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Discussion Starter #1
2001 OB, 206K miles, replaced almost everything in the cooling system including HG. OEM T stat.

I've been battling an odd overheating problem from some time. My latest riddle has to do with the radiator cap. I've had a LOT of gurgling noise and just recently the coolant that flows into the overflow wasn't being returned to the radiator. So, I took a non-OEM rad cap, bent the tabs in some to create a "tighter" seal. Drove to and on the highway, overheated like crazy. Coolant was boiling in the overflow tank, which was completely full and over flowing. The good news is that when I parked and it cooled down the overflow was emptied back into the raditor. Today to put the new OEM cap back on, no boiling but back to a LOT of gurgling.

Is it possible that the cap I "modified" was too tight?

Some additional info... I've used a new OEM cap, non-OEM cap w/ a pressure release lever and a 2nd OEM cap. with each of them I have gurgling.
 

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go to the dealer and get 2 bottles of the coolant conditioner $2.50 each, i think. add them to the coolant.

it sounds like you have a head gasket problem. the coolant conditioner may help if it is an external leak. if it is an internal gasket leak the only solution is to have the gaskets replaced.

these engines are also difficult to fill with coolant without getting an air lock. search for ''burping'' or ''burp'' to find info on the proper way to fill.
 

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Discussion Starter #3
I've gone through all the HG trouble shooting... replaced the HGs, water pump, timing belt, etc. Did block leak test, was negative.

Not sure why with the "modified" rad cap I'd have no gurgling but boiling / over heating then when I put the OEM cap on, gurgling but no boiling / over heating.
 

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Just a guess here
Water has a higher boiling point under pressure, the cap you modified may not be holding the correct pressure therefore allowing the coolant to boil whereas the oem cap my be holding back the pressure and only opening when it reaches higher pressure which could possibly produce a gurgling sound
 

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I doubt you could bend the tabs enough to significantly alter the function of the spring inside. You MIGHT stop a leak at the top of the radiator, directly under the cap.

If your properly filled and burped system (with a 170 deg OEM-style large t'stat, and functioning rad fans) is boiling while idling, you should try a chemical headgasket test available from the parts store.
 

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Discussion Starter #6
I doubt you could bend the tabs enough to significantly alter the function of the spring inside. You MIGHT stop a leak at the top of the radiator, directly under the cap.

If your properly filled and burped system (with a 170 deg OEM-style large t'stat, and functioning rad fans) is boiling while idling, you should try a chemical headgasket test available from the parts store.
Doesn't boil on idle, unless it was already boiling. Once it started boiling, continued ever with engine off until it cooled. Chem HG test was done and was negative.

I cleaned the radiator filler... I've read that if its not clean it might not alow for a good seal. I can't imagine that the small about of grit I cleaned off could make a difference but with this car, who knows.
 

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Discussion Starter #7
So, I picked up a new rad cap, 16psi, no more boiling but still getting a lot of gurgling plus once coolant is pushed into overflow not all of it is returning to radiator. My guess, the system is sucking in air from somewhere when cooling....
 

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A split hose on the overflow or split nipple on the radiator or leaky hose to radiator connection will cause the car to draw air back into the system as it cools vs drawing fluid back into the system from the over flow. Check those things first could be a very simple leak in the over flow hose set up causing issues.

Also as mentioned you need to burp the air out of the system and top it up. Common mistake. Take the cap off let the car sit idling for a few minutes while the coolant is circulated and any air trapped in the system gets circulated out - top it off at the radiator - and put the cap on. The over flow bottle should always have coolant in it with left over capacity for expansion ie overflow coolant - it should never be empty which case the cooling system will draw air into the system as it cools down.
 

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Probably a long shot, but easy to check: make sure the opening in the end of the hose in the overflow tank is not sitting flat on the bottom of the tank. Pressure from the cooling system can push coolant into the tank, then the hose opening seals itself to the bottom of the tank preventing return flow. To fix, either pull up the hose a bit or cut off the end at a 45 deg angle.
 

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Discussion Starter #11
I've replaced the overflow hose and snipped the end @ a 45% angle just to be sure. I've also place a small hose clamp on the overflow outlet from the radiator.

I think I'm going to try and burp the system AGAIN this weekend becuase I think theres definately some air in there. I've actualy been getting some return of coolant to the radiator recently but maybe only an inch or so from the overflow. When the over flow is at a normal cold level and I drive it for an hour or so the overflow fills within 2 inches from the top. It doesn't usually get so full that it spills out.

I've been manually sucking the coolant back in the radiator every couple of days. This helps reduce the "over heating" and makes it at least drivable.

We'll see.
 

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I've been having a very similar problem to this and what I've found is that my timing belt tensioner was Junk. It's about 2 years old now (not OEM my bad). With the tensioner shot the timing never skipped. The toothed belt still had plenty of tension to drive the cams on the toothed side , but the water pump is driven off of the smooth back side of the timing belt and with out proper tension it's not turning the pump and not circulating the water. another symptom of this was a knocking at start up that would go away shortly after. the tensioner is banging on the stop in there.
This is starting me to thinking that an awful lot of the Subaru gasket problems might be caused by poor circulation at the start of the problem????
Also just for the record I think Subaru wasn't thinking when the put the T'Stat at the bottom of the system Hot water rises.
 

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That seems a feasible option.

The placement of the thermostat is integral to controlling the temperature and as a safe guard. It's placement helps control head temperature more efficiently and if the radiator becomes low of coolant the engine will remain full even with the thermostat full open because the pump pulls from the bottom and forces it up.
 

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Discussion Starter #15
Solved?

