After the recent head scratching (continued) about the higher temps in my 2000 D2, I decided to take apart my old t-stat to see exactly what makes it tick. I did find out a few things (that could prove useful in my search for an answer), and figured I would share it out for the general public who might not know what the inside all looks like.
This is a 1.5 year old Land Rover thermostat.
Pic below of bottom of t-stat (off lower hose) when closed. Important to note it is spring loaded, and with pressure you can open it up at any temperature.

Here is the top view. From what I can tell, there is a flow through up here from the outlet on the block. When the stat is closed, some coolant is allowed through those holes to heat up the stat and open it if needed. It is a pressure sensitive valve though, so more flow can get through there presumably under higher RPM/pressure. With the stat closed, it flows through and bypasses the radiator and goes out the side outlet back to the water pump. When the stat is open, it seems like there is more pressure pushing up on that spring loaded piece, allowing radiator cooled coolant to flow to the water pump through the side outlet, and not back to the upper radiator hose.


This is the cutaway view of the insides. Sorry about the sloppy cut job. My Dremel decided it didn't want to work tonight, so I went about it carefully with a Leatherman and a drill. Not knowing exactly what was inside sections I made my test holes with the drill. On a side note, this part is super tough and not in the least bit fragile. At least the actual part is built strong!

After taking it all apart, I heated up some water to 210 degrees and put the stat in. It slowly opened from the bottom, going from sealed to open about 1" in 45 seconds. When taking it out of the water, it closed in about 30 seconds.

Here is another picture of the bottom of the stat. I wanted to note the little ball bearing which is what you hear rattling if you shake it.


Here is another angle of the t-stat. This was when it was taken out of the water, and already closing up.

Not sure if this will be helpful to anyone or not. To me, it was interesting to see that both sides were spring loaded and pressure sensitive. It makes a heck of a lot of sense why with increased RPM's my temperatures go down quickly. It could be because more coolant from the engine outlet is getting to the stat (instead of just through the sensing holes) and opening the stat, or that even though the stat might not be up to temp, the pressure is opening that side and allowing the radiator to flow. Or, could be a combo of both.
Here is the route of flow:
https://landroverforums.com/forum/at...ing-layout-pdf

 

Good pix. If you had water at 185, would have seen it open also, just not as far.

Imprortant question - is there any opening (hole or slot) between the chamber that has the disk with 4 holes and the outer chamber that leads to the water pump ? In the third pix it looks like a slot at the bottom to the left of the spring.

If the opening is there between the stat chamber and the outer chamber, there would be coolant circulation at all times, main stat closed or not, thru the metering holes and back thru the water pump. If passage exists, then increase number of sampling holes to increase hot coolant on stat. Remember that in D1 engine stat sits in hot coolant all the time.

Of course this flow design puts normal stat operation in a D1 upside down. The cold coolant enters the hot stat area and begins cooling it off - before cold coolant has traveled thru water pump and the engine block. In a D1, the stat opens, hot coolant leaves, cold is brought up thru the pump and block, and when it has cooled engine down, the stat closes back down (full or partial), repeat.


Not large quantity of coolant is designed to flow up through the port with the metering holes, just a small leakage.

 

Yes there was Buzz. Seemed like a sea shell almost with that chamber from what I can tell taking it apart. For the metering holes though, the thing that was an "ah-ha" moment for me though was that the entire thing was spring loaded / pressure sensitive. So, with pressure, the metering holes might not even come into play and it is pushed in allowing for more flow. If you put more holes in or enlarged, it could make it harder for pressure to push it it down maybe. I'm not sure really. I'm thinking that the only reason those holes are there are for a trickle down during idle. So, if you did put more holes in, you could help the stat out at idle, but maybe hurt it at higher RPM's?
Also, as a side note, being my truck was average 210 on the road I wanted to see exactly how wide it opened at that temp. I might mess around with it later at others temps for the heck of it!

 

That's a great write up. I have had a couple people mention they have drilled out or added more hole for better coolant flow, I can see how that might help but also wonder if it causes issues at higher RPM's also.

 

At idle, the truck would over heat if the trickle was not enough to open the stat. For the guys who have modest idle, but rise in temp at low speeds, I suspect that the cooler liquid entering the bottom overwhelms the trickle and allows the stat to close off more than it should.

IMHO anything you can do to increase heat to stat is a plus. The bottom side would be the suction side, and the upper side would have more pressure. So at high RPM, there will be more hot water coming that way, but the cross section of the holes (and a few extras) is still very small compared to that of the upper radiator inlet.

In the condition where main stat is closed and above idle, the bypass valve would be pushed in by water pressure, anf the stat would warm up quickly. In the state where the stat is not 100% open, and you are running at speed, the bypass top valve would be closed, but with only metered flow of hot coolant into the stat you are cooling the stat off more than it should be, so it won't be open as far as it could be. From the dissection, it may also appear that when the stat is partial open, it restricts the trickle flow (by blockins the path "down" to those open scallops at the base of the chamber). If that is the case, maybe a hole drilled in the side wall of the upper part of the chamber would allow the trickle to flow even if stat was open somewhat. Once it opens, cool water is coming by in short order. In a D1, once it opens the entire block has to be exchanged before the cool water reaches the stat to start closing it back down.

