Changed the thermostat on my 2001 P38.
Had an absolute b%$%ard time trying to get an air lock out of it. I did every single technique written about “burping” the P38 but no luck. Vehicle kept creeping up to overheat and the thermostat would not open.
Finally I decided to drill a 3mm hole in the shutoff valve on the thermostat. I made sure that the hole was at highest point in the thermostat where it sat once installed. This allowed any trapped air that was between bottom of radiator feeding into the lower chamber of thermostat to escape. Issue I had prior was that there was air trapped her restricting the thermostat from opening….. that’s my assumption….. It may just be luck.
BUT….Success and now the thermostat opens…

But a 2001 is a Thor not GEMS. As I mentioned in your other thread, the Thor always seems harder to get all the air out. I can't see it being the stat though as they are the same on Thor and GEMS. I've just had a look at a spare thermostat I have sitting here thinking that maybe an OE one has a hole and modern aftermarket don't, but no hole. In the old days a thermostat always had a hole with a 'jiggle pin' in it to allow air to bleed through and the pin was there to stop is getting clogged.

i generally drill a small hole in thermostats for that exact reason . started doing it on water cooled motor bikes to bleed air out . as Richard said they used to come with a small spring loaded relief valve built in .

Correction re engine type Gilbertd…… however, you are correct re the design feature lacking in the non OEM thermostats and owners drilling a hole in non units should be an absolute requirement. Had I done this from the start I could have saved myself about 6 hours labor.

There is a mod that is tried and true for the Disco 2 that is now available for the Bosch P38, that puts the t-stat in the position that the RRC had, at the coolant outlet of the engine. It should be available shortly for the P38 GEMS. I am actually the Beta tester for the GEMS kit (I get no money for saying this, but i did get my kit for free for being the tester). I’ve had mine for a few months now and am really happy with it. The temps are steadier (I monitor them with an Ultragauge plugged into the OBD2), and I get rid of the plastic t-stat assembly. I’ve felt for some time that the original location of the t-stat (eg RRC location) was where it should be. The bleeding of this set up is easy, though as Richard points out, the GEMS is easy to bleed anyway.

The link for the Bosch P38 kit:
https://landroverforums.com/forum/range-rover-p38-2nd-gen-56/new-p38-thor-bosch-inline-thermostat-kit-now-available-114106/

If anyone is interested here is also a link that contains the considerable experience with the kit on the D2:
https://landroverforums.com/forum/discovery-ii-18/official-extinct-inline-thermostat-mod-thread-105010/

I'm always wary when someone decides they know better than the man that designed it in the first place. Agreed, there is a suitable place to fit a thermostat that is a throwback to the Classic engine but why did the designers decide not to fit it there and go for the remote one? I've monitored the temperatures on mine with the Nanocom and never seen it vary by more than 4 degrees C under any circumstances, including sitting (at length) in traffic with an ambient of 40 degrees.

The reason GEMS & Thor use a different thermostat type & position to the Classic is to allow warm coolant through the heater before the thermostat opens, hence cabin gets warmer quicker.

But the Classic is the same? Heater feed comes off the inlet manifold before the thermostat so the heater circuit has flow before the thermostat opens. In fact, I don't know of any car that doesn't have this system.

Harv, this is a very interesting solution! Had I see it earlier, in my dreadful "hot engine" years, I would have run to it immediately. Now I am fine (see more later), but I will nevertheless keep this in mind when the time comes. I've got two friends with GEMS which I am sure will be interested as well, once your testing results prove satisfactory ...

I'm always wary when someone decides they know better than the man that designed it in the first place.
Well, in general - and in theory - I do agree wholehearted with you.
However, the more in the years I tinker with the cars, and the more I read and apply, assemble and disassemble, I reckon sometimes technical solutions obey more to a compromise (technical, budgetary, legislative, or else) than a true "optimal" solution. Me - actually, "we" - as final-end users, might choose not to be constrained by these parameters, to the limits of the reasonable obviously.
Personally, I have thus become wary of the "why" a certain solution was implemented in the first place - not because I think I know better, but simply because by analyzing, if there was a compromise (e.g. usually is choice of material, or position or shape), you can overcome shortcoming in that compromise and have a better, "improved" solution.
No need to make this post longer, but there are many proven 'workarounds' that make living with a(n) (older) vehicle a better proposition. We need to keep in mind the factory designs were also built around a finite shelf or work life, and I may suggest we are in some cases, way past that :-)

Agreed, there is a suitable place to fit a thermostat that is a throwback to the Classic engine but why did the designers decide not to fit it there and go for the remote one?
I actually think the P38 is one of the very first cars where the thermostat is not really "just" a thermostat, but instead a diverter (pass me the term) of coolant to allow as Pete says above, faster warmth to the interior and a 'variable' flow until the thermostat itself is fully open. To a certain degree, pretty clever. Many cars have a similar design nowadays.
I favor the "older" design, but I am old myself ....

