The thermostat doesn't have anything to do with the heater core.

 

Extranet, is the t-stat not modulating between the two paths to try to maintain its design temperature? I don’t think it’s all or nothing to the radiator. I agree, radiator fully shut off from the flow below it’s temp (eg during warm up), and fully receiving all the coolant flow above its temp (when very hot), but modulating during most driving conditions.

 

As the temp is transitioning, the tstat is in a transitioning toward open state. When the engine is fully up to temp the tstat is in a nearly completely stable state where it is balancing the heating of the coolant by the engine and the cooling of the coolant by the radiator. If your engine load or the ambient temp changes, then the "amount of openness" of the tstat will change to accommodate. When the engine is "really hot" the tstat will be all of the way open and the resulting temp will be a function of how well the rad is able to cool the coolant.

 

Hi. There is a point in the thermostat rated temperature where the flow is changing paths. With the engine running, it is of minimal duration as to say 'modulating' , as coolant is increasing temperature reinforcing the switching towards the radiator. Ambient temperature would have to be extremely cold to maintain 'modulation'
The LandRover and modern thermostats with vehicles featuring bypass routes are not designed to maintain a temperature; its design is to switch paths. The purpose of the thermostat is to avoid the engine from running too cold. (for whatever fuel economy, emissions, efficiency, or passengers comfort as LR claims.)

An in-line thermostat does not switch paths; switches from closed to open. With engine running, getting hotter reinforces the 'keep it open' state. With coolant above the thermostat rating; there is no action at all. Is open to flow and cannot open more. Above that point the running temperature depends on ambient temperature, air flow, coolant flow, restrictions, lead foot, climbing, towing, vacuum leaks, mixture, aerodynamics, speed, drag... If the thermostat cannot 'open' more, it does not control nor lowers temperature. It is already open, it is done with its job, and does nothing else after. Coolant would have to go cold for any later action.

For my point of view, for a thermostat to 'modulate' it would have to be designed to act not at a temperature of say 180F; but varying its action range of say 'from ~160F to ~190F. The manufacturer choice hysteresis.

 

The thermostat 100% modulates the temperature.
More so on the D2 system than D1, as the thermostat has coolant passing it 100% of the time in the standard D2 system.

This video explains it well in generic terms, using generic temperatures. That link starts at the most descriptive part, but it's worth watching the whole thing.

On a standard disco 2, the coolant is circulated by the pump 100% of the time, through the block and past the thermostat at a reasonable rate. It helps with evenly heating the engine, avoiding hot-spots in the system and gives quite real-time feedback to the thermostat. The thermostat blends the coolant circulating in the block, with coolant cooled by the radiator to aim for a fairly constant temperature.

As the temperature approaches "operating temperature", the thermostat starts to open and some of the "cold" radiator return coolant is mixed into the coolant drawn into the pump. Because the coolant is circulating all the time, the engine cooling jacket temp starts to reduce, and that coolant causes the thermostat to slightly, based entirely on what ever the temperature is of the circulating coolant. Because the water pump output is ALL circulating through the block, the feedback is very fast.

Then, once the coolant temperature begins to exceed the thermostats rating, the thermostat opens further and further (trying to add more cooled coolant), eventually causing the bypass plate to move back and block the hole from the block, forcing almost full flow through the radiator circuit, with just enough flow through the tiny holes in the bypass plate, that the thermostat gets feedback from the block coolant about when to start to close again, back to modulating the temperature. In most systems, its actually pretty rare that the bypass needs to close, unless the ambient is hot, the car isn't moving and the radiator cooling capacity is quite low for the system size.

The inline mod acts like the old D1 system - which (like my D1) suffers from spikes in temp, because there isn't the same amount of constant circulation of coolant, even heating AND passing directly past the thermostat bulb. On the D1 - when the thermostat is wide open, coolant isn't forced fully through the radiator - it can still circulate within the block at a reasonable rate and the slightly less restrictive radiator side gets "ok" flow... but a D1 can over heat easily, if the engine speed isn't high enough for the pump to circulate enough coolant, for the flow through the radiator to provide enough cooling flow. The inline system (like the D1) ends up getting sudden gulps of cold radiator return and closing, then opening again once the low flow from the hole makes the thermostat open again... then repeats until the temps are more even through the system. There are graphs posted, that show this. Inline mod is actually a bit worse, because the D1 thermostat location at least means the thermostat bulb is almost in the flow within the block - close enough to register the temp change fairly quickly.

The water pump isn't a pump, really - it's a circulator - it creates zero real pressure, is just swishes the coolant around, encouraging it to move through the system, as required... If there is too much restriction, the flow will easily stop. You could easily stop the flow from a car water pump with your hand.