Mysterious Coolant Leak
 

Hey Guys,

Hoping for help finding a mysterious coolant leak. Just got this car three days ago so information is limited.

'07 Mazdaspeed3
180k miles, Engine has 50k. Swapped by a local shop 10k ago.

The problem:
Once my car has been parked after being driven (so parked when completely to temperature), it will inconsistently drain coolant.

I first noticed the coolant reservoir was empty the other day so I filled it.

Car has never overheated in the 10+ hours I've driven it. And it's not drained coolant every time I've parked either. Only drained a significant amount of coolant twice, and then no drainage that I can see every other time I've parked. Only drained twice.

So here's my best shot at figuring it out. In the reservoir there's a little drain if the coolant level tops out. Pictures attached of where I found coolant.

What would cause the coolant to go that high in the reservoir....or even boil over? Looking to diagnose and fix this in the next couple days, but I'm not quite sure how to fix that. Doesn't seem like a sticky thermostat would cause this, or some bad seal....
Just now digging into the service manual, will update with findings. Thanks for any tips or help.

Sorry for the unhelpful potato pics, just trying to show exactly where the coolant dripped. Just looks like it dripped out the little hole on the bottom of the reservoir.

Top it up, go for a good drive to get it nice & hot, turn off the engine, carefully remove the reservoir cap, attach a pressure tester, pump it up to 20psi and start looking for signs of leakage.

Thanks for the tip. Don't have a pressure tester on hand but that's a good next step.

No new info except that everything above without the pressure test (topped off, bleeding cycles, all good) and parked it, just to watch it the coolant level rise and overflow again. Drained out the reservoir.

Took a pic of the reservoir but I don't think I can post it while mobile--definitely oil in the coolant reservoir. The odd thing is that the oil is clean, just dumped it all back into the engine. No coolant in the engine oil.

There is only 2 ways oil gets into the coolant,

1: Somebody accidentally adds it or, 2: Blown headgasket.

Yep, you're speaking my language. Thanks for the tips.

I have the same problem
 

Hi i have the same problem with my mazda 3 2008. were you able to fix the problem?

I believe I found the problem, at first I changed my thermostat which I knew was going bad but wasn’t the problem, I then after changed the water pump which was also in bad shape, but wasn’t the problem, it was the coolant reservoir I believe haven’t had any leaks but it’s only been 2 days so I’ll update you if anything changed, I seen cracks on the reservoir and assume that when you turn off the car the pressure builds up in the reservoir and with the cracks it pushed all the pressure out and forces your coolant to be drained from the overfill line.

3 main problems
coolant reservoir
thermostat
waterpump

Hi @Madrich99 ,

I was able to fix the problem just by changing the repository of the coolant and cap.

https://cimg0.ibsrv.net/gimg/www.maz...626e515b86.jpg
old and new coolant reservoir