WRX Info

Frequently Asked Questions

Engine FAQ: Cooling System

What type of coolant should I run in my car? You can never go wrong by using Genuine Subaru Coolant available through your local dealer.

Is the Subaru coolant pre-mixed or not? They have both kinds, but BE SURE to read the labels carefully as they are nearly identical in appearance except the wording on the package!

What aftermarket coolant can I use? This Subaru .pdf document states Subaru's position on which type of aftermarket coolant you can safely use.

Is there a required coolant additive? Yes. Subaru Cooling System Conditioner has recently been required to be added with every coolant replacement to prevent coolant system leaks. This recommendation applies to every Subaru model for every model year.

What is the best coolant? The "best" coolant differs from stock SOA advice, so use at your own discretion. Ensure you choose one that contains no silicates and no phosphates. Silicates act as a cleaner, but also eat away at water pumps and seals. If you mix a coolant containing phosphates and water with a high mineral content, the phosphates in the coolant may “drop out” and form deposits in your cooling system that can lead to corrosion. Only recently have aftermarket coolant manufacturers started producing silicate and phosphate free coolant. Generally speaking, the types of coolant that are compliant are the newer "extended life" coolants. However, you should carefully review the product labels and/or product websites to ensure they are silicate/phosphate free.

What is the best coolant ver 2.0? As far as the best coolant for removing heat from your engine, it would be 100% water. You might even see better heat removal capability if there is a coolant additive mixed with it. Obviously, 100% water should be used in racing only applications and only by advanced users. Its use in a normal passenger car is NOT recommended even for short use. Water has a very high heat capacity. The reason it is mixed with ethylene or propylene glycol is to lower the freezing point below 32F and raise the boiling point above 212F. Both water and ethylene or propylene glycol are essential to the coolant's function.

Can I use the new blue colored Subaru Super Coolant in my pre 2008 Subaru? My edited for clarity response from SOA when I asked this question on 5/29/2008 is below.

We can only recommend that you use the coolant that is recommended for your pre 2008 Subaru. This is because we have not tested or researched using the Super Coolant in previous year models.

Best wishes,
Nancy Quinn
Subaru of America, Inc.
Customer/Dealer Services Department

What types of coolants are available? Ethylene glycol (EG) and propylene glycol (PG). PG is a newer coolant and can either be marketed as one of the new "long life" coolants or "environmentally friendly" coolants. Performance characteristics between the two types are fairly close, so either would be fine for use. PG coolants could be considered to have a "slight edge" based on comparative data though.

What about coolant enhancers? Coolant enhancers are made by several different companies and have several different uses. The main use is to provide increase surfactant action of the water/coolant mix. Surfactants allow a better contact between the coolant and the engine wall surface, which aids in the heat exchange process. Enhancers are fairly inexpensive, and may bring down the temperature a few degrees. These products tend to overstate their claims though. Actual temperature reductions are significantly less when used with coolants. They often advertise "minus XX degrees" on their packaging, when in fact these figures come from using straight water instead of coolant. When used with coolant, temperature reduction of enhancers is greatly reduced. Popular manufacturers include:

Neo Synthetics' Keep Cool
Royal Purple's Purple Ice
hy-per lube's Super Coolant
Red Line Oil's Water Wetter

Should I use tap water, deionized water, or distilled water when mixing? Best waters to use are in this order:
1. Deionized water
2. Distilled water
3. Tap water

Never use straight deionized water in a cooling system, as it by itself, it is VERY corrosive and caustic.

Are there any advanced coolants? Evans and Engine Ice could be considered advanced. These should only be used after considerable research on the pros and cons of each fluid.

What about the freezing point of my coolant and the proper mix ratio? With a 50/50 ratio (Subaru recommended) of coolant to water -37 C / -35 F is the freezing point. Different coolants using different ratios will result in different freeze protection levels. Check the coolant product label for specific information as each coolant manufacturer utilizes additives that can affect freeze temperatures. If you live in an area where temperatures may go below these values, check with your Subaru service center for their recommendations on coolant dilution ratios.

