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Possible solution to preventing blown HG!


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Surfing around and found possible solution and the real reason why HG blow in Subaru's


This is from the Toronto Subaru Club(TSC)


FAQ - Blown Head Gaskets on Phase I&II EJ2.5 - Toronto Subaru Club


Originally Posted by [b

Quote[/b] ]

by mutant_dan@ Jan 14, 2005 at 09:42 AM




From: *Gene Goldenfeld <genegold@

Thu Jan 13, 2005 *8:06 pm

Subject: *Re: [outback] Head Gasket failure info


There is an interesting discussion about head gasket failure in 2.5L

Subaru engines over on the subaruvanagon mailing list. (This is a

mailing list for people with VW Vanagons who have swapped in a Subaru engine to replace the VW water boxer.)


Indeed. In the interest of getting the discussion over here (and

providing me a copy to archive), I've copied Al Wick's posts on head

gasket failure from the Subaruvanagon group. I let him know.





I was in unusual situation where I was able to find the head warp

"gasket fail" problem before the dealers were getting any failures.

Many years ago I adapted a new 2.5 (10k miles) to my airplane. I have

sensors galore on the plane, so when the head first started leaking, I

was able to look at the data on my laptop and see that the head leaked

pressure to coolant system 3 seconds after I hit full throttle. It then

dissipated 5 seconds later. I really learned a lot and deliberately

operated at full throttle for long periods (hour or two). All the time

logging the pressures and temps every few milliseconds.


So here's the deal. All failures are caused by air in the cooling

system. No air, no problem. If you have air bubble AND you operate

at high throttle settings, the head will warp. It takes a long time to

show up. So if there was trapped air 6 months ago, then you might

now start to see discolored coolant, overheating. Or it might only occur

climbing a hill in hot weather. Only the 2.5 has this marginal

condition. All other Subaru's bullet proof.


My flying partner makes a living replacing gaskets on 2.5's these days.

Number one repair item. *It appears that the 2.5 has an area at center

of block/head interface which doesn't have enough coolant flow. When a

bubble passes by, it boils locally. This eventually causes head to

warp, gasket to give out. Subaru has tried 3 different style gaskets,

even adding coolant conditioner to improve heat transfer. Still a

problem. But absolutely all failures caused by trapped air in system.

All models of 2.5 liter the same.


The solution is very simple. Just drill and tap your coolant crossover

pipe and add a fitting that allows air to leave engine and rise to your

swirl pot. You will never have a problem. I operate my engine full

throttle for hours at a time. Fabulous engine.


-al wick



I got a couple other private messages regarding the same. I'll try to



The crossover tube is the aluminum coolant tube that lives under the

intake manifold. It's rectangular in shape and the main coolant hose

attaches to one end of it. It's the one everyone reverses. I'm unable

to say there is a "best place" to add a fitting to the tube. Likely it

does not matter where you place the fitting. Just somewhere in the top

of this tube. The fitting needs to be on this cross over tube because

this is the highest point in the ENGINE cooling system. Your goal is to

purge any air that happens to be in the engine. This is different than

purging air from your radiator or from your heater core. Air in the

engine causes head warp. Air in other components just reduce their

efficiency. We imagine that air flows with the moving coolant, but

actually it only does to small degree.


Drill and tap this tube. Install a fitting, and run hose from that

fitting to your swirl pot. No dips in hose, it must gradually rise to

swirl pot. As long as the swirl pot is above the engine it will

automatically purge all air from the engine. This results in a "robust"

cooling design. Robust meaning it handles unusual conditions. If your

brother in law borrows your vehicle, blows a hose, and doesn't realize

he needs to bleed air, no problem. Because your system automatically

purges all air from engine.

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Swirl Tank:

The coolant flow in automobile engines is not always fast enough or "up-hill" enough to ensure that all air will be purged from the system. Some automobiles have a bleed valve to release trapped air when filling the cooling system. Engines also generate "air" when they are running at high power settings. The "air" is steam caused by small areas in the block or head where boiling occurs. A lot of builders (different engines) have found that they do need to run coolant/air bleed lines.


