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Subaru Cooling System Solutions

Since Subaru’s 2.5L engine is especially noted for cylinder head gasket failures, most of them in the over-100,000-mile service category will have had the head gaskets replaced or will need a future replacement. There are many reasons that radiators should be replaced, especially following a cylinder head gasket failure on a 2.5L Subaru.

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Photo 1: I discovered this stack of scrap radiators at a local Subaru specialty shop. My question was, “Why?”

While researching this month’s Diagnostic Solutions, I discovered an old file photo of Subaru radiators that I took at our local Subaru specialty shop. As you can see in Photo 1, several of the radiators don’t appear to have any external leaks and are otherwise undamaged. With this picture in mind, let’s explore cooling system overheat diagnosis and repairs by using a typical 2000 2.5L Subaru Outback as a focus vehicle (see Photo 2). Since the 2.5L engine is especially noted for cylinder head gasket failures, most of them in the over-100,000-mile service category will have had the head gaskets replaced or will need a future replacement. But why the stack of Subaru radiators in otherwise good repair? Keep reading because there are many reasons that radiators should be replaced, especially following a cylinder head gasket failure on a 2.5L Subaru.

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Photo 2: The 2.5L Subaru engine is known for the cylinder head gaskets failing between 100,000 and 150,000 miles of service.


Symptoms are important. An overheat problem symptomatic with slow driving indicates that the cooling fans aren’t activating or aren’t running at full speed. On many modern import applications, a fan module that pulse-modulates the fan motor circuit controls the cooling fan speed.

In other applications, the cooling fan speed is controlled by the PCM, which activates low- and high-speed fan circuit relays. The easiest test for cooling fan function is to activate at least one fan by turning on the air conditioning (A/C) system. But the most direct and accurate method for diagnosing any modern cooling fan system is to connect an enhanced-level scan tool.


While scrolling through parameter indicator in display (PID) values displayed on the scan tool, check for trouble codes, observe applicable data streams and, if available, use the bi-directional feature to manually activate both low- and high-speed operating modes. One of the most important PIDs is the engine coolant temperature (ECT) sensor data. Use a non-contact pyrometer to compare actual water outlet temperatures with the PID values displayed on the scan tool. Allowing for some technical inaccuracies, both readings should be within 10 degrees of each other.

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Photo 3: Thoroughly inspect the space between the radiator and A/C condenser for accumulations of leaves and inspect for coolant leakage around the tank gaskets, as well.


If a Subaru’s overheating problem is symptomatic with highway speeds or climbing a steep incline, it’s usually because coolant flow is restricted through the radiator. While we’re discussing coolant flow, be aware that some cheaper aftermarket replacement thermostats can restrict coolant flow on Subarus because their physical dimensions are too small, or their opening temperature isn’t calibrated correctly. The diagnostic solution for this condition is to install either an OE thermostat or a reliable aftermarket brand of thermostat.

Most Subaru radiators, as with most import radiators, use cooling tubes with very thin cross sections that easily clog with scale or even some types of granular cooling system sealants. Although the service lives of most OE coolants extend well past 100,000 miles, the coolant additive packages designed to prevent both chemical and electrolytic corrosion eventually become degraded to the point that the coolant itself can corrode the radiator core. Following Subaru’s coolant maintenance schedule will drastically reduce radiator clogging and leakage.

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Photo 4: This Subaru A/C condenser is relatively clean. When the air flow through the condenser or radiator is restricted by organic debris, use soap and low-pressure water to remove it.

While not immediately visible to the eye, airborne organic debris like cottonwood lint or weed chaff caught between the A/C condenser and radiator can seriously restrict air flow (see Photo 3). Since aluminum condensers and radiators are easily damaged by high-pressure compressed air, one diagnostic solution is to use soap and low-pressure water to flush away organic contaminants. If the debris can’t be removed or the cooling fins are severely damaged, the radiator should be replaced (see Photo 4).

In practically all cases, it’s more cost effective to replace a Subaru radiator with a new one rather than try to clean or rebuild the old one. One reason some Subaru specialists sell a radiator replacement with a cylinder head gasket replacement is because loose sediment from the engine’s water jackets can be transported into the radiator core during a catastrophic boil-over.


With most Subarus having 100,000 or more miles on the odometer, this loosened scale and sediment will severely reduce coolant flow through the radiator. When replacing a clogged radiator, prevent additional failures by replacing hardened radiator hoses and rubber mounting grommets.

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Photo 5: A new radiator cap will maintain specified levels of cooling system pressure and will help prevent air from entering the radiator.


Pressure cycling takes its toll on all cooling system components. During pressure cycling, the radiator forces coolant into the coolant reservoir during warm-up and draws it back out during the cooling process. The upside is that this “inhale/exhale” process caused by pressure cycling also bleeds air from the cooling system. The downside is that this process, over time, tends to fatigue the radiator core.

The radiator cap contains three components that allow coolant to freely travel between the radiator and the coolant reservoir during pressure cycling. The first is a pressure valve that vents coolant to the reservoir during a warm-up cycle. The second is a vacuum valve that allows coolant to be drawn into the radiator during a cool-down cycle. The third, and most important, part of the radiator cap is the rubber gasket located under the cap, which seals the cap to the radiator or coolant reservoir filler neck.


When the gasket hardens or cracks, air will be drawn into the cooling system, which can accelerate the degradation of the coolant additive package. When replacing the radiator, the diagnostic solution is to also replace the radiator cap (see Photo 5) to prevent abnormal levels of pressure, vacuum or air from building up in the cooling system.

subaru cooling sidebarBefore Installing A New Radiator On Any 2.5L Subaru: 

Make sure that the cylinder head gaskets aren’t leaking. Keeping in mind that the degree of head gasket leakage can vary according to engine temperature, leaking cylinder head gaskets on Subarus and other nameplates might exhibit one of two different failure symptoms.

Our first symptom occurs when the coolant is cold and the thermostat is closed. During cold operation, a leaking head gasket can cause combustion gases to be trapped under the closed thermostat. With the thermostat closed, the combustion gas pressure increases inside the engine block, which forces coolant to back-flow from the engine block through the radiator and into the coolant reservoir. In some cases, the reservoir will overflow, causing a net loss of coolant, which will then cause the overheating condition.


Our second symptom occurs when the coolant reservoir constantly needs to be refilled, but there is no evidence of external leakage. In this scenario, coolant is being drawn through the cylinder head gasket and being expelled through the exhaust. In most cases, a P03XX misfire code will be stored for the cylinders involved. The freeze-frame data accompanying the P03XX code will indicate if the intermittent head gasket leakage occurred when the engine was hot, cold, at idle or under load. A green or brown coloration of the spark plug insulator will confirm the presence of coolant in the cylinder. If the engine oil looks like a chocolate milkshake due to being mixed with coolant, the leaking head gasket diagnosis is confirmed.


After a new radiator has been installed, check the engine coolant sensor and the thermostat for accurate calibration, especially after a severe overheat. In most cases, replacing with new is the best insurance against a future comeback.

Last, but not least, run the engine through a complete warm-up and cold-soak cycle to ensure that the water pump, thermostat and engine coolant sensor are working properly and to vent any air trapped in the cooling system. All of these steps are important to prevent future service comebacks.



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