Thermostat d40

[Genghis]

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]

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]

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

 

[OldManBeard]

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.

I know this post is a bit old but I've only just noticed it. I have replaced a lot of thermostats over the years, both in my own vehicles and other people's. Never have I come across one that failed in the open position. Every one of those failed because the wax had at least partially escaped. The wax pellet expanding is what opens the thermostat. No wax, no opening.

 

[Azi]

Quite common stuck in open position. Engine runs cold, uses more fuel, ecu senses cold engine so delivers more fuel

 

[OldManBeard]

Quite common stuck in open position. Engine runs cold, uses more fuel, ecu senses cold engine so delivers more fuel

Next time you have one stuck open you need to investigate why, as it really is quite unusual, not common at all. There are only a few reasons why that can happen and most point to other problems. e.g. Incorrect fitting, buying poor quality rubbish, lack or of effective coolant additive, resulting in rusting up.

 

[Azi]

Hey champ, Im no expert, but I suspect the wax loses its ability to contract, expand etc. Replaced a few in Ford 6 cyl running rich.
Regards

 

[OldManBeard]

Hey champ, Im no expert, but I suspect the wax loses its ability to contract, expand etc. Replaced a few in Ford 6 cyl running rich.
Regards

The nature of the wax is that it will expand under heat and contract when it cools. It can't lose that property without undergoing a chemical breakdown. When the wax expands it pushes up what is essentially a small piston, which opens the thermostat. Like any other kind of seal in moving components, the seal in that piston assembly will wear. When that happens wax will slowly leak out, usually very slowly. Low quality thermostats will usually experience seal failure faster than higher quality ones. Given how infrequently good thermostats require replacement, this is not an item that's worth trying to save a few dollars on.

It is true that when a thermostat seal leaks the wax could potentially be affected by chemicals or minerals in the coolant but it will leak out before that happens, as the piston containing the wax will be under positive pressure in relation to the coolant. i.e. The wax is pushing out more than that faulty seal will let something in, at least until so much wax is lost that it makes no difference either way.

 

[Cobez]

Had the same happen on my D40 a month or so ago.

Easy to replace. Drain your coolant from the radiator. Take the passenger side wheel off and it's right there. Undo the turbo pipe clamp, push pipe out of the way and with a long 1/2" extension or a few joined together, gently crack the nuts on the thermostat housing (making sure you have a big container underneath to catch the coolant.

Take old thermostat out of the housing, slam the new one in with the rubber ring, put everything back together, refill coolant and bleed.