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Everything You Ever Needed To Know About Cooling Systems

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176K views 64 replies 46 participants last post by  choi0706  
#1 · (Edited)
On Subaru cars there are a lot of problems that could happen to your cooling system. Head gasket failure is probably the biggest and least understood problem that comes up on RS25 at least once a week. So I am taking it upon myself to create a Cooling System Write-Up. I will try to list and explain all components of your cooling system and how critical they are to your car. I’m hoping it will give people a better understanding of their cooling system and help them diagnose their own problems in the future. Hopefully I don't crash and burn.

First things first. I recommend only Genuine OEM Subaru Parts. OEM parts were designed specifically for our cars. They fit perfectly. They are designed to the exact tolerances and specifications. There is no substitute for OEM quality. I have heard to many stories about people buying aftermarket parts and having them not fit correctly, not work properly, and cause more problems than what the owner had before. In my opinion, it's just not worth the hassle. But if you are upgrading to a high quality performance part, go for it!

Coolant: Subaru recommends using their own Genuine Long Life Coolant/Antifreeze in all Subaru vehicles. According to Subaru, this genuine coolant contains antifreeze and anti-rust agents that are especially designed for the Subaru engine. Primarily because of its aluminum crankcase. They claim that other coolants may cause corrosion within the Subaru engine. They also do not recommend using any additional anti-rust and corrosion inhibitors.

Color of Genuine Subaru Coolant (SOA868V9210 or SOA635002) is a darker forest green. It is far from the normal neon green antifreeze that is commonly bought at your local parts store. Also when mixed 50/50 with water, it is thicker than any other coolant on the market. It has an extremely sweet smell to it as well.

Also Subaru recommends running the new Cooling System Conditioner (SOA635071) in all Phase II EJ25 engines. Due to a poor design in initial head gaskets, internal and external coolant leaks may occur. They recommend adding 1 bottle of the conditioner every time you have the cooling system serviced. Never add this product to the coolant reservoir. Always remove coolant from the radiator and add the bottle straight into the radiator. Then top the radiator off with coolant until full.
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Water: Subaru recommends using pure tap water when mixing your coolant. And since pure tap water doesn't exist, Distilled water is the next best thing. Tap water or spring water contains many minerals that can form deposits within your cars cooling system. Over time they will deteriorate your cooling systems capabilities to keep your engine at optimal operating temperature. Deposits can plug your radiator, heater core, thermostat, and other areas of your cooling system that has small passages.

Ok, your cooling system is consists of your radiator cap, radiator, radiator hoses, thermostat, thermostat housing, water pump, passages through your engine block and cylinder head, head gasket, coolant temperature sensor, heater hoses, and a heater core.

Your cooling systems main function is to reduce heat within the engine. A normal combustion chamber will experience temperatures from 1500-4000 degrees F. Normal operating temperatures for a Subaru are 158-212 degrees F. So as you can see, there is a lot of heat that the cooling system needs to get rid of. And only a properly functioning and well maintained cooling system will due.

There are a lot of different problems that can arise within a cooling system. So I will break them down by individual component and try to explain why things happen the way they do.

Radiator cap – The function of the radiator cap is to let pressure build within the cooling system to a safe level. Stock radiator caps will hold pressure up to approx. 16 PSI before it will for open the spring and release coolant into the overflow tank. The purpose of keeping the coolant under pressure is to increase the boiling point of it. The higher the boiling temperature, the more efficient your cooling system will be. When the engine is shut off and is cooling down, the radiator cap will let the coolant that ran into the overflow tank go back into the radiator to keep it full and not let any air enter the system.

When a radiator cap isn’t functioning properly numerous problems can arise.

1. If the radiator cap is stuck shut, it will raise the pressure of the cooling system considerably. In this case it is very possible to have a radiator hose burst or even have the radiator burst as well. Any pressures over 16 PSI can be fatal to cooling system components.
2. If the radiator cap is stuck open, it will cause overheating problems. Without having the cooling system properly pressurized, the coolant will boil over into the overflow and not be as efficient as it should be. This will raise the coolant temperature and cause your car to overheat.
3. If a radiator cap is not opening properly during the engine cool down, it can cause radiator hoses to collapse. Pressure is built within the hoses whenever the temperature is hot and the coolant is expanded. Once the coolant cools, it will try to suck extra coolant into the system through the radiator cap. If the cap does not open and allow that coolant to flow, the hoses will pull a vacuum and collapse. If this happens, remove the radiator cap and you will find the hose will “inflate” again.
4. A stock radiator cap is designed to only hold 16 PSI of pressure. The STi cap holds around 19 PSI. The extra pressure created by these caps will most likely cause radiator failure when used on a stock cooling system.
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With any of these symptoms, it wouldn’t be a bad idea to just replace the radiator cap and give it a shot. Radiator caps are relatively inexpensive and do go bad over time. It’s probably the cheapest and easiest part to replace on your cooling system.

