Pictured below is a radiator cap that created a caused the cooling system in a second generation Outback to no longer function as designed.  This is a commonly overlooked cooling system component and should be inspected at a minimum of every year. If you look at the seal on the cap pictured to the left you can see where it has grown in size from new.

Failed Subaru Radiator cap

One challenge is, the waiting oil change  does not always allow for removal of a radiator cap if the system is hot and under pressure as besides a potential burn it can also create an air pocket in the cooling system.

A job of the cap is to regulate pressure to allow for higher boiling temperatures as coolant under pressure boils at a higher temp than coolant that is not under any pressure at all.  The cap should vent pressurized coolant to the coolant over flow bottle, and as the temps come down allow for the coolant to be pulled back into the radiator as the decreases and instead is now under some vacuum.  It’s this constant push and pull that is crucial for the proper function of the cooling system under extremes and also when trying to recover from a heat event.

In this situation the swollen seal  blocked the overflow tube port in the neck of the radiator, not allowing for the proper venting under pressure or the removal of excessive pressure and expanded coolant.  The port being blocked at times also prevented any coolant that did make its way under extreme pressure back into the radiator when the system was in vacuum.

This excessive pressure over time coupled with low coolant levels and air pockets did in fact damage a set of head gaskets.  One potential warning sign is a collapsed radiator hose.

If you have no idea how long it has been since your radiator cap was replaced, perhaps its a good investment to do it as maintenance or ask at your next visit to a service center to have it done, or if it can be inspected.   This is another one of those grey area situations where there is no mileage suggestion only a suggestion for inspection.

With the cooling system intervals being stretched out to ranges that are good for the environment on the 2008 and newer models, you can bet this will come up more and more as it truly becomes the forgotten component  of your Subaru’s cooling system.

Thanks for reading

Justin