Steam from the overflow pipe (overheating)
 

I have a 2004 rx8 with 45k klmtrs on it, had it for like 2-2.5 years, and had an overheating experience which I dont know how it was caused.

So I was doing a casual run the other day for about an hour stuck in traffic for like 20-30 mins and red lined like a handful of times the other day and the thermostat was fine and as soon as I came home saw the thermostat was 4-5 notches past the half way point (3/4 of the way to red). Radiator fans came on, and stayed on for a long time.

Service history is nothing unusual, broken cats getting replaced with straight pipes. Oil change every 18k.

Only thing was had an oily orangish fluid in my overflow tank, thought it was an O-ring failure sign. So just today, put in a bars leak powder in the overflow topped it off with 200 ml coolant and 500ml distilled water (its really hot here).

Started the engine, waited till it got to operating temp, then turned on the heater. After like 10-15 mins the thermostat needle kept fluctuating past the halfway mark, and then steam would come out of the overflow pipe. turned it off waited a few, and then repeated with same result.

This puts me at a disadvantage because I am a noob so I dont want to seem like I am experienced in any of these area's. But I know the bars leak product is a temporary fix. I also know it may have clogged a pipe. Or the problem may have been the water pump, thermostat, or something else I am overseeing. As long as its not a O-ring I will be happy.

I will get a combustion tester just to be sure about the o-ring. But has this happened to anyone else? Any experiences I can learn from? Any other things it can be?

Get the coolant pressure test.