1KZ Coolant Temps

I'm a new 1KZ, auto, full time 4WD Hiace owner. A digital coolant temp gauge had been added to the van. What is considered to be the threshold between acceptable and too hot for coolant temps.?

On my first brief trip with the van going up Snoqualmie Pass in temps a little below 0dC and with speeds below 100km/hr, often much slower due to slow traffic from snowy road conditions, the coolant temp rose to 95dC at the top. The stock gauge in the dash never went above about half way.

Searching the internet I haven't been able to find an answer. I saw some reference on the internet that the standard thermostat start to open at 82 and is fully open at 95. I've seen another state it is fully open at 88. If the fully open temp is intended as 95, I'm not too worried. On the other had if I easily surpassed a fully open temp on a cold day at moderate speeds, I'm pretty worried about what might happen in the summer.

The original dash temp guage electronics are designed so it sits rock solid center even as the temperature fluctuates within the acceptable limits of normal engine operation. Only when it goes high (or low) outside the limits will it then skyrocket up. If it does, there is definitely something wrong.

Having a supplementary temp guage is essentially superfluous but many 4WD guys with these engines love to be control freaks over every possible variable they can imagine. The big question is: which of the trio (?) of temp sensors in this engine is that guage hooked up to?

Looks to be on a thermocouple attached to the elbow by the pressure cap.
https://photos.app.goo.gl/UjQPnNasuEf8BQvC7

There's some info I added here, as the 1Kz thermostat placement was 'trendy' at the time, but non ideal. Porsche copied the system in their first water cooled 911s, most engines of which were replaced more than once. Landrover/MG too.

BMW also did it, but cunningly made their engine so efficient that it didn't matter what the thermostat did, the worst that happened was the cabin heating stopped.

If the 1kz gets too hot the heads crack, so the add on thermometer is there for that reason. The trendy thermostat location (on the inlet - doh!) gave rise to wildly varying temperatures that 'might upset owners' so they damped out 'quite a bit' of the guard with a zener diode, on all of thise design.

That, can also be modified

https://hiace-super-custom.forumotion.com/t3430-cooling-the-1kz-te-in-the-hiace-super-custom

Thermostat placement was trendy?
Fitting the thermostat in the bottom pipe is an idea started by the Germans in the 1970's in VW, BMW's, Mercedes etc. Yes, the main reason behind this was to split the cooling system and have auxiliary flow that feeds the cylinder head and interior heater system continiously, even if the thermostat is closed. It’s not a “trend” it’s a functional change followed by most modern egines.

The reason that the thermostat reacts slow is exactly to prevent heat surge and doesn't cause it. It’s not a water flow problem nor is it caused by other cooling component problens. Heat buildup in the head of the 1KZ is from design mistakes Toyota made (it’s their first modern high power turbo diesel) that starts showing as cracks in the combustion chambers generally around 200,000kms. No amount of temperature monitoring will solve the head design problem and the heat issues. It’s just spectating. They got up to about the 5th version of the head trying to solve it but only fixed it with the next gen 1KD engine.

Well, in practice the inlet thermostat relies on a bypass of hot coolant from the head, and the water returning from the radiator, mixing each side of the thermostat, to decide what to so.

Landrover, and my fix, decided to take this mixer right outside of the engine in the form of a 'shower thermostatic mixer', so the previously small 'head bypass' becomes a full flow pipe, and the valve mixes that with the cold side radiator output, to aim a suitable temperature coolant at the engine. This eliminates delay of opening and closing as the temperature is always the correct one.

I say it's a trend as it seems to have stopped now, but is it really still in use? IMO it's madness to use this unstable system.

The reason it failed so many engines was the slow reaction of the inlet thermostat.

In Porsche this created a huge heat soak (a hot drive up to traffic lights), the inlet thermostat was blissfully unaware of the baking engine, and then when the traffic lights changes the (now softened with the heat) bores scored and wrote the block off. The Boxster suffered less as the engine was detuned, but the 911 failures were epic.

In other cars such as MG/Landrover and some Toyotas etc, it was the cold that was the danger. The cold slug of water outside of the engine sat there while the engine warmed up. Thermostat opened! Engine got cold. Thermostat closed. Engine warmed up, coolant cooled down, thermostat opened - new slug of cold coolant! And round and round it oscillates until the whole cooling fluid mass is warm enough.

Someone actually mapped the temperature variations and it was epic to see the graph, which caused the head to contract and expand, wrecking the head gaskets and cracking some heads. It was a big problem - fast violent heat cycling of the head, because the inlet thermostat was too slow to react, so the temperature oscillated. Cost manufacturers an absolute fortune.

That's why all this design damped out the middle range of the gauge: so the driver didn't see the needle sway about. But invisibly of course, temperatures did sway, and many engines failed. The proper, outlet thermostat design was fast and gauges were normal and worked fine.

Essentially a thermostat forms a negative feedback loop. similar to those in RC servos and audio amplifiers. There is something called 'phase margin', if the amplifier is too slow, it oscillates.

This (temporarily) trendy system was too slow for the job, a basic control theory mistake, so it oscillated which damaged heads and head gaskets.

The 'solution' was to fit a really low temperature thermostat, supplied by Harteck for old Porsches, and often fitted to the 1kz. This however only masks the problem by 'speeding up' the opening. But a lower temperature also uses more diesel, and the system is still marginally unstable, especially in cold weather and in heat-soak situations.

Nope it hasn’t stopped. No reason to, it works. The 1KD, 2KD and the successor 1GD & 2GD all have the thermostat in the same location.
All the theory in the world can try an explain why it doesn’t work but the the fact is it does. Those engines see huge mileage in extremely hot and harsh environments (Australia & South Africa for example) without issue.

sooo.... what is the top end of temperature it could safely continue driving at?
I've set my engine guard to 102....can't remember how i came up with that temp, however... when it starts creeping to 95, i start blasting the heater which makes it drop pretty fast...
what is the 'oh shit' we have a problem temp range?

My engine is a 3l and my temperature warning buzzer is set to 95. It normally sits in between 84 and 87, bouncrs up to 90 then quickly comes back down when I presume the fan clutch kicks in.

I have fitted an extra fan where the air con was and if I see a long steep hill I put the fan on as I start to ascend not when it is already heating up.

I have hit above 95 a few times but most often when I stop for something, switch the engine off and lose water circulation. So you return start the engine and it bleeps until you get some water flow happening. One time it hit 104 doing this and I was a bit scared !

I have found it better to let it cool down before switching off but you don't always have that option eg. a petrol station or when the outside temperature is very high.

I've just taken it through the Atacama desert and that's quite high and hot with lots of ups and downs and only a couple of bleeps. This is pretty much when you go up for so long that you slow down and lose air flow.

Personally I think 102 is too hot for a warning but my engine is different and maybe turbos run hotter

yeh it really seems to be a unicorn of info to find... Seems to be all 'old wive's tales' of where it should sit...
As a hypothetical .. If we go off the factory gauge... the needle doesn't move until say 100 degrees..
If thats the temp in which toyota would allow the temp to get to before the needle moves.. surely in toyota's wisdom, they would pre-empt this... thus meaning.. the engine could get to say 120ish? before catastrophic demise.
If that makes sense?..