I've monitored the temperatures on mine with the Nanocom and never seen it vary by more than 4 degrees C under any circumstances, including sitting (at length) in traffic with an ambient of 40 degrees
After my many repairs and fixes to the cooling system, now I got a situation as you describe. I am usually on 84-86 on traffic with peaks of 90, whereas when the rad was partially block it would jump from 96 to 110 in city driving.
A good functioning cooling system should not require strange changes, however an in-line simple thermostat is not a bad idea.

I also support a switch to activate the AC fans at will, due to the very high underbonnet temps ...

Well this is a great post and some fantastic feedback and opinions.
Riddle me this…… After fitting the new thermostat, water pump, temperature sensor, drilling small hole in same said thermostat and changing a number of hoses I now find the vehicle runs about at 88C +/- 2C all day long regardless of traffic or slope. NANOCOM tells me I’m putting 64% load on engine at times but engine temp is 88 +/-2……
Here’s the bit that’s strange. I just drove 45 miles at about 70MPH on highway and after reaching destination I decided to feel the bottom of thermostat and radiator. The top section of thermostat is as hot as you would expect it to be and the bottom half I would label a lukewarm. I attribute this to heat transfer from the thermostats water in upper section transferring its heat to the lower section. The bottom of radiator is the same. I’m not able to feel the middle section.
How is it possible for this situation to exist? Engine and temps showing normal. I’m thinking there is a blockage in radiator but I am able to flush water through it freely (water runs through it with no apparent restrictions).
I’m ordering a new radiator as I cannot think of anything else that would impede the flow of water through the radiator then thermostat….

Isn’t that just indicating that your radiator is effectively cooling the coolant? The top is hot and the bottom is cool?

HARV, I don’t know enough about radiators or the coloring system to comment on that

The top of the radiator will be hot after a drive, as it gets the hot coolant from the engine. The coolant flows down through the radiator, with the ambient air also flowing through the radiator fins, absorbing the heat from the coolant. This is why the bottom of the radiator ( and the bottom of the t-stat) will be considerably cooler than the top.
If I’m understanding your post correctly, your radiator and cooling system are working properly.

Thanks Harv

Whilst your post and comment makes sense, I somehow thought that water exiting the radiator would be hotter than the 35C reading at hose leading from radiator to thermostat considering than the temp of water in rubber hose going into radiator was 89C. That would be a very efficient radiator to cool water 50C in such a short space

It will vary quite a bit with ambient temperature, and how hard you’re running the engine. If it was very cold outside the bottom of the radiator would be much cooler than when it’s hot out. With the engine running, the bottom of the radiator will always be at least a little warmer than the outside air temperature. When the engine isn’t working very hard (not generating a lot of heat), the t-stat won’t be letting much coolant flow through the engine (t-stat will only be a little open, as it’s trying to keep the engine at its set temperature) and therefore there won’t be much flow of coolant through the radiator, the ambient air flowing through the radiator will be able to extract a lot of heat from the coolant, resulting in the bottom of the radiator being only slightly warmer than the outside air temperature.

R&R the radiator. Turns out the radiator was fcuked…..
After running vehicle 24 miles the pipe at top of radiator reads 76C and the pipe at bottom 45C.
Alls well until the next series of problems……

I thought that when you said it was hot at the top and cold at the bottom I had my suspicions of the radiator being blocked. The bottom temperature should be lower than the top but not by that much. That may also explain why you had so much trouble bleeding the air out of it. I know they can be a bit of a pain but not impossible.

@Gilbertd, it didn’t add up for me either…..
But it was original radiator so 22 years out of the thing is a result.
As an aside, a laser temperature sensor is a tool everyone should have in their locker…

RE: Hole in stat and jiggle valve...

Small (and possibly very obvious to some) point but I have encountered quite a few replacement thermostats now where folks have fitted a stat. (or drilled it) ... but with the hole at the bottom so the air (lock) can't pass through it easily .... !!