If you new stat doesn't fully open, then it's faulty.

Also remember when fully open the stat only blocks the bypass hose (in the centre at the top), so all the coolant flows via the radiator. The flow from heater back to pump still flows because these two are offset & not blocked.

V8 Thermostat.jpg
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Got my old thermostat off today and I can confirm that the old one does fully open and it fully shuts off the top 18mm inlet.
It also opens quicker than the new one so I'm going to refit the old one and send the new one back.
Despite looking crusty on the outside it was very clean inside so might run it for a couple more years then replace it with a genuine item for peace of mind.

Lesson (which I'm sure most already knew): don't bother with aftermarket thermostats for the P38. If you decide to risk it, fully test it to ensure it not only opens, but also isolates the bypass as it should.
 
Got my old thermostat off today and I can confirm that the old one does fully open and it fully shuts off the top 18mm inlet.
It also opens quicker than the new one so I'm going to refit the old one and send the new one back.
Despite looking crusty on the outside it was very clean inside so might run it for a couple more years then replace it with a genuine item for peace of mind.

Lesson (which I'm sure most already knew): don't bother with aftermarket thermostats for the P38. If you decide to risk it, fully test it to ensure it not only opens, but also isolates the bypass as it should.
Stats are a really dodgy area these days, all made in China I suspect.
 
Got my old thermostat off today and I can confirm that the old one does fully open and it fully shuts off the top 18mm inlet.
It also opens quicker than the new one so I'm going to refit the old one and send the new one back.
Despite looking crusty on the outside it was very clean inside so might run it for a couple more years then replace it with a genuine item for peace of mind.

Lesson (which I'm sure most already knew): don't bother with aftermarket thermostats for the P38. If you decide to risk it, fully test it to ensure it not only opens, but also isolates the bypass as it should.

I replaced the one on my Jag as preventative maintenance. No original available so best aftermarket I could find fitted. Utter ****e. Refitted original and just hope to hell it never fails.
 

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