Radiator – Your cars radiator serves one purpose. Reduce cooling system temperatures (on AT cars, it cools the ATF temperatures as well). Your car needs to be between 158-212 degrees F to operate as efficiently as it can. So the radiator is basically a piece of pipe that is coiled in rows from the top to the bottom. Those coils are covered in fins that act as heat sinks. As the coolant passes down through the radiator, the heat dissipates from the tubing to the fins. Then as air passes over the fins, the heat is removed from the cooling system. (When the car is idling and there is no airflow across the radiator, your electric radiator fans will kick on to help remove heat from the radiator.) And if you ever touch your upper radiator hose, you will find that it’s hotter then the lower radiator hose. That’s how efficient a cars cooling system is.
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Problems that can arise from a radiator.

1. A radiator can cause a car to overheat for several reasons. If the radiator is plugged, it won’t let the coolant circulate as fast as it needs to. When the coolant isn’t being cooled properly, heat will rise within the cooling system. Corrosion and rust are the two main reasons why radiators get plugged.
2. Also if the fins on the radiator are bent, proper airflow will not dissipate the heat as well as it should. This will also cause cooling system temperatures to rise.
3. Anytime you see coolant leaking from your radiator, have your radiator replaced immediately. NEVER trust any kind of radiator stop leak in a bottle. It will only clog up the rest of your cooling system creating more problems for you in the end.
4. Also, stock radiators can only handle up to 16PSI of pressure. By adding an aftermarket radiator cap, it’s been know to burst the radiator end tanks.

Radiator hoses – There really isn’t anything spectacular about radiator hoses. But their condition is vital to the proper functioning of your cooling system. Anytime you find a crack or bulge on a radiator hose, replace it immediately. It only takes a little bit of extra pressure to cause that hose to break and leave you stranded along side the road. Always check the condition of your radiator hoses and heater hoses during oil changes and any other under hood maintenance. And if one is bad, replace them all. They aren’t that expensive and replacing them now will save a lot of headaches down the road.

Thermostat – Your cars thermostat serves one purpose. To get your engine to operating temperature as quickly as possible. The quicker your engine gets to operating temperature, the better efficiency your engine will have. When your car is initially started, the coolant temperature is the same as the ambient air temperature outside. Your engine runs it’s best around 190-220 degrees F. So the quicker the temperature rises, the better your fuel economy will be and better performance you will get out of your engine. The idea of the thermostat is to stay closed and only leave coolant circulate in the engine until operating temperature is met. Once the coolant reaches a certain temperature, the thermostat will mechanically open and allow that coolant to circulate through the rest of the cooling system.
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Problems that can arise from a bad thermostat,

1. If the thermostat is stuck shut your car will come up to operating temperature normally. But once operating temperature is met, the thermostat won’t open as it’s intended to. So your car will continue to build heat within the engine causing an overheating condition. Some times you will see your temperature gauge rise and if a worst-case scenario, you may blow a radiator hose off. Either way, it’s not good for your engine.
2. If the thermostat is stuck open, you will realize your car won’t come up to operating temperature as quick as it should. In some cases, operating temperature will never be met. You may drive your car and find that it will build up some heat, but once the car is left sitting at idle, all heat that was built up will be lost. The thermostat is letting coolant circulate through the entire cooling system at all times never allowing the engine to build up the heat that it needs to run efficiently. Any time your thermostat is stuck open, your car should never overheat. It’s acting at all times as if the engine was at operating temperature and it won’t let any heat build up.
3. Anytime you have your cooling system drained, it’s a good idea to just replace the thermostat. It’s a small price to pay for assurance that you have a new thermostat. The thermostat is located on the bottom of the engine block where the lower radiator hose enters the engine. The plastic joint that connects the hose to the engine is the thermostat housing. Just drain the cooling system and remove the 2 10mm bolts and pull the housing off. Clean all of the corrosion off of the housing and install a new thermostat with a new gasket.

