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Discussion Starter #1
Hey guys hoping I can get some advice as I'm completely new to Subaru and just bought a 2006 Subaru Impreza WRX STI. Here's what happened:

Started driving my car after it sat all day long for about 10-15 minutes driving up to 55-60 mph, only revving the car to around 5500 rpms. This is stop and go traffic btw. All of the sudden I pull to a red light and see smoke barreling out of the hood scoop. I look at my thermostat and its reading few notches above the second line. I immediately pull over to the side of the road, lift my hood and find coolant/antifreeze all over the engine bay. I wait a little bit open up the black reservoir to see I'm completely empty of coolant. Of course I look under the car to watch for leaks or drips but there wasn't any.

Get coolant at the gas station, fill the reservoir back up, fill my radiator back up. (I have an after market Mishimoto radiator). Looked for leaks, didn't find any. After about 30 minutes of waiting I hop back in, watch the thermostat all the way home, drive on the interstate at 80mph with ZERO issues. Thermostat rose to the normal right above the first mark and stayed there. Drove it again a few times without any issues.

Today now, I drive it again, driving the same way and noticed the thermostat was reading back above the second tick mark. I pulled over, car still running and look under the hood. No issues whatsoever. Hop back in drive the car for a while and it eventually cooled back down slowly to wear it normally is.

UPDATE: Today driving in the pouring rain the thermostat went higher, close to the three marks by the H indicator on my thermostat. I quickly tuned the car off, stopped and let it rest a little before driving it again. Just put coolant inside the night before. No coolant explosion or anything just reading super hot.

WHAT IS THE PROBLEM? Bad thermostat? The dealership thought it was initially a blown hose or a leak from somewhere but I looked and there is nothing leaking under the car. Car drives perfect besides for that issue. They also thought that it was pressure built up and it blew through the cap or overflow. Any advice or thoughts let me know.
 

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Could be a bad thermostat. Probably is if there are really no other symptoms. Inexpensive and very easy to replace if you feel that might be the problem, or even just to rule it out.

Be clear:
Are you regularly adding coolant to keep it full when it has not overheated?
What does you exhaust look like at startup?B
You didn't mention having just drained the coolant prior to thus issue and sometimes it is fine.
 

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1: the car is overheated enough to blow its cap (like 20psi or maybe idk what those caps are rated at)
2: make sure the gauge is not reading an air bubble
3: thermostats can be tested in boiling water for function

the first thing you want to make sure of is that its not a blown head gasket or cracked block. A compression test will rule that out, or they make color changing fluid that senses exhaust gases you can put on your radiator. As stated a replacement thermostat is still cheaper than those two options if you dont have a compression tester. Its quite the simple system; the pump is shot, the thermostat is shot, there is air in it, or there is a leak(could be head gasket, hose etc - anyway that makes the system unsealed). (maybe blockage/serious corrosion but i doubt that)
 

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Discussion Starter #4
Thanks for the reply guys. So I just bought the car last Monday from a dealer here in Sarasota FL. Car has 152,000 miles but looked like it was really kept up with. The guy did put a bigger Garret turbo on the car so could that be an issue?

So far I have put in almost two 5 qt jugs of coolant, one Thursday evening and one on Saturday evening. I bought a new thermostat hoping that's the issue. Like I said car runs fines. Ive noticed when I start it, the revs reach 1500 then kinda fluctuate around 700 to 1300 rpms. I just bought AMSOIL signature oil which should arrive today to do a complete oil change. Also have an appointment to take it to the dealership Friday fro a free inspection over the whole car.

I haven't done a complete coolant flush or anything. I did let it run with the cover off of the coolant reservoir to help deplete any air bubbles in the system. I let it run for about 5-10 minutes then put the cover on and drove home. I'm sketched about driving it now so its just gonna sit as much as possible.
 

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I have no ldea what this issue is, but I will say Subaru radiator caps do wear out. I replace mine with every coolant change. Additionally 2006's have two caps and it's imperative they are in the right location. Also I would use only Subaru OEM caps as they are a critical part of the cooling system and aftermarket caps could introduce new problems.

The intermittent gauge readings probably mean there is air in the system. The way these systems are filled is critical. You take off both caps and fill the system slowly (from empty) from the upper reservoir. Sometimes an air bubble will form in the radiator so it needs to over flow a bit to make sure it bleeds out. Then cap the radiator and finish filling the system. There might be a pocket left here or there, but it should bleed itself out. Just make sure you keep the overflow at the fill mark until the system stabilizes.
 

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SubySal`s advice is excellent. Add it to the thermostat.

But, and its a big but. Garrett turbo? This changes everything. Not because its Garrett, but because it's not stock. The really bad part of this is that it makes bigger problems far more likely.

I suppose there are dealers capable of fixing a "built" car somewhere, but none that I am aware of. Dealers near me don't want to touch em, and just return them to stock. You need to find a good Subie shop that can deal with the car.

Check for oil in your coolant and coolant in your oil and for coolant in exhaust.
Rule out the caps and thermostat and simple air bubble.
Then be prepared to tear into it or get it to that shop.
 

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I agree with SubySal. First you need to make sure the cooling system is properly bleed first before going any further. Its also a good idea to let the car heat up and then completly cool off and adjust your coolant level after. Then if this issue continues to happen, then proceed with further diag.

The fact that you bought a car with a large turbo and now having cooling issues worries me.
 

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Update. I found the leak. The upper radiator hose had a nice little cut in it and began spewing coolant out once warmed up. I replaced the upper radiator hose, put new coolant back in and started driving again. ANOTHER ISSUE...

Now I'm driving the car here in Bradenton FL, it was 102 degrees. I'm driving up to 6,000 rpms through stop and go traffic. Everything seems fine. I then get to the Subaru dealership (getting my fluids checked to see what needs replaced. I just bought the car two weeks ago), when I put the car in reverse and park. I come back about 5 minutes later and notice coolant is in a puddle coming out under the passenger side. I then noticed the leak started dripping once I put the car in reverse, you could still see the drips on the pavement.

I'm not sure what it is or how but there was a decent size puddle of coolant all over the ground. I then manage to get home with no real issues. Pop the hood, my coolant is low and ultra low or non existent in my radiator. I've then filled the radiator up with water and driven the car yesterday and today with no issues. Car heats up and stays right on target. I've driven it hard for a few minutes hoping I could get it to react again.

Any ideas?
 

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A blown head gasket will cause these problems. I had issues close to what you are describing I replaced my radiator caps and the car ran great. I thought I was in the clear. Until I drove the car really hard and the coolant overflowed. What happens is a blown head gasket will allow exhaust gasses to be pushed in the cooling system then cause it to over flow and the thermostat not to work.
 

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I recently had an air pocket which overheated the car. Bled the system properly the second time and coolant temps are averaging between 195 - 205 now.

Question, is it normal to "top off" the overflow reservoir tank more often in the summer heat if radiator and expansion tank has the correct amount of coolant and no signs of leaks or air pockets? Currently the overflow tank has coolant a bit under the L line when cold start or driving for two - three hours. Sometimes the tank looks empty when looking from the top of the engine bay. Makes me nervous...
 
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