cap should be fine unless it was faulty. I blew the original AST in half, so I had to replace it.

The thermostat won't work correctly if there is an air pocket in the engine at the thermostat. Your symptoms are a definite clue that the thermostat did not have coolant around it until the engine overheated. Bleeding air out of an FD engine can be a PITA. Many threads on this subject.

Where do you buy thermostats? None of the shops near me can get them. Just emailed Ray, waiting to hear back. Rock would be my next option I guess?

Mazdatrix is not exactly close to San Diego, but they are likely to have a thermostat in stock. Mazda Motorsports USA ships some of their parts from Ontario California, but it might take some time to get signed up and be able to place orders with them. I would try a local dealership if you need one quickly, they might be able to get parts from the same Ontario warehouse overnight. If you do swap the thermostat, I would check the new one in a pot of boiling water before installing it. Also be sure to install the jiggle pin correctly (12 o-clock) so it can allow air pockets to escape. I like to drill one or two very small holes in the thermostat (slightly smaller than the jiggle pin hole) to help air pockets escaping and to allow a little more flow through the radiator when the engine is cold.

I agree with the others who suggested an air pocket in the cooling system could be the problem, and it's wise to check that the pressure caps are holding pressure. If the overflow tank was problematic or the engine was not sucking coolant back into the system from the overflow tank/reservoir, I would expect to hear the low coolant alarm before the engine even starts up.

Whatever you do, a funnel like the Lisle 24680 Spill-Free Funnel is a great way to get the air out of the cooling system. Some of the local auto parts stores usually carry them. I forget if you want to put the funnel on the filler neck or on the AST, I removed the AST from my car a long time ago.

In southern california at this time of year, do you need a thermostat, really?

[Ignore this.]

Does the water pump become an issue here?

Is it spewing coolant by any chance?

Are the fans coming on?

Running without a thermostat is a very bad idea on these cars. Just removing the thermostat will send less coolant through the radiator when the engine is warm, because the bypass port won't get closed without the thermostat to close it. I agree it's a good idea to check if the fans are working.

It could be a sticking thermostat but idle miss is a classic water seal symptom. As is blasting coolant.

Fans are always on, some glitch with my relay or PFC. Not an issue lately because it’s been mid 90s so it’s hot enough to need them all the time.

it was more of an uneven idle, not missing. The pattern of the idle wasn’t consistent. Instead of brap brap brap it was more brap brap brrrrr brap brrrr brap brap.

The fans come on in stages. Are all the stages working?

They are always on, even when it’s just turned to ACC they kick on. PFC issue. I need to get the software and adjust the settings. And/or check the sensor to make sure it’s functioning correctly