Subaru head gaskets are about as hot of a topic as national health care lately. If you own a Subaru you know what I mean.
Since the 2.5 liter Subaru engine was put into production there have been thousands of head gaskets replaced across the country. We’ve done a good portion of those ourselves. In another post I’ll go into further detail about the details of Subaru head gaskets but for today I would like to offer some suggestions on how to help prolong the ones you have.
Subaru head gaskets can fail for numerous reasons. Failure of sealant, improper torque, surface imperfections in the cylinder head or engine block and of course heat or excess heat.
I’m going to focus on excess heat. Within the engine block the coolant is circulating to keep the engine cool due to internal combustion occurring. The coolant remains in contact with the metal and is able to absorb heat, travel to the radiator and release the heat to the atmosphere.
Two important things must occur to for the coolant to do it’s job. It must have adequate flow to move the heat away from the internal areas of the engine and it must remain in contact with the areas it’s trying to cool.
There are 3 very important items that may individually have a negative impact on the coolant’s ability to do it’s job. If all 3 components are bad or inferior, problems could develop even sooner.
Subaru Radiator Caps OLD vs NEW
Keeps coolant in a sealed system, allows overflow to exit and return as coolant expands and contracts, but most importantly it raises the boiling point of the coolant in the system by keeping the cooling system pressurized. Most radiator caps for stock vehicles keep the system pressurized between 13-15psi. This can raise the boiling point depending on the mix of coolant/water an additional 35-40 degrees. A 50/50 mix of anti-freeze and water has a boiling point around 265 degrees. Add to that a radiator cap that holds 13psi and now you have coolant that won’t boil until 300 degrees .
There are areas throughout the engine where the coolant circulates that become very hot. So hot that it can boil coolant. Since we must have the coolant in contact with the metal to perform it’s heat transfer duties we now have a problem. Scenario: Radiator cap is weak (which we run into all the time on Subarus). A weak radiator cap not holding pressure may not let all of the coolant get hot enough to boil but there are areas within the engine that are now boiling. Boiling coolant has air bubbles that now keep the coolant from contacting the cylinder walls and other extremely hot areas within the engine. This heat is more than the engine and gasket were designed to withstand on a regular basis and thus a situation that will accelerate the failure of the gasket.
Flow of the coolant is important also. To keep from boiling the hot coolant must be quickly moved away from the hot cylinder walls up to the radiator so it can release it’s heat. Below is a picture of a Subaru water pump and also a quality Japanese aftermarket water pump. Although we for the most part believe in genuine Subaru parts, here’s a case where genuine Subaru part’s may not be the best choice. Note the stamped steel vanes on the Subaru pump vs the quality cast and machined impeller on the Japanese counterpart. The tight clearances and defined impeller vanes are very efficient at moving coolant through your Subaru engine. (an interesting side note that older Subaru water pumps were made nearly identical to the pump on the right).
UPDATE: WATERPUMPS WITH CAST IMPELLERS AS IN THE PICTURE ARE NO LONGER AVAILABLE. ALL WATERPUMPS HAVE SUPERSEDED TO WATER PUMP SIMILAR TO THE LEFT. UNFORTUNATELY PROBABLY THE RESULT OF A MANUFACTURER TRYING TO LOWER THE COST OF PRODUCTION.
Another important part of keeping the hot coolant flowing out to the radiator and away from the internal hot spots in the engine is a high quality thermostat. I’ve shown below the comparison between a generic aftermarket brand on the left and a genuine Subaru thermostat on the right.
Note the Subaru version has a much larger spring, larger diameter central area for coolant flow and is made of steel and brass. The generic brand contains copper, a big no no with Subaru. Subaru actually states that copper in a Subaru cooling system is ill advised and may cause excessive electrolysis and corrosion.
Even after trying to be dilligent about providing the best possible cooling for your Subaru you still may need to cross the head gasket bridge some day. If you ever get to that point, Smart Service will have a solution for you. We now offer new and improved aftermarket head gaskets which we believe will be the last ones your car will ever need. I’ll make a future post with more details about their construction.