Experience with the Ford 3.8 Liter V-6 indicates the need for bleed lines in some installations. The cooling system flow and pressures must be understood to know where to connect the bleed lines.


The size of the bleed lines is important as any coolant that flows through them is really shunting around the heads. A good size for the line is 1/4 inch inner diameter.


When steam is generated within the engine it is important to keep the steam from getting to the radiator. It is better to bleed the steam off to the atmosphere if necessary to avoid having it go through the radiator. Generally, steam is not produced for very long periods and only a small amount of coolant will be steamed away.


A swirl tank may be needed if coolant is being blown overboard but the average coolant temperature is well below the boiling point (including the effects of a pressure cap).


The swirl tank acts as a centrifuge, keeping the heavier material (coolant) to the outside of the tank and the lighter material (air and steam) to the center and top of the tank. There is a 0.10 inch diameter hole drilled in the top center of the tank for the air/steam/coolant to bleed off. A 1/4 inch diameter hose takes the air/steam/coolant to the header tank. A typical swirl tank is show below. This method is also used to "de-foam" oil.

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I'm actually very tempted in doing this to my own car. The only other people that have done this that I have found on the web are airplane guys that use the EJ25 for small aircraft.


I'm going to do some more research into this one.....


I don't see any good reason why you couldn't just vent it into the overflow reservior. Maybe the hot coolant gas is too much for the plastic tank?

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That's another thing that would have to be figured out. I guess an easy way to figure if the valve is doing anything is by the ease to replace coolant after a flush. I've haven't personally replaced the coolant yet but I've read that it can be difficult to fill with correct amount of fluid.

It seems that alot of HG failures occur after a coolant replacement.

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What fluid are you suposed to use? Just regular antifreeze with distilled water? My subaru mechanic says that the head gaskets go from people not using distilled water. He says that hose water has stuff in it that eventually eats at the gasket or drying it out? Dont know if this is right. Mine has 132,000 miles and no problems with this yet, "knock on wood", and I live in Phx,Az where it gets hotter than s*%t. Iam in the process of replacing alot of part and dont know if I should just do them now or wait till it starts running hot. By my temp guage this car runs cool even in the summer. What do you guys think?
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I use the premix 50-50% anti-freeze. When replacing the fluid, I use the bleed valve on the top left of the radiator. Remove the plug and pour slow. (pee stream) An old Subaru mechanic taught me that. Next time I will also use the prescribed Subaru treatment added to the new anti-freeze.
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  • 3 weeks later...

Hate to be the spoiler here, but not sure if you can prevent it! I have an '03 Legacy SE Sedan which was NOT supposed to have any HG problems (they were miraculously fixed). I am suspecting, once again, that my HG is leaking. Here's the skinny on my repairs:


8/2004, mileage = 21K: Left side, external leak found. Dealer put that SUPER DUPER conditioner in to fix the leak.


1/2005, mileage = 26K: Smelled coolant (maple syrup), taken to dealer. They pressure test and of course, no leak found.


2/2005, mileage = 27K: Still smell coolant, I bitch, I'm right! They find the leak (finally) and replace left side HG.


10/2005, milage = 36K: Smell coolant again, I'm right again. Left side replaced a SECOND time.


8/2006, mileage = 45K: Smelling coolant again, resevoir low. Guess what I think it needs?


There seems to be a pattern developing. The last two gaskets only lasted 9K. I told Subaru I kept my last Legacy up to 136K. That means 10 more times for a HG replacement for me!

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There seems to be a pattern developing. The last two gaskets only lasted 9K. I told Subaru I kept my last Legacy up to 136K. That means 10 more times for a HG replacement for me!



You try a different dealer? Maybe they aren't using the correct amount of torque.

Grand Ams, and other GM vehicles with the 3400 V6, have had similar problems with their LIM (Lower Intake Manifold) gasket leaking due to that reason. Main cause is poor design.


At least Subaru doesn't use Dex-cool, that stuff will eat your engine in a hurry if it gets in your oil.

Friends don't let friends drink cheap beer.
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It seems that alot of HG failures occur after a coolant replacement.


Our previous generation NA Outback was a real biotch to bleed when replacing the coolant. I had the dealer do it the second time.

It is still ugly.
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