So, I think I figured it out. After replacing the heater core inlet and outlet hoses and the bypass hose I still had air in the system which lead to the coolant being pushed into the overflow and overheating. After replacing the hoses the coolant was returning from the overflow into the radiator though so that was a step in the right direction.

Picked up a leak test kit and tested the **** out of it today, no exhaust in cooling system. Tested it after idling for an hour, RPMs up and down, negative. Drove for about 15 minutes, until it started to overheat, still negative.

I put in a bypass to take the heater core out of the loop and no overheating! There seems to be a lot of pressure in the radiator hoses but it is released some when the termostat opens. I am also getting a few bubbles in the overflow. Still tests negative for exhaust gasses though. I my guess is there is some air in the system form when I opened it up to install the bypass.

Next step, install new heater core. Anyone have a "how to"?
 

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This is my first time here so excuse me if I make a mistake. I was wondering if you replaced your lower radiator hose? Sometimes the lower rad hose will go bad on the inside and look good on the outside. As you drive faster it will collapse and cause overheating. The lower hose should have a spring inside it so it does not collapse. The lower is also usually bad for sucking air because of a bad fit or loose or misplaced clamp. When the engine is cool squeeze the lower hose by hand. If it collapses replace it. Also is there any chance your cooling system is contaminated with soap or cleaning solution or oil. Any of those things will cause the system to foam up and overheat. If you flush a system you have to rinse it out with clean water before adding the Anti-freeze. Oil will coat the inside of the cooling system and prevent the anti-freeze from transferring heat to the radiators metal surface. Good luck!
 

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Discussion Starter #17
This is my first time here so excuse me if I make a mistake. I was wondering if you replaced your lower radiator hose? Sometimes the lower rad hose will go bad on the inside and look good on the outside. As you drive faster it will collapse and cause overheating. The lower hose should have a spring inside it so it does not collapse. The lower is also usually bad for sucking air because of a bad fit or loose or misplaced clamp. When the engine is cool squeeze the lower hose by hand. If it collapses replace it. Also is there any chance your cooling system is contaminated with soap or cleaning solution or oil. Any of those things will cause the system to foam up and overheat. If you flush a system you have to rinse it out with clean water before adding the Anti-freeze. Oil will coat the inside of the cooling system and prevent the anti-freeze from transferring heat to the radiators metal surface. Good luck!
I did replace the lower hose, upper hose, heater hoses, bypass hose (small rubber hose off water pump), Tstat (oem), water pump, etc. Flushed the system a couple of times, the most recent with a chemical flush additive and ran water through to rince as instructed.

Since I put the bypass in to take the heater core out of the loop I've had no overheating issues at all. It's been perfect (knock on wood). I'm wondering if it could be a clogged heater core... i'm going see if I can flush it out this weekend and give it a shot. i'm willing to take the 10 minutes to avoid taking apart the dash, especially since winter is just about here in NE.
 

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I'm wondering if it could be a clogged heater core...
A few years ago there was a post here that suggested exactly this.

The heater core is an essential part of the coolant path. The core connections are at the crossover pipe at the top of the engine (hottest part, going to the heater) and at the intake to the water pump. At the pump, the returning coolant passes over the thermostat sensor element, and it is alleged, has a significant impact on the thermostat.

When the engine (thermostat) is cold, coolant flows through the heater core, providing a continuous path for coolant to circulate through the engine block. As the circulating coolant heats up, the hotter returning coolant will eventually cause the thermostat to open, and from then the thermostat and coolant returning from the heater maintain a balance.

If this is correct, then what happens if the heater core is partially, or completely blocked? The engine will initially warm up but there could be hotspots in the block because there isn't the flow of coolant through it. Eventually, the coolant at the bottom of the block, at the pump inlet will heat up, and the thermostat will open somewhat. At slower speeds the engine block itself is very efficient in shedding heat through the aluminum. But at higher speeds, although there's more air passing through the engine compartment, there's also more cooling through the radiator. The thermostat is more dependent on the flow from the bottom of the radiator to the pump, and because this tends to be cooler, the thermostat might not open as far as it should. The coolant in the engine can move above normal, but this isn't detected, except by the temperature sensor in the crossover pipe, because the heater core isn't providing an adequate return path for the overheated coolant to hit the thermostat sensor. The "balance" that is so critical, has been upset, and the engine overheats. (The radiator fans can come on, but because there's less flow through the thermostat, it has less effect on engine temperature than it would if the t'stat were wide open, which it should be when the engine temperature goes above normal.)

Okay, so that's "the theory". There was another post that did exactly what you did -- loop the input to the heater core directly to the output. This resulted in the same effect -- no overheating, supposedly proving the theory. However, as I recall, there was another post (not sure if it was in the same thread) where, instead of bypassing the heater core, the pipes to it were terminated -- in other words, the loop from crossover pipe to pump inlet was opened. This should have led to overheating, but apparently there was no impact.

Without more "scientific" tests it's not clear, one way or the other. I tend to lean toward "the theory", but not to the point of arguing for it. Perhaps others here will recall those posts, or have their own experiences to support or refute "the theory".

in the meantime, if you find that the flow through the heater core is initially slow or blocked, that a flush clears out noticeable deposits, and that after flushing the overheating doesn't re-occur, then that would be one more weight in favor of it.
 

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I hope this will help save you some headaches. Check and see if your heater works(puts out heat) and leave it alone. If you flush it you could cause a leak.
The heater core plays no part in cooling an engine. Think about it, if it did than we could only drive our cars in winter? Because we do not use the heater in summer right? Sounds to me like you might have fixed it by something you did and just didn't know what it was. The last heater core leak I had cost $600.+ on a Jeep Grand Cherokee.
 

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If the heater core is clogged, you could overheat the engine before the thermostat opens as it will delay when it opens.
 
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