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Thread: 1HZ 1HDT series engine coolant flow.

  1. #1
    Join Date
    Feb 2007
    Perth, Australia

    1HZ 1HDT series engine coolant flow.

    I was previously uncertain of the way to coolant flows around these engines until I took it apart. Hopefully someone else will find this interesting and useful when working out how to plumb up any new coolant lines.

    This is where the thermostat goes on the side of the 1HZ/ 1HD engines behind the alternator. The thermostat housing goes down and is the suction for the bottom radiator hose. The pipe seen at the top goes to the back of the engine (under the exhaust manifold) is a suction from the heater.

    The suction from the heater is never sealed off and will always have coolant moving through it. When cold, the thermostat is designed to allow water to circulate around the engine and be sucked out the block (through the smaller round hole in the middle) and as well as the heater. Once the thermostat warms up, the hole in the block becomes restricted and the inlet from the radiator opens allowing cooler coolant to be sucked from radiator. All water runs across the back of the thermostat so it has a good idea of total engine temperature.

    The pump is always moving water around the engine, but the inlet and outlet are on the block (not the head) so the water must be flowing through the radiator to take the hot water in the head since the top hose goes to the head at the front. This is a common design fault of most engines where there is no way for any steam in the head to get out and recondense.

    The cavity between the two surfaces goes to the water pump in the middle for suction and is pumped back into the engine block at the front.

    The head on the 1HZ has minimal coolant volume around the exhaust ports and when this engine is run under load (many by adding a turbo) the cooling system is unable to cool down the head in this area. As a result the temperature variation across the head causes it to crack. This is why I am running the direct injection 1HDT as there is no history of these heads cracking.
    Toyota Landcruiser 80 Series 1990 VX Ltd - 12HT powered- diesel for run-in period (no longer 1HDT).
    190L vege oil LR tank and 90L diesel, Vormax, 30 plate FPHE, two automated 3-way ball valves with delay to prevent mixing on purge.

  2. #2
    Join Date
    Dec 2006
    Townsville, North Queensland

    Re: 1HZ 1HDT series engine coolant flow.

    This new design is not only used by Toyota.

    Yesterday I decided to change the coolant in my 1998 Mitsubishi Triton. (4M40 motor also used in Pagero)
    On removing the top radiator hose from the fitting on the head I discovered the thermostat is not located in there as one would expect it to be.
    It's on a housing on the side of the block, connected to the bottom radiator hose. I can see several advantages to this design. First thing you notice is that it is much easier to refill the cooling system, as the thermostat does not trap heaps of air inside teh head. On starting the engine the water pump pushes all the air out of the heater circuit and it can bubble up and out the radiator cap immediatly and not hane to wait for the thermostat to open.


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