BTW, old Merecedes owners have a stat that looks just like the metal part, with top "umbrella" as well, just no extra holes. It has been around awhile.

 

The hot idle issue happens when the sensing holes become blocked. 4 holes = 100% sensing ability = opens at the correct temperature when the system is "bypassed" (aka at idle speeds with tstat closed) it will only open when it senses warm enough coolant. If 1 hole is blocked, you lose 25% right away - the tstat won't open nearly as soon. This is usually made obvious by the fact that your aux fan will turn on when you're sitting idle, even if it's freezing outside and the clutch fan is disengaged - the ironic thing is that there's no point to the fan even turning on, since no hot coolant is making its way through the radiator in that state anyways lol.

Screwed up system, which is why they don't use it anymore. With many aluminum parts and these vehicles getting up there in age - the odds of some form of contamination or deposits plugging one of those holes is very high. I replaced my tstat and one hole was blocked. I pulled another at a friends LR shop and one was also blocked. It fixed my problem for a few months, but now she does the same thing again.

If it's really hot out and I'm stuck in traffic or something, I'll just put it in idle and rev very slightly.. enough to engage the bypass spring, at which point the sensing holes lose their purpose and the tstat can correctly open - fan usually shuts off in 5 seconds.

I've done lots of thinking about this system, if anyone has questions feel free to PM me - and no, the "DIY in-line tstat mod" is NOT a good idea, as the heater core loop was never designed to handle high-RPM coolant flow, which is why the bypass spring exists in the original design. With the mod, when your car is cold and the tstat is closed.. it all flows through the heater core loop (which is fine and all..) till you rev the engine, then it's still forced through the loop as long as it's cold. No bueno.

 

OK - so if at 2000 rpm, engine warmed up to operating temp, would bypass valve be open or closed? Seems like if it were open, this would provide a dramatic shunt of hot coolant around the radiator, in effect, a bypass of a significant part of the cooling.

I would suggest that at idle, low pump volume, the light spring has it closed and the meter holes simply warm up the stat. The in/out of heater core are across the water pump also, and designed to take the idle pressure from the pump (important point - maybe not from the new bronze impeller improved pump on the aftermarket).

When engine is slightly revved to move in traffic or in "N" position, pump pressure squirts up, light spring opens to allow byapss and protect the heater core and O rings at the core. This does heat up the stat and open for more cool water from lower hose (I know it is coolant, but dislike writing cooled coolant). Engine cools down. But as the stat opens, it closes off the light spring more and more.

Eventually, the stat is partial open, with what trickle of hot coolant is coming in thru the metering holes to warm up the cooler water from the lower radiator hose is trying to keep the thermostat open.

IMHO the stat does not open to the percentage that would match coolant temps in a D1 engine with similar stat (open at 180, full open at 204F stat for the D2, that would equal a US 180F in a D1). Keep in mind they have same water pump, don't know about pulley diameter and belt speed.

Once we are under constant speed, the trickle of hot water thru the metering holes must counter balance the colder water from the lower hose. If we had a few more holes, we should be able to shift that up a bit, making the stat open more, and cooling down the block some more.

Page attached from STANT on the conventional bypass design.

You will notice in the pictures that the bypass valve position does not change with heat.

 

I was having a tough time understanding the points you were trying to make here - if you study the RAVE manual or others for long enough, the design becomes fairly easy to understand - sounds like you've got a good grasp of the situation though!

To answer your question, at 2krpm steady (bypassed for pressure relief) you're right, the coolant flows through the "elbow" and is allowed through the "tee" at the top of the shroud. As the tstat opens, pressure on the spring will drop and coolant will be redirected through the radiator.

That's more a question of understanding however, the real issue is the fact that when the tstat is closed and the engine is at idle, EVERYTHING on the cooling system forward of the oil fill cap stops doing anything

...except for the trickle of coolant through those 4 holes.

As temps build up they act on the tstat which enables more water to flow over it and get a more accurate reading of the temperature. Keep in mind that when its closed, the flow is almost zero - so it must take a while for the hot coolant to make its way from the engine block through those large diameter hoses via only those 4 little holes! Especially at idle pressure. THIS is the issue with the system (in my mind) since it's worked perfectly for me in every other scenario except at idle.

 

Well, sounds like if holes were added, the idle cooling would improve, and the operation at speed would also improve, as more hot water would make the stat open a little more. Everybody wins.

On the D1's, here's the shade tree heater bypass (lol - from our man in San Juan). The later D1's also don't have a heater valve, and just get their water constantly near a bypass hose on the block, with a machined-in size restriction, to keep water pump pressure at bay. Many D1 owners also drill one small hole in the face of their stat, to pass gas (err gas bubbles) that might be trapped.

 

Agreed I think that's the least intrusive way to fix or improve the system. Though I must say, I laugh every time I get in my rang rover sport and it takes 3x longer to warm up than my 13 year old disco