Technical Info

Subaru Cooling System Basics

This last week, a friend of mine was experiencing issues with his cooling system and his car began to get hot. His car is a 99 Legacy L Sedan with the 2.2L, so it was highly unlikely that a head gasket would be the issue, unless he overheated it, which was not the case.

I went through basic troubleshooting steps with him. Thermostat, radiator, water pump, etc. His follies prompted me to think more about the common misconceptions of Subaru cooling systems, despite him being knowledgeable about working on cars. So here is my list of the top 4 things you need to know about Subaru cooling systems.

1.) Only use OE Subaru parts

OE stands for Original Equipment. This is not something most owners of Subaru really know about. Most DIY mechanics, and even most shops, will purchase their parts either wholesale or from the local parts store. I cannot tell you how many times a car overheating was due to not using OE parts. Although it is tempting to save a few bucks and get a part advertised as “better than OE”, you’re not doing yourself any favors. My buddy purchased a failsafe thermostat from the local parts store and couldn’t figure out why his car was still getting hot. Here’s a side-by-side comparison of the Stant versus OE.

2000 Subaru Outback OE Thermostat|2000 Subaru Outback “part store” Thermostat

As you can see, the thermostat on the left (Subaru OE) has a much larger temperature sensor and has a wider primary valve (evident by the size of the lip area) than the parts store thermostat on the right. In my friend’s case, the parts store thermostat wasn’t opening fully and not providing enough flow causing the bottom part of the radiator to be cold and the top was hot.

Thermostats aren’t the only component that should be purchased OE. Another critical component is the radiator cap. An OE radiator cap has a stricter pressure tolerance than a parts store cap. This is important and will be explained next.

2.) There can be no air bubbles or opportunity for system leaks

It may sound like a no-brainer, but many modern cars rely on a closed pressure system to help the proper flow of coolant. Referring back to the previous point, a parts store radiator cap has a wider margin of error than an OE. Typically, when you shut off your car, the cooling system builds much more pressure than when it is running. This is due to thermal expansion and the conversion from an active cooling to passive cooling. This pressure builds up at the highest point of the system, the radiator cap, and vents into the overflow tank. If you notice, the radiator cap has a spring and valve, similar to the thermostat. That spring is a fixed pressure spring that only open up at the right pressure and closes when the pressure drops. It is designed to open when the pressure is the greatest at engine shutdown. Caps with lower tolerances will open up during the drive cycle, releasing pressure and causing the thermostat to not have enough pressure to cycle colder coolant back into the system.

The same principle exists for air bubbles in the system. Air bubbles cause similar issues, but also have dangerous implications if left in the system for too long. The cooling system relies on pressure expansion of the coolant to help bleed air. Liquid is not easily compressed, so it vents through the coolant overflow tank, whereas air is easily compressed so it prevents the pressure from venting into the overflow tank. This causes extra strain on your hoses and, if severe enough, causes hot spots to form in the water jacket and causes cylinder detonation/ping/knock, which can be deadly to an engine, and cause the cooling system to heat up faster than the radiator can cool it causing overheating.

If you have a hole in your radiator, a leaky cap, or air in your system, take care of it ASAP. I have a video on how to burp your cooling system.

3.) Use quality coolant and additives during maintenance

In the first step, I outlined the importance of using OE, but coolant can go either way. Using a quality coolant at the proper 50/50 water ratio is key to staying cool. Back in the early 2000’s, Subaru Coolant Conditioner was introduced as a fix to stopping premature failure of the head gaskets, and while it worked, it wasn’t the solution for many engines that ultimately failed. Today, Subaru still uses the coolant conditioner during maintenance intervals and should should be used in all engines, except those using the new Subaru blue coolant, as it is already mixed in. You can purchase it here for around $6.00 a bottle or a two pack for less than $10.

Sticking to the recommended maintenance schedule is also important. Over time, the coolant loses its pH balance and becomes acidic due to the presence of a slight electrical current in the engine. When the coolant becomes acidic, it starts to corrode the head gaskets and seals internally until they fail. The primary purpose of the conditioner listed above is that it helps to maintain the proper pH of the coolant, but does not do so indefinitely. The secondary purpose is to serve as a stop leak when gasket and seal corrosion being to happen.

4.) Never use any stop leak/gasket sealer/liquid block sealer.

On many older vehicles, when a gasket starts to show signs of failure, a stop leak or sealant in the coolant can help fix these issues or slow their progression. For Subaru, this is not the case. Subaru engines have very small tolerances, as low as 5/1000th of a millimeter, before failures begin to appear. At this low of a tolerance, conventional parts store cocktails do not work, not to mention can cause costly damage to an engine.

For example, I worked on a Subaru that someone had used a head gasket sealant that contained silica. The Subaru experienced the worst kind of head gasket failure where it mixed coolant and oil together. Silica hardens when heated, which is what makes it effective as a head gasket sealer, but the granules of silica are not fine enough to get to where they would prove effective. Worse yet, the sealing surface between the oil galley and coolant journal experiences no excessive heat, like the coolant journal to combustion chamber does. In turn, the silica mixed with the oil and found its way into the oil journals and eventually to the bearings causing it to severely damage the crankshaft, main and rod bearings, as well as damaging the camshafts. The whole long block was garbage. Don’t do it. Not even for a day. It makes the difference between a few hundred dollar repair and a one way trip to the junkyard for your car.

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