I have a 2004. The car significantly overheated on the highway few days ago in stop and go traffic. It was a hot day. In the 90's. Did not max out the needle, but got close. I turned on the heater, the ac off and cruised to get air over the engine. The temp went up and down for the remainder of the trip home, most often in the normal range. Both the rising and falling temperature happened quickly. Less than a minute up once the needed started to move, and 5 min down when it started to fall.

I had not been seeing coolant under the car. There was no indication of a leak.

When I got it home, there was some vapor coming out of the overflow, but the coolant level was not low. I read the posts here, and have some experience with cars. It sounded like a thermostat problem to me.

I replaced the thermostat, the water pump since I had things torn down, and both belts. Refilled the system with coolant, let idle with the cap off to bleed the air out. It came to normal temp and stayed there. I put the cap on but not all the way - unpressurized- and drove the 4 mile square around my house as a test drive. The temperature never got above normal. Getting back to the house, steam was coming from the overfill cap.

I took it off and coolant shot out under high pressure.

Two questions:

1) Shouldn't I be able to run the engine with the cap off without it boiling over?
2) Since i replaced the water pump and the thermostat, and it still seems to be getting too hot to boil the coolant off, what other components should I be looking at? How should I test them?

 

If the coolant temp needle moves on an RX-8 (or nearly any other Mazda for the past 15 years), then your coolant temp is already past critical levels.

Our needle starts to move at a temperature that damages the coolant seals and warps the engine, and it can do that on the first event. Not always, but most of the time. Repeatedly doing it is playing russian roulette with a loaded gattling gun. If you got the needle nearly to the far right, there is no way your engine escaped without damage.

 

Regarding testing components--it seems a compression test to check engine damage may be the place to start if it got that hot. Changing other things will not help if the engine is bad.

 

Why are you replacing parts if you haven't diagnosed the cause of the problem?