Question about 1990 Mitsubishi Montero

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Coolant blowing out of radiator into overflow bottle at start up.

1990 mitsubishi montero 3.0L V6, engine rebuilt, cooling was fine before, now cooling system pressurizes at statup and blows coolant into over flow bottle. i thought it might be a cracked head or the head gasked installed wrong,swapped heads and another new gasket, still doing it, engine does not overheat and no coolant in the oil.

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  • renshaws Dec 12, 2009

    i had the work done at a shop that rebuilds motors and took it back when i noticed this problem, they are the ones that swapped heads, apparently they have another montero doing the same thing and they are stumped, i thought there might be a more exotic answer. if it were a cracked head or block, wouldn't it also overheat?

  • renshaws Dec 12, 2009

    I had assumed they would know this, the only other info i found was that if the o-rings or tubes from the heater core are damaged during the rebuild, air can get into the cooling system, but i wouldn't think it would be pressurized like this. if i fill the radiator and then start the engine, the overflow immediatley fills up and if i take the radiator cap off at this point the system is pressurized and the coolant in the radiator is still cool. is there a way to test the pressure in the radiator and see if it is over specs? any other place this pressure could be coming from other than the cylinders?

  • Mark Egan May 11, 2010

    Did you have the replacement head magna-fluxed to check for cracks not visible to the eye? Does the water seem to percolate out of the overflow bottle, like a coffee pot? If so, you've got a cracked head, or possible cracked block, which does not necessarily result in water in the oil. Microscopic cracks can occur between cylinders on the head or block, and not lead to an oil passage.

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Typically would not overheat until enough coolant was lost. Find out if they mag fluxed the heads, or had it done. A lot of shops rebuild without doing this step as it's expensive. If they got a rebuilt head, again they need to know if it was mag-fluxed.
It's harder to mag flux the block while in the vehicle and assembled.

Posted on Dec 12, 2009

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  • Mark Egan Dec 12, 2009

    No place the pressure can come from except a compression stroke of the pistons.

    Try this - with a cold engine, leave the radiator cap off, and start the engine. Watch the coolant, and at some point you should see flow begin to occur. When that happens, if you get a big flush of coolant and "air", then the "air" came from the compression of the pistons leaking into the water chamber somehow (the crack I've mentioned.).
    If you don't see that, then just watch the water, and see if you see constant bubbles coming up. Again that is the compression leaking into the water chamber.
    If the water level falls at any point, refill with water (you don't have to use coolant until you get the problem solved.).
    Another way to test is to leave the radiator cap lose and drive around. If the problem is a cracked block or head, or bad head gasket, then you will eventually overheat, as you will have pumped all the coolant out.
    Finally, radiator pressure should not exceed 10-12 pounds (if I remember correctly.). It's not easy to measure what pressure is building inside the radiator though. And if you think it's the radiator cap, buy a new one. They are cheap.


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    Item Part Number Description 1 8100 Radiator Cap 2 - Nut (1 Req'd) 3 - Bolt and Retainer (1 Req'd) 4 - Nut (1 Req'd) 5 - Bolt (1 Req'd) 6 - Coolant Recovery Hose 7 - Coolant Return Hose 8 - Coolant Recovery Hose 9 8A808 Degas Bottle A - Tighten to 9-12 Nm (80-106 Lb-In)
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