Ok, I cannot figure this one out for the life of me. Two weeks back I had been driving the gst all day before work...when I pulled into my parking lot, I noticed the temp gauge creeping towards the red. I immediately shut it down and let it cool off. I noticed that I heard coolant bubbling in the overflow tank. So I checked that and sure enough the overflow tank was full and a ton of hot air was being pumped into it(causing the bubbling). So I thought it might have been the thermostat getting stuck shut...had it changed the next day. Things seemed to be going well after changing the thermostat. I noticed a couple times that I still heard bubbling in the overflow tank...but didn't think much of it.
Fast forward to today, I drove the eclipse maybe a total of 50 miles...pulled into my driveway and parked it...noticed that while the turbo timer was counting down my temp was going up...so I shut it down immediately. Once again, the coolant overflow tank was overflowing some and a ton of hot air was being pumped through the return line into the overflow tank.

Main facts:

No other symptoms of leaky or blown headgasket

No oil in the coolant

No coolant in the oil

Both times the car has had this problem, after it's had a chance to cool off, I check to see if the coolant has returned to the system out of the resevoir...both times it has.

Temp remains a touch under half way under normal driving

Only seems to overheat once I park it after running it semi hard for 50 or so miles

No coolant leaks anywhere that I could see

Thermostat was changed maybe 500 miles ago

Water pump was replaced in January

Nothing blowing out of the exhaust upon start up

Both fans are working properly

Cooling system was flushed a few months back

No smoke whatsoever

Oil pressure is perfect

Temp sensor has not been changed since I've had the car(which leads me to believe it's never been changed)

Radiator cap looks fairly clean and new...but also has not been changed while I've owned the car

Mods to be noted: dejon piping from smic to intake mani, dejon intake pipe, type rs bov(recirc'd obviously), hallman mbc, 5zigen catback, autometer boost gauge.
Running ~13psi right now


Only things I can think of is that I've either got an air pocket or something else trapped in the system or the radiator cap has taken a shit on me
Throw some ideas at me...I'd like some input before I do anything else.
Thanks in advance for any input!

2007 wrx

"Both times the car has had this problem, after it's had a chance to cool off, I check to see if the coolant has returned to the system out of the resevoir...both times it has."

Does this mean your coolant is always full?

It means that I'm not losing coolant anywhere and it's being drawn back into the radiator.

I got under the car today and checked all the lines for leaks...didn't find anything.

One other interesting thing I discovered today while I was inspecting everything...DSMotorsports neglected to change my water pump(I still got charged for it) when I had them do my timing belt service. So that might have something to do with this whole mess. I've never met anybody as lazy as those bastards...never take your cars there

2007 wrx

Ok, one more update...

I just went out and let the car idle for about 15-20 minutes. There was no overheating, passenger side fan kicked on for a little while and then turned off, no coolant overflow when I turned the car off. There didn't seem to be any steam pushing through the return line that leads to the overflow tank. Both fans kicked on when I tried the ac, which by the way shot out nice and cold. There was no new coolant on the driveway after I shut her down. Also, there was no smoke when I revved it a few times.
I did notice two things I wasn't too sure about though. First, I noticed a very very small puddle of water in my exhaust after shutdown. Two, the upper radiator hose did not get hot and did not feel like it had any pressure(maybe because the car was just idling?).

So, do you guys think it could just be a faulty radiator cap that's causing the problem after being driven for a little bit?

2007 wrx

my guess would be air pocket somehow. i would also go back to dsmotorsports and get your money back for not changing the waterpump. how do you know though?

just wait for it to overheat, as soon as it does pop the hood and bust that cap off and let the coolant go everwhere, let it cool down with the cap off. top off, continue on your way. wash, rinse, repeat until it doesnt overheat anymore.

now, dont forget to have a big rag or old shirt to wad up and use to take the rad cap off/block your body from the ensuing volcano. it gets hot in there.

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It could just possibly be the cap, it's cheap so go for it.

It's sounds like the coolant isn't flowing properly though, or there's not enough in there. Have you added any coolant since it overheated? Did you flush the system yourself? Have you taken off the cap while it's cold, then start it and watch to see if it starts flowing through? Are you sure the T-stat isn't on backwards?

I'll tell you what I did when I had an overheating problem in the GS-T.. I replaced the T-stat, upper and lower radiator hoses, and the cap. I filled it all back up and ran it for awhile. I started looking around after awhile and I finally noticed coolant squirting out of the TB to T-stat housing hose a small amount, There was no leak on the ground because it was landing on the tranny. I don't know if it was leaking before or if the old cap was leaking and then by the added pressure from the new cap made it crack or what. I also then got a HUGE hole in the water pipe to the oil filter housing hose. After all that I have had no more problems.