...I'm running out of things to test and the problem persists. The car is a 2003 WRX, I have just finish rebuilding the engine and the car has been on the streets for a couple of weeks now. Doing a very good research, I found the following problems that might be related to this problem:

Faulty Thermostat
Faulty Water pump
Clogged Radiator
Clogged heater hoses/radiator
Air lock

Short long story, the car came with a few issues including a bottom engine issue. I rebuild the engine at home, and everything sounds and runs fine no oil/water leaks or mixes. I replaced all coolant and radiator hoses with Samco ones, and found the water pipe that runs above the radiator clogged - replaced it too. So clogged hoses discarded...

I ran the car with no thermostat - works great, both top and bottom radiator hoses get hot, the car takes a bit more to warm up but everything looks fine after that, no overheating at all. So faulty water pump and clogged radiator discarded...

Both heater hoses get hot, so clogged heater or pipes discarded (?)

Tested 3 different thermostats: an old one, a new one borrowed by a friend and a new one I bought yesterdey straight from the dealer. Same story: with the thermostat in place, top hose hot and bottom hose cold...

Air lock avoiding: filled the system through the turbo hose, some times through the top radiator hose, filled every single tank after that. Filled the car with the rear in a higher level than the front...

...and nothing worked. With the thermostat (any of three) in place, top hose hot, bottom cold... And I also boiled two thermostats just to make sure I didn't get three bad ones ! Both oppened without any problems in the hot water.

So, what am I missing?

BTW, I use the same procedure to replace coolant in my old STi and I have never had this issue there...

Thanks for any ideas
Marcio

 

Not sure it will help but this is the method I use (copied and pasted from elsewhere) and it has worked find so far.

Disconnect top hose (rad end) and slowly fill engine through top port until full.
Reconnect top rad hose.
Disconect the small hose that goes from the H/T to the rad (highest point of rad) at the rad end.
Fill Via H/T whilst keeping finger over the hose, when coolant comes out the rad pop the hose back on.
Continue to fill via the header tank untill a couple of cm from the top.
Fill expansion tank.
Massage top and bottom hoses to get air bubble out.
With rad cap left of, set the heater and fan controlls to medium and start car.
Allow the car a few mins for hot air to start to come through the fans, keep checking for this as you keep an eye on the level in the header tank and top up a little when required.
Massage top and bottom hoses to get air bubble out.
As the hot air is now coming through the fans, and the level in the H/T is stable with no air bubles be expelled, its safe to put the rad cap on.
Job Done.

 

Great. This is slightly different from everything I tried so far, so it is worth a test. Thanks.

I drove the car for some 60 miles without the thermostat, in a very hot day with some traffic and the temps kept good all day long. Another weird stuff is that the car doesn't overheat even with the thermostat in place without the water circulating in the system... is there a way to test the water pump properly?

Thanks,
Marcio

 

Dont drive the car without a stat in place you can cause other issues hot spots
and flow/pressure issues
given the heater works and car dont over heat with the stat removed the water pump sounds fine
1.refit the stat making sure the little bypass hole sits at 12 clock.MAKE SURE THE STATS THE RIGHT WAY ROUND....
2.remove both rad caps
3.fill via the turbo until water comes out the rad cap hole refit that cap
4.keep filling via turbo till header tank is full .
5.fill over flow bottle between min/max
6.run car with heaters on hot on full.
7.after few mins fill header tank if needed refit cap.

Note the bottom hose does not get hot really as cools very quick and will only do so after hard blast on boost etc

To test the bottom hose get car up to temp by driving it run some boost then pull up and feel it right away as it cools quick

 

TBH, if there WASN'T a difference in temperature between top and bottom hoses, the rad wouldn't be doing it's job, would it?

The only time I'd be worried is if the heater stopped working, or the system was over-pressurising.

 

Yeap, valve is with the bypass at 12 clock and with the spring end pointing inside the pump.

You both said interesting stuffs regarding the temp difference between top and bottom hoses. In my STi (only reference I have) the top hose is very hot and the bottom one is warm with the stat open and fans on.

In the WRX the top hose is extremely hot and the bottom one is cold, very cold the whole time with the stat fitted - with or without fans on, etc - so I believe that the radiator is not that efficient to make the water so cold at the bottom end (isn't??). I'm expecting a behavior similar to the STi, at least bottom hose warm to touch.

And I drove the car with stat for a few miles, same issue: hot engine, extremely hot top hose and cold - not even warm - bottom hose. Another thing I forgot to mention: with the stat installed once the fans are on, they don't come off, never. Had the car with the stat fitted with fans on for 20 minutes before giving up. Without the stat, fan came on and off without issues...

Thanks for the help
Marcio

 

SOLUTION: let me separate the two issues that were going on with the coolant system:

- top hose hot, bottom cold.

As it is a recent "rebuilt at home" engine I started to wonder if the water pump was ok - the only thing that I didn't replaced... So I removed the pulleys, timing belt cover, timing belt, cam pulleys etc and removed the water pump. Inspected it and sent a video of it to a friend who confirmed it is spinning like the way it should. Re-checked every single hose by filling through it and checking if there was water coming out of the pump place and everything was fine too. So put everything back together, new water pump gasket and filled the engine with water again, through the turbo water hose. Did a couple of cycles of turning the engine on and let it heat up, turn on the fans, turn engine off then waiting it cooling down and refill the system.

Tired of my 'blindness' I removed the oil temp gauge from my STi and installed it in the WRX. After driving for some 12 miles I could see that the oil temp was always oscillating between 88 and 93º, so no problems at all and the cooling system was just doing its job. Problem one fixed but fans still never turning off once on!

- fans that never turn off.

Even with the cooling system ok, the fans never turned off with the engine warmed up (this plus the top hose hot and bottom cold is why I was wondering if the system was working properly at the beginning). Knowing the system was doing its job, my research changed to "radiator fans won't turn off" and found this:

http://legacygt.com/forums/showthrea...ng-87297.html?

Decided to do the simplest one first: removed all 4 Fan relays that I'm sure that were working fine from my STi and replaced the WRX ones with them (the ones in the engine bay fuse box). Result: issue solved!!!

Thanks for the help again.
Marcio

 

Well done, another problem put to bed.

 

So you replaced all 4 relays? I have a 2006 Subaru tribeca acting the same way. I am going to check to see if the fans stay on. My car starts to overheat when I'm at idle. But acts better and temp gauge shows that the heat drops while I'm driving.