What about coolant colors? Currently, there is no color standard. This means that if you have green coolant in your radiator, you should not necessarily buy green coolant to top off your radiator. In this situation, you can do two things if you do not know the specific coolant type:
1. Use a multi-compatible coolant. Some manufacturers have coolants that will work with many varieties of coolants. These products should also be silicate and phosphate free. Carefully read the manufacturer's label for these details prior to purchasing.
2. Perform an engine flush and replace with new coolant. Note coolant manufacturer and type for future reference.

Are there compatibility issues with coolants? Yes. Some coolants are not designed to be used with other types. Ensure you read the product label for compatibility.

What about higher pressure radiator caps? Higher pressure radiator caps increase the boiling point of your coolant. Higher pressure = higher boiling point. Higher coolant pressures also transfer heat from the cylinder heads more efficiently. There have been several reported cases of higher pressure radiator caps causing ruptures in the endtanks on any OEM Subaru radiator. These should generally be reserved for aftermarket radiator users.

What about decreasing the temperature rating of the thermostat? These are generally not needed by the average or above average user. These are more for advanced users with greater cooling requirements due to racing applications or highly modified vehicles. When you get to this point, you will find the shortcomings of the OEM cooling system and will determine which thermostat best suits your needs. This link and this link provide lot of good information on switching to a cooler thermostat. Many also consider this to be a reliability modification even on a stock vehicle.

What about aftermarket radiators? These are generally not needed by the average or above average user. These are more for advanced users with greater cooling requirements due to racing applications or highly modified vehicles. When you get to this point, you will find the shortcomings of the OEM cooling system and will determine which radiator best suits your needs. They are a great upgrade for the weak plastic end tank equipped OEM radiator though, so if OEM failure occurs, an aftermarket unit is worth looking into. Popular manufacturers include:

Gimmick Motorsports

What about aftermarket or "slim" fans? Just say no. Always use OEM fans as aftermarket fans usually negate the good effects of an aftermarket radiator. This thread shows the basics of stock fans = good, aftermarket fans = bad.

How often should I change my coolant? Refer to and use the maintenance schedule in your owner's manual for Subaru coolant. For other coolant, refer to the coolant manufacturer's recommendation.

How do I replace my coolant?

Page 4 of this Subaru .pdf document also provides coolant replacement tips.
This webpage details coolant swaps, and radiator and thermostat swaps that may be helpful.
This link also has a step by step fluid replacement guide.

Any tips on avoiding air pockets in the coolant system? This thread has some great tips.

What about a DIY coolant flush? There are generally three ways to perform this:
a. After the system is drained, many fill up the radiator with regular or distilled water alone. Then heat cycle, cool, and drain.
b. After the system is drained, fill up the radiator with a "radiator flush" product and water. Then heat cycle, cool, and drain, and then run clear water through to wash out any traces of the product.
c. After the system is drained, fill up the radiator with a 50/50 ratio of white distilled vinegar and water. Then heat cycle, cool, drain, and then run clear water through to wash out the vinegar. The vinegar is acidic enough to remove any scale or impurities, but not harsh enough to harm your seals and gaskets. Many users believe this type of flush to be gentler than commercially obtainable "radiator flush" products.

Flushes as described above should only be performed by advanced users.

Should I perform a coolant flush? Not needed at all and in fact SOA recommends against it according to this TSB. However in a later TSB, 01-166-08, says "Subaru of America, Inc. is proud to announce the approval of the Wynn's PowerFlush III coolant exchange machine. The PowerFlush III was chosen for it's three separate tanks that allow the ability to perform a clean water flush of the cooling system in Subaru vehicles. A clean water flush is possible by filling one tank with fresh water, one tank with new coolant and one tank used for removed liquids." Other machine can/do re-use fluids and as discussed above mixing fluids is a bad thing. So....if you want a machine flush, ensure the shop has a Wynn PowerFlush III, otherwise just say no.

What about topping off my system? Don't sweat it. Just add water and be done with it as the minor amount of water you add isn't enough to dilute your overall coolant capacity enough to change much. If you are super worried anyway, you could top it off with pre-mixed coolant, but that's a little on the "it puts the lotion in the basket" side. And if you think you need to add .1 ounces of Subaru Cooling System Conditioner when you top off...you might want to get a new tin foil hat.