Hey, I have a 2005 Mazda 3 2.3 hatchback. It's a long read so I'll sum up the symptoms at the bottom if you don't feel like reading a wall of text.

A couple months ago, I was driving to work and got a check engine light. I used my coworkers OBD scanner to find the error codes and discovered that my thermostat was stuck open and I decided to replace it.

I went to Advanced Auto Parts and bought one of their thermostats and had my mechanic put it in as I was busy. I noticed that the thermostat didn't come with a gasket and looked like it had a rubber gasket built into the thermostat body. My mechanic had put the new one in and discarded the old one.

After he was done, I drove the car a little ways from the shop and it begin to overheat. I pulled over, let the engine cool and drove back to the shop. I explained what happened and he offered to drive around with me to see if it would overheat again.

After a couple of minutes, it started to overheat. We pulled over and looked at the engine, the reservoir was FULL of coolant and it was completely still while the engine was running.

We just kept driving it and the problem seemed to fix itself, or so we thought. About a month later, I was driving to work and it started to overheat again. Same thing all over again.

I did some research and discovered that Advanced Auto's thermostats differ from Mazda's as the spring is thicker. So I took that one out and got one from Napa auto parts who claims their thermostats meet exact factory specs. I put the Napa thermostat in myself and again, did not use a gasket as it didn't come with one and had a rubber seal on the thermostat body. After installing the new thermostat, flushing the radiator, and filling it up with new coolant, burping the system and letting it idle to normal operating temp. The car will not overheat will idling, I let it idle for what seemed like an hour and everything was fine.

After driving it around my neighborhood for about 20 minutes, it started to overheat again, I forgot to mention that the car runs fine for about 20 minutes before overheating.

I've noticed the heater blows extremely hot air when I accelerate, and goes completely cold when idling, blows cold air when overheating regardless driving or idling.

When it does overheat, I turn off the engine and screw the radiator cap slowly to release the pressure within the reservoir. It seems to reset when I do this so it leads me to believe that the cooling system may be over pressuring. It lets me drive for another 20 minutes then I have to repeat this.

After trying with no success, I took it to another mechanic who vacuumed out the air in the cooling system, it worked for about 3 weeks but then started to overheat again.

I'm suspecting it's the radiator cap as I've tried everything else, but I figured I would try here and see what this community has to say or if other mazda3 owners have experienced this same problem.

Too long, didn't read symptoms:

New Thermostat but old radiator cap.

Car drives normally about 20 minutes, then overheats.

Heater gets extremely hot when accelerating and goes dead cold when idling but stays cold regardless when overheating

The reservoir is completely filled with coolant when overheating and the return lines are completely still.

If I slowly open the cap, let some of the air pressure escape, the reservoir coolant sucks right into the engine and the hot coolant comes out of the return lines and fills the reservoir and it begins circulating normally for about another 20 minutes then overheats again.


Any help would be greatly appreciated.

Thanks,

-Mike

 

I think I posted this in the wrong section. sorry!

 

other post is at https://www.mazda3club.com/mazda3-mazdaspeed3-98/2005-mazda-3-2-3-over-heating-nightmare-56366/

 

Test in this order:
- Is coolant level low? Pressure test for leaks
- Bleed your coolant again
- Pressure test rad cap
- Confirm fan is working
- Confirm no debris blocking radiator
- Test thermostat to exact degree - if good go onto next step, if fails replace with Mazda part
- Test ECT sensor

If issue still not found report back! Could get into more serious issues if the vehicle has over heated a bunch and none of the tests above solved your problem.