Did you see any leaks when you did your pressure test? If not, it's leaking internally (since you are losing pressure, there is a leak)

You had something in your coolant system go snafu. The question is when you overheated, did you melt your coolant seals.

 

There are no more coolant expansion tanks in US Mazda parts inventory according to the dealer I use for parts, they got one of the two remaining in Mazda Canada's inventory for me just last week. There is an online seller that is offering them on ebay for a little price premium, and Atkins is doing the same. So don't delay if you are in the market for one...

 

I think DaveW is probably right. I have symptoms consistent with getting combustion gas in my coolant. Coolant is filthy, I have a pressure leak without any external leak, and an overflow from the reservoir. I guess I’m just not leaking much coolant into the engine though. It’s running no differently than when I bought it so I don’t think my overheating incident caused it, unless that’s the reason I can’t get the car to cool now.

My plan is to replace the thermostat, radiator (aluminum Koyo? Fluidyne?) and caps to see if I can stop it from overheating, then I’ll keep a close eye on coolant level, condition, smoke, and watch for air in the system too. If I continue to have symptoms of a bad seal I’ll start preparing for the rebuild. Sound like a decent plan?

And thanks for all the help, and quick responses

 

Yes, that sounds like a logical way to proceed.

 

Alright, I got a used Koyo radiator, new thermostat, and new caps installed (with a little fabricating) and it looks good initially. The car ran for 20 minutes in my garage, mostly idle, but the fans kicked on and temp gauge didn't budge when I revved it. Except for a small coolant leak I think my problem is solved. I'll drive it close to home for the next few days to be sure.

I am still thinking I have coolant seal issues, but time will tell for sure. I'll post back if that winds up being the case.

Thanks for all the help here

 

Yes, I also think "Dave" is right

 

You called it. I just elaborated...

 

Yep, I definitely just quoted the last person who posted there. Thanks to both Adam and Dave.

Unfortunately I see more sweet, white smoke on startup than I used to, so I’m going to do another coolant pressure test but it’s almost definitely rebuild time. At least I got to enjoy it for a few hundred miles...

 

If it gives white smoke in the summer, the writing is on the wall :/

 

Out of interest, what is the maximum safe limit? Mine has read 100degrees C a few times on the pfc commander. Mind you, this is when I've turned the engine off and back on again after going to the shop or refueling etc.

It sits around the 85C mark when moving but quickly rises to the mid 90s when in traffic. This is with a fmic

 

100 is hot, but ok if stable. 110 is getting rowdy.

 

When you turn off a hot engine coolant stops flowing, and the coolant heats up. This is normal. Once the coolant starts flowing again, you will see temps drop back down.