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Old 19 Sep 2013
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1KZ-TE engine - how it works; specifically, the cooling system

I don't much like linking to other websites but even less do I intend to ever join a whole string of bulletin boards, thereby condemning myself to far too many hours spent online.
However, I can't resist the thread linked below, which gives an interesting insight into how the 1KZ-TE engine cooling system works; but are the theories therein correct??
Cooling the 1KZ-TE motor:

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Old 19 Sep 2013
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seemed ok to me , I can also understand why fitting the lower thermostat, will keep the head temp down , as the stat is sensing the block temp lower in the engine which would be lower than head temp , it may be that the head temp is getting above the flashboil point with a hightemp, low fitted thermostat. This would be most likely when working very hard . toyota must have a reason for fitting the thermo where it is but could be due to emissions as that seems to govern most things these days .
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Old 1 Oct 2013
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A good write up, however running a 76 degree thermostat isn't necessarily the silver bullet that Bob seems to think - it is more likely the fact that he completely rebuilt the cooling system that he didn't get any further problems

All fitting a 76 degree thermostat does is make an engine designed to run at 82 degrees run cooler than designed. How much difference to the internals and general running I don't exactly know.

In most cases the head gasket problems tend to be down to a number of factors like corrosion in the cooling system from not using genuine Toyota coolant, leaks from the rear heater pipes leading to over heating, radiator leaks (more often from fitting cheap radiators), viscous coupling problems, etc.

The bottom line is that the cooling system and head gasket are very sensitive on the 1KZ-TE so they need to be looked after. We have rebuilt three in the last month including our own being done now and all due to coolant leaks.

Julian Voelcker
Overland Cruisers - Specialising in Land Cruiser preparation and servicing.
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Old 11 Oct 2013
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Originally Posted by tacr2man View Post
toyota must have a reason for fitting the thermo where it is but could be due to emissions as that seems to govern most things these days .
So, it's likely that the long, detailed discussion in the link has no substantial errors (such as the direction of the coolant flow).
I wonder if any other Toyota engine designs have the same, or similar, location for the thermostat?
Maybe the replacement D-4D engine??
It seems to have the even longer name, the "1KD-FTV".

Originally Posted by JulianVoelcker View Post
The bottom line is that the cooling system and head gasket are very sensitive on the 1KZ-TE so they need to be looked after. We have rebuilt three in the last month including our own being done now and all due to coolant leaks.
There's a lot of comment online about the 2.4 litre engine (as fitted in the "Surf" for example) with regard to head gasket problems, but this is the first time I have read about this issue with the 3.0 litre engine; it is also fitted in later versions of the "Surf" vehicles, I believe, + it is fitted in lots of "Colorados" and "Prados".
Does the D-4D engine suffer from the same sensitivity to the overall maintenance of the cooling system?
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Old 21 Oct 2013
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I cant comment on using a different or the location of the thermostat etc but you should be ware that the 1KZ-TE suffers from another potential problem when it is coupled with an automatic gear box.

The radiator cools the ATF as well as the engine. It has been known for the ATF cooling pipes in the rad to corrode allowing coolant to mix with the ATF causing major problems in the auto gear box. Toyota coolant helps minimise this risk, but it is possible, as a preventative and relatively cheap fix, to fit a separate ATF radiator by-passing the standard rad and eliminating the risk.

As Julian says, the cooling system on the 1KZ-TE needs to be looked after and the use of Toyota coolant minimises the risk of corrosion.

AFAIK the D4D does not suffer from these problems.
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Old 22 Oct 2013
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I sold my SWB 90 recently with over 275k miles on the clock.
I only ever had one issue with the cooling system, on the motorway the needle would rise until it was at 10'oclock it usually sat just below 9 oclock.

Flushing the system did nothing to stop this problem however replacement of the radiator cured it.
The engine never gave any trouble with the cooling system apart from that one instance.
Points to note: This was a Manual SWB with base spec, no Aircon and no automatic.
With the Aircon the flow through the radiator can be quite slow especially if the area between radiators is not kept clean, also Automatic versions put extra heat into the cooling system.
On their own these are probably not enough to cause an issue, together with a radiator that is getting slightly blocked and extra heat load from speed or load this could cause the head to overheat.

I understand that Toyota fit resistors in the temperature gauge to slow the range of movement to avoid customers complaining about the temp needle moving during normal operations, this could also cause problems as many people never check the dash and by the time the needle is in the upper range it could be damaged.
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