In early 2000 Subaru had some issues with external coolant leaks from the Head gaskets. Subaru issued a Campaign (WWP-99) instructing Subaru Dealers to install a bottle of what they called coolant conditioner, which was essentially stop leak. This kicked the can down the road if you will at best.
What we have seen over the last few years and even more so lately is that once the cooling system is drained and left empty for any length of time the conditioner in the cooling system is hardening and clogging the small cores in the Radiator and some times worse the heater core. The job of Stop leak is to bond, so one it’s there, it’s there, and you can’t expect that draining the coolant will also remove the stop leak in its entirety.
This is why it is so important after servicing the cooling system for any reason at all you must take the time to check the system for restrictions even if you did it prior. We do this here, every time we service the cooling system, but even at simple oil change services the techs will break out the infrared thermometer and check for temperature variances in the radiator if we are allowed the time to do this.
Pictured below is an example of a restricted radiator that showed no signs of issues until after the system was drained to repair oil leaks at the head gaskets
A restricted radiator may not show up by watching the temperature gauge until the damage is done, so we feel it’s just good maintenance to keep an eye on this for you as the car ages. Any good shop should be willing to do the same.
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