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What's the release valve on thermostat for?


NAXJA Forum User
Chicago IL
There is a release valve on the stock thermostat, but the Stant heavy duty 195 doesn't have one. What is it for? Does it matter if I install the one without a valve?

My XJ is a 93 4.0L AT.
I am not sure what valve you are talking about. If it helps, I believe the superstat is designed to fail in the open position instead of failing closed.
Should be fine. The "jiggle valve" is designed to prevent air from being trapped under the thermostat. Generally not a problem on the 91-2001 XJs but you can drill a hole if you want, I usually do with a stat that doesn't come with it.

Here is a link to an excellent article that gets into this and other cooling issues.

Are you talking about the radiator cap?
The Stant SuperStat Premium has its own built in bypass.

All thermostats are made to "lock" open following an overheat situation.

Back in the early 90s I put a FailSafe brand thermostat in my Dad's F250 460 engine--freakin' thing BROKE and I ended up overheating on a Spring Bear hunt.