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Thermostat d40

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Genghis

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Hey out there,
Wondering if anyone else has experienced this?
I have an 08 d40 4.0l and it seems the thermostat has failed in the closed position. I know they are supposed to fail open. At the moment I'm getting coolant boiling and top hose hot. Bottom hose is still stone cold. I'm assuming if the water pump has failed that even though there isnt water flow (which there seems to be) thermostat would be open and bottom hose would be hot?
All symptoms point to thermostat but it seems this one has failed closed. Am I completely wrong here or does it happen?
Regards Sam
 

Old.Tony

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I haven't heard of either failures. There might be other reasons for that: thermostat failures might not be considered unique to the Navara (go on, groan, after all I am a dad). It might also be that the water pump is rather reliable - over 350,000km on ours and it's still the original pump (and it works just fine).

The easiest way to test the theory is to remove your thermostat and chuck it in a pot on the stove and watch it. It should start opening as it approaches 85C where it should be fully open.
 

Genghis

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I haven't heard of either failures. There might be other reasons for that: thermostat failures might not be considered unique to the Navara (go on, groan, after all I am a dad). It might also be that the water pump is rather reliable - over 350,000km on ours and it's still the original pump (and it works just fine).

The easiest way to test the theory is to remove your thermostat and chuck it in a pot on the stove and watch it. It should start opening as it approaches 85C where it should be fully open.
Hey Tony, thanks for your reply.
I agree, i doubt its a water pump. I am inclined to think its the thermostat. Only reason I posted was in this day and age thermostats are supposed to stay open in the event of failure. In my case it seems it is closed. Will find out in the morning as I have a new one to pick up.
Hoping this is what the issue is other wise its looking serious.
Once again thanks for the reply
 

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