Sounds like a silly question...but bear me out.

I've checked the radiator cap numerous times at the track yesterday and today and it never gets that hot. I can leave my hand on it while the coolant temp is 206 degrees. This doesn't seem normal to me, but I can't remember ever really checking the radiator cap in the past...

I figure that there is some air in that little channel that the cap connects to, but when I filled the system last night, I filled the radiator all the way up the first time (and have done so numerous times in the past--this is what the service manual says NOT to do).

I'll be ordering a new radiator cap, just to rule that out.

Car runs like a champ and I can 5th and 6th gears pulls on the autobahn with no problems. Not leaking coolant anywhere I can see...doesn't burn it. Oil is good. No smoke. No coolant fumes in exhaust. No gas fumes in overflow tank.

I'm stumped.

Todd

Yes my rad cap deffinitely gets hot, too hot to keep my hand on it anyway...

On our 02 WRX there is only one cap--on the coolant tank. That one definitely gets hot on my 04 STi. The 03+ have a cap on the radiator too...that's the one I'm talking about.

Todd

Yes, it gets too hot to touch when it's fully warmed up.

If the cooling system is operating properly, the radiator cap WILL get hot.

There are actually three (3) caps in the U.S. cars (see comment on person from Germany).
1. Radiator cap
2. Reservior cap (reservoir tank near turbo)
3. Overflow cap.

Two (2) possibilities:
1. Bad radiator/reservoir caps and/or caps not properly secured.
2. Bad thermostat/improperly installed thermostat

From your description, it sounds as if the coolant is not flowing properly. The radiator and reservior caps should get hot, especially the way you are describing your driving.

Sounds as if the coolant is boiling in your system, hence all the bubbles.

More likely your thermostat is not working properly. Either it could be installed wrong or it is bad. You can get bad ones even if they are new. If the thermostat does not open, the coolant will not flow through the radiator and cool. The hot coolant flowing throught the radiator WILL make the radiator and reservoir caps get HOT.

I don't know where the temp sensors are located. If they are in the radiator, and if the thermostat is not opening, or not opening properly, you will see cooler temps in the radiator, and the fans will still come on.

Well, there are only two caps that hold any type of pressure. The coolant tank and the radiator. The overflow tank is not pressurized. The coolant tank one has been replaced and tested to be good.

My thermostat is fine. It was tested before installation and opens on cue (easy to see while watching the temp gauge--not the stock one--when the coolant temp drops after steadily climbing during warm up). The radiator itself gets very hot during normal operation. Under normal operating conditions the system operates as normal, and you'd never know there was an issue. I've had bad thermostats before...this isn't one.

That leaves the radiator cap...which more and more I think may be bad, so I'm ordering a new one tonight. If it is allowing an air bubble to form in the channel that goes into the radiator top, as the hot coolant flows in from the block, it may be boiling in that local area (due to relatively low pressure). I may tie off the rubber tube from the radiator overflow outlet to see if that stops the bubbles. Only problem with that theory is I can let is idle all day and there aren't any bubbles. Temp gets up to 204/206 and the fans come on...temp goes down to 188. Cycle repeats.

There are some other possibilities, but they seem like long shots - head gasket, water pump, constricted turbo cooling line superheating coolant...


Thanks.

Todd

Yes, it gets too hot to touch when it's fully warmed up.

Yea, that's what I thought. Thanks.

Very weird to have the radiator hot, but the metal cap only "warm."

Todd

Well, there are only two caps that hold any type of pressure. The coolant tank and the radiator. The overflow tank is not pressurized. The coolant tank one has been replaced and tested to be good.

True. Guess I should have said "There are actually three (3) caps in the U.S. cars coolant system" but obviously, you knew what I meant