Very like the dodgy aftermarket VW header tank caps, there are also dodgy Subaru aftermarket thermostats. These can also be sold in the packaging of usually reputable brands, but not perform anything like the correct OEM part. The good Subaru thermostats are not expensive, and the dodgy aftermarket ones sometimes sell for as much, so using them is a false economy. Just use a genuine Subaru part. Subaru thermostats very rarely fail in our experience, and we do not recommend ever replacing them as preventative maintenance, unless you have good reason to, such as you have tested one in boiling water and it either didn’t open or close properly.
Many customers have had significant problems with their cooling systems which have turned out to do due to an aftermarket thermostat not working properly and / or not fitting properly. Often after a lot of wasted time. In most cases they didn’t previously have a thermostat problem. They’d replaced the original believing they were doing ‘preventative maintenance’, but instead crated themselves a world of hassle.
Testing Thermostats:
To test a thermostat, ideally you need a temperature controlled water bath. The controller on ours allows you to set the temperature in 0.1 degree C increments, with the temperature being measured and controlled by a K type thermocouple. But it doesn’t actually hold the temperature 0.1 degree increments. It holds it to about +/- 0.5 degree. You don’t need that level of accuracy though – 1 or 2 degrees is as good, as thermostats are not precision devices. You can sort of do it with a pan of hot water and a thermometer on any heat source, but it’ll be a lot harder, and much less accurate. This is because thermostats open very slowly. To find the exact temperature that they open, you need to be able to heat the water up to that temperature without overshooting, and then hold the temperature at that level for at least couple of a couple of minutes to give the thermostat a chance to respond. This is easy with a reasonably well tuned PID temperature controller – you just set the target temp, and come back later. It’s virtually impossible with just a pan of water on a heater and a thermometer. Although testing with a pan and a thermometer, or even just with boiling water and no thermometer is 100% more useful than not testing at all. With a temperature controlled bath, you start a few degrees below the temp the thermostat is supposed to open, then keep repeating the test with the temperature set a degree higher.
A good thermostat should start to open with in about +/- 2 degrees C of it’s stated temperature, and be fully open by something like 10 degrees hotter then that. Many are fully open by about 6 or 7 degrees hotter then their stated temperature. For genuine Subaru thermostats, fully open is when the disc valve has lifted by 8 – 9 mm.
If the thermostat never fully opens, it needs replacing. Similarly, if it fully opens but at a drastically different temperature then expected, or fully opens but then does not fully close on cooling down, it should be replaced.
One failure mode which we have seen twice is that the thermostat opens, but at a higher temperature then it should, then never gets to fully open no matter how hot it gets. One time was with an EZ30 thermostat, and the other with an Audi one. Both were used OEM parts, and were compared to new OEM replacements.