Since fitting waterless coolant, I have noticed some rather odd temperature fluctuations, which I think is because I fitted a standard aftermarket thermostat, rather than the proper Toyota one. This wasn’t deliberate, it was a pure cockup on my part. Still no damage done, I just have to replace it. This did lead me on to attempt analyse the entire cooling system in order to explain this phenomenon though, and I thought I would share it with you.

Thermostat position
When I was changing the coolant, I was surprised to find some kind of blockage in the thermostat opening on the engine block. As I couldn’t actually see it, I got out my camera and took some photos. What I found, was that the blockage is in fact a metal cap, shaped to enclose the very end of the thermostat, and mounted on the engine block, just inside the thermostat housing. Now I have read references to the end of the thermostat being very close to the block, but this is not just close, its touching. This leads me to draw the conclusion that Toyota have mounted the thermostat so that it will operate directly from the temperature of the engine, not the temperature of the coolant. Ultimately I guess this would result in lower engine temperatures, as the thermostat is no longer reliant on the transfer of heat between the engine and the coolant. It also explains why the thermostat has Totally different responses in the car, when compared to the saucepan test, as posted by Dave Miller some time back. (ace answers post #6681)

Radiator cap.
The coolant exits the top of the engine down a pipe. At the end of the pipe it has to turn 90°.
To one side is the hose to the vertical radiator, and to the other is a pipe leading to the expansion bottle via the radiator cap. Anything that flows, will always take the path that has the least resistance. This means that although the radiator cap must allow coolant out to the expansion bottle, it must also present more resistance to the flow of coolant than the radiator system. Otherwise the coolant will prefer to overflow the expansion bottle.

Horizontal radiator.
The power of the engine is small when compared to the weight, particularly if its automatic, 4x4 with twin aircon and heating. This is why, for a diesel engine, we see relatively poor MPG returns. It needs to burn a lot more fuel than a car in all driving conditions. More fuel means more heat, hence the need for more cooling.
The small radiator behind the bumper is fed in series from the horizontal radiator, and both are fed in parallel to the vertical radiator. This means that coolant can flow through the vertical radiator, or through the horizontal radiator. Coolant that flows through the horizontal radiator has to flow through the bumper radiator. I can’t be totally sure why they have added such a small radiator to the system here, but my guess is that if the pipes that feed the horizontal radiator had to run close to the front bumper, then it would be a wasted opportunity if they hadn’t. After all, there is very little natural airflow through this system, so exploit it where you can.
In order for coolant to flow from the vertical radiator to the two additional ones, the two “In series” radiators must present marginally less resistance to the flow of coolant, than the vertical radiator. If they don’t then they are largely redundant or will require an additional pump.

Main engine fan.
The main engine fan has to be big enough, and strong enough to pull air through, the horizontal radiator, two aircon radiators, and the main vertical radiator. This is why there is such a large fan connected to a Viscous coupling, that can use most of the torque of the crankshaft to spin it quick enough to move the required volume of air.

Aircon fans.
There are two aircon radiators mounted on the top of the horizontal radiator, and both are equipped with an electric fan. This is very useful if the main fan is struggling to keep the temperature of the engine down. if the temperature rises to 105°C switch on the aircon fans to help it out until the temp reaches normal.

Coolant pump
When the engine is running, the coolant pump is always pumping. It needs to do this to circulate the coolant within the engine to absorb as much of the heat as possible. This means that there must be a small waterway within the block of the engine that forms a link from the top end of the engine to the pump. In effect, this bypasses the thermostat, and entire cooling system. If it did not then the coolant would only flow round the engine whilst the thermostat is open. When the engine temperature has triggered the thermostat to open, The pump has to draw in cooled coolant from the radiators into the engine, rather than through the thermostat bypass. This is important otherwise the coolant just bypasses the cooling ducts and the engine overheats. If the by pass waterway presents a greater resistance to the coolant than that of the cooling waterways in the engine, coolant will prefer the cooling ducts rather than the bypass. This is possibly why the Toyota thermostat has a bigger opening than standard aftermarket stats. A smaller opening will present a greater resistance, and may unbalance the whole cooling system

To summarize:
Anything that flows, will always take the path that has the least resistance.
The coolant waterways within the engine block will have a given resistance to the flowing coolant.
The heater matrices must provide marginally less resistance to the engine, or the coolant wont flow through them, and there will be no heaters.
There must be a thermostat bypass waterway to allow the coolant to cool the engine, whilst the thermostat is closed.
The bypass waterway must present a greater resistance than the engine cooling waterways.
The bypass waterway must present a greater resistance than the radiator system.
The correct thermostat is crucial. If a smaller aperture increases the resistance of the radiator system to that of the bypass waterway, then fit as many radiators as you like, it wont cool a thing.
The radiator cap is crucial. insufficient pressure and the coolant flows out the expansion bottle.
Phew!

- (#8266) dave Bright, 1 May 03 6:32

couple of observations, Dave.
This object inside the block that the thermostat touches, the stat is touching it when cold?, it'll stop the stat from opening then- the valve moves IN towards the block...

re. the rad cap, you note the two small hoses attached to the filler neck. Only the one to the expansion tank is controlled by the cap. The one from the rad (it's an air bleed)is inside the closed, circulating part of the system, the only pressure differential being that created by the water pump.

The little radiator behind the bumper you refer to IS NOT A RADIATOR! it's an aircon condenser...

Be aware of differences between 2x4 and 4x4 models, different a/c configurations, and ONLY the 4x4 auto has the horizontal rad.

Re the water pump, there is NO internal bypass. The rear heater, and the cold start thermo wax in the injection pump act as bypass.

- (#8267) david miller, 1 May 03 7:29

Hi Dave.
On the end of the thermostat is a sprung loaded disk that touches the object in the housing. The object has a small hole in the centre to allow the thermostat some movement into the block. If you like, I can send a copy of the photo, when I get home. Its a bit fuzzy I’m afraid but you can still see it.
the pressure differential created by the water pump, is what will cause the coolant flow to prefer the expansion bottle at reduced or zero cap pressure. This is why I have dropped the idea of using a zero pressure cap with waterless coolant.
I was confused by the small radiator behind the bumper because Oblsud picture 00806522 appears to show a dotted line from the horizontal radiator to a pipe that shows a dotted line to the “aircon condenser”.

Thanks for the info on the bypass, I was wondering just how that would work. Obviously if they have used the rear heater, that also explains why it doesn’t have a shutoff valve like the front one.

- (#8268) dave Bright, 1 May 03 8:14