Water Pump – Your water pump is the heart of your cooling system. It’s run off of the timing belt under the timing belt covers. The only part of the water pump that is visible is the bottom of it where the thermostat housing is bolted to it. Whenever your car is running, the water pump is working at all times. It circulates the coolant through the engine at all times to ensure that your coolant doesn’t build up any hot spots when the thermostat is closed. Then when the thermostat is open, it circulates coolant through the entire cooling system. There is a bypass hose off of the water pump to allow the coolant to circulate throughout the engine while the thermostat is closed.
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Problems that can arise with the water pump,

1. The most common problem with our water pumps is that they will start to leak over time. Since your water pump goes through so much stress, over time the gasket will start to leak, seals around the pulley will start to leak, or the water pump may even crack. If you are experiencing coolant loss, look under your car where the lower radiator hose meets the water pump. If you see coolant dripping off of the housing, chances are it’s the water pump. Also as the coolant dries, it may leave a dried green crust around the thermostat housing. Anytime you see these symptoms, your water pump probably needs replaced.
2. Water pumps may start to make noise over time. Since they are always spinning at about the same RPM as the engine, the bearings within the pump will start to go bad. Normally you may hear a faint humming noise when your car is sitting at idle, but when the RPM’s are increased, the humming noise will increase with the speed of the engine. Anytime this happens, your water pump needs replaced. If the bearings are bad enough, it may cause the pulley to wobble and throw the timing belt off. This can be catastrophic to your engine.
3. Anytime you replace your water pump, it’s recommended to replace your timing belt as well. If you have the work done at a shop, there shouldn’t be any extra labor costs due to the fact that they will have the timing belt off to service the water pump. Also it wouldn’t be a bad idea to have a new thermostat and thermostat housing installed. Even new radiator hoses wouldn’t be a bad idea. This is a really good time to do a lot of maintenance to your cooling system.
 
#2 · (Edited)
Coolant Temperature Sensor – The Coolant Temperature Sensor allows your ECU to monitor the temperature of your coolant. This reading helps your ECU decide how much fuel your engine needs at a given time. Whenever your coolant is cold, the ECU recognizes that and will allow more fuel to enter the combustion chamber through the injectors. This extra fuel helps the engine idle better and run better at cold temperatures. Once the ECU recognizes that the engine is building temperature, it will lean out the fuel making your engine more efficient and give you better fuel mileage. It also relays the temperature back to the instrument cluster letting the driver know the operating temperature of the engine.
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Problems that can arise with a faulty coolant temperature sensor,

1. A coolant temperature sensor may leak coolant over time. It is screwed into the engine underneath the passenger side of the intake manifold. Most of the time the threads have some sort of thread sealer on them to keep the sensor from leaking coolant. If this occurs, you can either get a new temperature sensor, or try to reseal the threads of the old one.
2. The biggest problems with coolant temperature sensors are bad readings. Like any other electrical sensor on your car, it can short out over time or have an open in it. Anytime this happens, you will find your car may be hard to start or idle rough. A bad temperature sensor will relay false information back to the ECU claiming the engine is running extremely cold or extremely hot. Either way, the fuel measurements will be severely off and your car will act as if it’s flooded or not getting any fuel at all. Sometimes a CEL will be set with this problem. Other times it won’t be. The best way to get an accurate reading of your coolant temperature sensor would be to use a scan tool and see what information your ECU is actually reading. Common sense will pretty much tell you whether the reading is right or wrong. Any astronomical number such as –500 degrees F or 500 degrees F should automatically tell you whether or not you have a faulty temperature sensor.
3. I personally recommend using a Genuine Subaru OEM temperature sensor. I replaced a faulty sensor with an aftermarket sensor and found that it did not fix the problem at all. The car still had a hard time starting. But after an OEM sensor was installed the car ran perfect. Aftermarket sensors must have a different calibration than an OEM sensor.

