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.

Also, the car WILL NOT overheat when idling.


Any help would be greatly appreciated.

Thanks,

-Mike

 

It's not the water pump as the coolant does circulate for the 20 minutes before overheating. The Radiator fan does turn on, at least when idling. No visible leaks as well.

 

stuck open code would mean the engine thought it was running too cold. so a thermostat or possibly the temp sensor itself could be what was bad. Using the OBD tool, you can read the temp sensor and see if it looks like it's heating up normally (reads ambient temp when sitting overnight, and should max out at about 190F at idle).
You can then look at it while driving to see when it starts overheating.
BTW, does the cooling fan come on when at idle? or when it's in its overheated condition? I'm not sure if the first thermostat replacement may have ended up with the fan disconnected or something.


You said that the coolant resivoir was full? like overflowing or just to the normal level?
I haven't really looked at mine to see, but I think it should be still when running.
If it was well above 'full', there could be a head gasket leak pushing air into the cooling system, displacing coolant, and ending up with an overheated condition.



"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."

This seems very strange, like maybe the first mechanic that replaced the thermostat put the tubing back incorrectly?

 

The reservoir is well above the max line. Filled up to the cap. I always fill to the max line and let the first fill get into the engine and radiator then fill to the max line again.

Im pretty sure it's a pressure problem, as I'm guessing the pressure between the thermostat and reservoir becomes so great that it doesn't allow the thermostat to open, hence all the backed up coolant in the reservoir. I'm guessing when i open the cap and let some pressure escape, it takes the stress of the thermostat and allows it to open.

But they may be leak somewhere in the head gasket letting the air in. If that is the problem, what do you think I'm looking at to repair it?

 

I'm having the exact same list of problems, but in my case I have changed the radiator cap, thermostat, coolant sensor, Radiator (this actually blew out on me), and my radiator fan (this was not running even when my engine was overheating).

Tonight I will be pressure testing the cooling system and doing a flush before the weekend. I have already done the block test and there are no exhaust gasses in the cooling system from that unless they are getting in during my drives around the neighborhood but I am not sure how this would happen since it does not at any other time I have tested it.

 

hmm have to checked if the waterpump is working fine?

 

Forgot to list that, water pump has been changes as well, flushed my lines and problem persists. I'm just going to tear it down and change the head gasket as that is my last resort.

 

i'm not a profesional, so my info could only be taken as a grain of salt.

but it sounds like the fluids are not circulating in the system, that's why the when the engine over heats, your heater is not hot because the fluids are not being circulated into the heater core.

Like I said I would double check if that water pump is faulty. Next thing I would check is pressure. Are you losing any radiator fluid at all?

 

New pump and old pump are good I flushed the entire system and know 100% my thermostat is opening. My only concern now is once it is apart I may not find anything wrong with the gasket and actually have a cracked head or even worse the block may be toast.

 

Before tearing into the engine, make sure the radiator isn't plugged / fins bent that's preventing cooling.
use a clear hose to see the fluid flow

 

Its a brand new radiator, whole system has been flushed as well.

 

Hopefully you've fixed this problem by now.

The thermostat needs to be place with a rubber gasket. The thermostat is sold without one , go figure. If you replace the thermostat without the gasket ,coolant will leak on the side of the thermostat. Air will also get in the line ,thus affecting pressure. You need to find the old gasket, mine was amazingly still in place but not set properly.By luck it didn't end up in the engine.

 

I replaced my stuck open thermostat with the OEM one..... like you said many people have had issues with aftermarket ones, and it was ~$50 shipped for the improved OEM one. The new one IIRC did have a gasket mated to it already, so that's all I put in place with it. I imagine the one your mechanic put in had one too.

 

Ja, laugh . I actually learned to put it myself. I have a problem paying 150 for just three screws .

 

Well three screws for the thermostat, three for the tensioner pulley I removed to get to the thermostat, those PITA retention clips on the hoses, and refilling the coolant.

Still not worth paying someone else to do IMHO.