My 2002 WRX began overheating today. My coolant was low, so at first I thought that was the problem. After filling it up and driving home, the temperature was still in the red.

 

There is no visible leaking so I'm guessing it's not a hose problem. I took the cap off the radiator and started it up but didn't see any flow. The fans do come on.

 

It overheats both when driving and idling.

 

I'm thinking this is a water pump problem or a head gasket problem but I have no idea how to check for either. Any tips?

My 2002 WRX began overheating today. My coolant was low, so at first I thought that was the problem. After filling it up and driving home, the temperature was still in the red.

 

There is no visible leaking so I'm guessing it's not a hose problem. I took the cap off the radiator and started it up but didn't see any flow. The fans do come on.

 

It overheats both when driving and idling.

 

I'm thinking this is a water pump problem or a head gasket problem but I have no idea how to check for either. Any tips?

 

My vote is for the water pump. I had a similar thing happen with my Legacy, I replaced the water pump and thermostat and the problem went away.

My recommendation would be to throw in a fresh thermostat, refill and COMPLETELY purge the system of any trapped air.

 

After that, make any further assesments about possible "big" problems. During testing, avoid pushing the engine temp past a mild overheat event unless you want to damage it. e.g. If you see that the temp is still tending to rise above normal, that's enough. :-\

You can also leave the the thermostat out and check the system, and it will usually run cooler than normal. Make sure you do burp it. If it does work, I would have to say the pump is fine.

i vote for the thermostat.

 

possibly the radiator fans not coming on. when it starts to run hot, pop the hood or listen real good and see if the fans turn on.

You can also leave the the thermostat out and check the system, and it will usually run cooler than normal. Make sure you do burp it. If it does work, I would have to say the pump is fine.

You can't leave the thermostat out, the system will not seal without a thermostat in there. I took an old thermostat and gutted it so that at least the system would seal. A hint about getting rid of the air bubbles. On the thermostat that is a little "one way flow type" wire thing, take a pair of side dykes and cut it off, that will leave a small hole that any air can pass through, this hole must be positioned at the top of the hole for the thermostat.

Now things are a bit more complicated. I was sold on the water pump idea until I saw some flow through the radiator. My original post was after I had the car running for a while with the radiator cap off and no coolant leaking.

 

A few hours ago I tried the exact same thing except this time coolant began gushing out. This means that not only does the water pump work, but so does the thermostat. The fans also work no problem.

 

So now I'm fresh out of ideas. I was hoping it was the water pump as that would have been easy to fix...

Now things are a bit more complicated. I was sold on the water pump idea until I saw some flow through the radiator. My original post was after I had the car running for a while with the radiator cap off and no coolant leaking.

 

A few hours ago I tried the exact same thing except this time coolant began gushing out. This means that not only does the water pump work, but so does the thermostat. The fans also work no problem.

 

So now I'm fresh out of ideas. I was hoping it was the water pump as that would have been easy to fix...

 

I think it will be even easier to fix. Agreeing with previous responders, I'm certain you're airlocked, Search "airlock", "burp" (this really should be a sticky).

 

Have you had your cololant changed recently?

 

You have all the classic symptoms of an air bubble. FIX it, or you will have a blown HG. BTW, I LIKE the little thermostat trick.

 

Also, 'kamaloakid', are you from Hawaii?

Nothing much to add except that you are vaporlocked, and need to burp the system. the coolant gushing out was what got pushed out when the air trapped in the system expanded and pushed the coolant in front of it. . .

The secret to filling these engines: Undo the top rad hose from the rad and fill directly into the engine block.