Heater Core – The heater core acts almost like your radiator. Coolant runs through the core heating it. That is really all the heater core does. Then whenever you want heat inside your car, you turn the blower fan on and it takes the heat off of the heater core and circulates it through the passenger compartment. It is very similar to a cars radiator except smaller.
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Problems that can arise with a heater core,

1. The heater core can get plugged over time with mineral deposits and corrosion just like a radiator. Whenever you experience a lack of heat within the passenger compartment, a plugged heater core may cause it. Coolant won’t be circulating as it should through the core so once the fans blow off any existing heat, it will just be cold air. To check for a plugged heater core, you will have to remove both heater hoses and run water through them. If water freely circulates through the core, then it’s probably not plugged.
2. Heater cores can develop leaks over time like a radiator. Sometimes you will smell coolant within the passenger compartment. Other times you will have coolant leak into the passenger compartment and saturate the carpet on the passenger side. It can also leave a green film on your windshield when the HVAC controller is set to defrost.
3. Heater hoses should be checks like radiator hoses. If any cracks or bulges appear, replace them immediately.

Ok…the moment you all have been waiting for.

Head gaskets – A head gasket is a thin metallic gasket that goes in between the engine block and cylinder head. All Subaru’s have 2 head gaskets. One on the driver’s side and one on the passenger’s side. Head gaskets are what keep coolant, oil, and exhaust separated within the engine’s long block. Some head gasket failure is more severe than others, but all should be repaired immediately to prevent more detrimental problems.

1. A head gasket can leak just coolant. Normally you will see a green crust form between the cylinder head and engine block. If it’s bad enough, it could also have liquid coolant dripping from it. A head gasket failure of this sort will need repaired soon, but it’s not going to leave you stranded somewhere. You may just smell burning coolant dripping from the gasket to the exhaust manifold. Just make sure you carry a gallon of coolant or water with you at all times incase your radiator gets low on fluid.
2. A head gasket can leak just oil. This should be treated similar to the coolant leak. You will probably notice a burning oil smell coming from your engine and when you check under your car, you will see that oil has been blown from the gasket and has created an oily film on anything in it’s path. Just carry an extra quart of oil around with you until you can get the engine repaired.

Ok, both problems listed above are just like any other fluid leak on your car. Gaskets and seals will go bad over time. It seems like it’s a really common problem with Subaru engines. Unless your coolant becomes extremely low, you shouldn’t have any issues with over heating. But you will have visible signs that the head gasket is going bad. You will probably also be able to smell fluid burning off of your exhaust. Definitely don’t put off having this issues repaired, but it should be ok to drive your car normally.

Head gasket problems that will cause overheating.
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(Green is water jacket, black is combustion chamber. See how relatively close they are. Just one break in the gasket will cause overheating)

This is probably the most common head gasket failure on a Subaru and most people don’t understand why they actually happen. Head gaskets don’t necessarily have to have external leaks to be bad. Most overheating engine issues do to head gasket failure will show absolutely no visible signs at all of failure. You can inspect your car all day and not find a single drop of oil or coolant dripping from your engine. This is an internal gasket leak. Whenever the gasket fails between a coolant jacket and combustion chamber, your car will overheat. What’s happening is that the exhaust in your car is pushing past the head gasket on the compression stroke and is creating air bubbles in your cooling system. A few air bubbles will be ok, but whenever those bubbles start to multiply, those large air pockets won’t let your coolant absorb the heat that is being created by your combustion chamber. As a result, you will notice the gauge in your car will shoot straight to hot. You may also smell the scent of burning coolant coming from your exhaust. The extra air in the cooling system will push coolant from your engine into the coolant reservoir and eventually blow the coolant from the tank into the engine compartment. You will probably notice air bubbles blowing from the radiator into the reservoir. That is exhaust gases escaping from the engine into the cooling system.

Head gasket problems that will cause mixing of oil and coolant.

The last form of head gasket failure is whenever you find oil mixed in your coolant, or coolant mixed in your oil. This type of problem should be fixed immediately like the overheating issue. Anytime coolant and oil mix within your engine, it can cause catastrophic damage to your engine. Once coolant is mixed with your engine oil, it will almost look like a milky substance. The oil inside your engine will drastically thin, and push past your piston rings. You will start to notice smoke coming from your cars exhaust. The coolant also takes away the lubricating properties of your oil reducing the protection the oil has on your engine. Also, the addition of coolant into your engine will raise the oil level within the engine. This will cause the oil to start foaming and may even build up enough pressure to blow seals out of your engine. Never drive a car that is showing these symptoms. Have it immediately towed to a shop or dealership.

Overheating due to an air pocket,

Sometimes air pockets will form in the cooling system for reasons other than a head gasket. Anytime you have your cooling system drained, you run the risk of leaving air pockets in the system. Also, if the coolant level is low, the system may ingest some air.

The most effective way to "burp" the cooling system is as follows:
1. Make sure the coolant is at ambient temperature.
2. Start the car and leave it idle.
3. Turn the blower fan on high and mode setting to floor.
4. Remove the radiator cap and allow the water pump to circulate coolant through the engine.
5. Anytime the coolant level drops in the radiator, top it off with proper more coolant. Make sure it's going in the radiator and not running into the reservoir.
6. Once radiator is full and no air bubbles are present, install the radiator cap.
7. Test drive the car and make sure it doesn't overheat.
8. Once back, check the coolant reservoir to make sure it is filled to the full mark. If not, add coolant til it's full.
9. Once the car has cooled back down to ambient air temperature, remove the radiator cap and top the radiator off with coolant. And make sure the reservoir is full.
10. If overheating still occurs, repeat these steps again. If it still doesn't help, you may have a bigger issue.

Testing the cooling system – There are many tools available to test your car for cooling system issues. All will help you diagnose your cars problems in some way or another. I will try to explain the 3 most common tools you will use.

1. Cooling System Pressure Tester. This device will screw onto your radiator in place of your radiator cap. When the cars cooling system is cold, there isn’t any pressure built up in it. Therefore it may be hard to find an external leak. By using this tool, you can pressurize the cooling system to any PSI to check for leaks. This tester works great for checking radiator leaks, hose leaks, heater core leaks, water pump leaks, and thermostat gasket leaks. Not only does it force coolant to leak, it will also show how much pressure you are losing as well. This tester is not really that accurate in finding internal head gasket leaks. If the leak is bad enough, it may cause coolant to fill up your combustion chamber. If this happens, you will run the risk of hydro locking your engine. But it will show if there is a pressure drop within the cooling system.
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2. Cylinder Leak Down Tester. This tester takes the place of one spark plug on your engine. Whenever you hook an air line up to the tool, it will pressurize the cylinder like your car was on the combustion stroke. When running this test, you be able to see how much pressure is dropping within the cylinder. You may also be able to see bubbles in your reservoir tank. This tool can also check for piston ring leaks as well.
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3. Compression Gauge Set. This tool acts similar to the leak down tester except while it is installed in the spark plug hole, you have to crank the engine over a few revolutions. It will show you the actual compression pressures within the cylinder. If a head gasket is leaking, you may notice a drop in pressures on certain cylinders. The ignition or fuel system has to be disabled when running this test.
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4. Combustion Leak Tester. This tool almost looks like a turkey baster. It also comes with a special fluid that can show signs of a combustion leak into the cooling system. This tool actually sniffs the coolant in your radiator. Whenever hydrocarbons are detected, it will turn the fluid in the tester a different color. This tool can almost prove that you have a head gasket failure.
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Well I hope this post will help at least one person understand their cooling system a little better.
 
S
#60 ·
solbey11 said:
EDIT:I want some people to read over this and PLEASE tell me if anything is wrong! I really appreciate constructive criticism. If you see anything I missed, or anyplace I am wrong, let me know and I will fix it. Thanks guys!
Click to expand...


great write up if you also have the knowledge in ac systems i believe that is another system people dont understand as much and can really help with a write up
 
#14 ·
Woki Toki said:
I think we should have a write-up section or like an informational section in the forums for these kinds of threads.
Click to expand...
I agree. And it shouldn't be a normal anyone can start a thread forum. I think threads should get nominated in as useful and informational threads. All info and no BS. That's a really good idea Woki Toki!
 
#21 ·
Wrexz said:
good stuff. a trick i just learned the other day to deal with this problem is to loosen the clamp on the upper radiator hose where it joins the radiator and stick a screw driver under the hose to let the air bleed out, when coolant comes out remove the screw driver and tighten the clamp and you should be good to go
Click to expand...
Do RS radiators have the "screw" on the top of them to bleed air out? I just checked my 96 Legacy and it has one.
 
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