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Thread: Instant Overheat Under Load

  1. #1
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    Default Instant Overheat Under Load

    Hi Mech-Warriors,

    I recently bought an '87 Kia Besta (identical to a Mazda Bongo Brawny, drivetrain of a Mazda B2000) and it's having a couple of problems.

    First, and most pressing, is the overheat issue which started shortly after I purchased the vehicle. Idling, the car runs great, does not over heat. Put the engine under load, and within ~45 seconds I'm north of the red.

    I've checked the coolant, it's topped up, running the car with the rad cap off and a funnel full of coolant, the pump and thermostat both appear to be doing their jobs.

    I did an oil change on the weekend, and have driven 300km since. The oil looks as clean as when I put it in. I didn't do a filter change as I was in a rush and couldn't get the old one twisted off.

    Would the symptoms I've described align up with a problem with the oil system or should I still be focusing on cooling system? There is minimal white smoke after I'm up to temp.

    Could there be a blockage in the filter? Failed oil pump?
    I'm not seeing coolant in the oil or vice versa so i think my head gasket is safe, for now.

    Appreciate all input.
    Last edited by bituerbo; 01-21-2013 at 02:04 PM.

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    Is the clutch fan engaging properly? Once the car is warmed up, shut the vehicle off and immediately try to spin the fan with your hands, it should not move (as the clutch shouldve been heat activated).

    If the fan spins freely, then youve likely found at least one of the problems. New clutch packs arent too expensive.

    (I owned a B2200; I had a slipping clutch on the fan).

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    I've watched the engine while the overheat occurs (beauty of cab-over-engine) and it's not the fan. Also it spikes too quickly, the rad would be dissipating SOME of the heat. It spikes up in ~45 seconds and can return to normal in under a minute of idling, also.

    Oil light is on, I had dismissed this earlier as the car is also having electrical problems, but it could be relevant.
    Last edited by bituerbo; 01-21-2013 at 02:16 PM.
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    Bad wiring or coolant temp sensor?
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    Originally posted by gretz
    Bad wiring or coolant temp sensor?
    Doubtful, when the temp sensor is pinned in the red I get reduced power (overheat symptom).

    Another theory from forum reading:
    Head-gasket failure causes blow-by of combustion gasses into cooling system, that creates a 'bubble' of hot gasses, which makes it's way to the top of the rad, where the temp sensor is?

    Is there any way I can test my oil circulation? If that's the culprit I don't want to be driving it at all. If it's just the coolant I can stop as needed.
    Last edited by bituerbo; 01-21-2013 at 02:32 PM.

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    Blown headgasket?
    Any bubbling in the resevoir? hydrocarbon smell?

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    Originally posted by sr20s14zenki
    Blown headgasket?
    Any bubbling in the resevoir? hydrocarbon smell?
    Not that I can see or smell.

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    You can add UV dye to the oil and see if any is making to the coolant. Might take a few days or might happen immediately.

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    .
    Last edited by Rat Fink; 12-04-2020 at 12:57 AM.
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    When the engine is cold, take off the rad cap and zip tie a rubber glove (like the blue nitrile ones) to the opening of the rad. Start up the engine and rev it a bit and put it under a little load. If the glove inflates even a little you have a head gasket leak.

    To check to see if the water pump is pumping simply pull off a heater hose when the engine is running. You should get a good stream out of it.

  11. #11
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    Originally posted by HO2S
    When the engine is cold, take off the rad cap and zip tie a rubber glove (like the blue nitrile ones) to the opening of the rad. Start up the engine and rev it a bit and put it under a little load. If the glove inflates even a little you have a head gasket leak.
    Awesome test!
    Diagnosed last night as a blockage just upstream of the thermostat housing. Calcium and mineral deposits, combined with bits of shop towel.

    Electrical problems (only had lights when I was on the gas) was resolved by a new DIY grounding strap from the engine to the body. Previously the throttle cable was the ground.

    Thanks everyone for the help, a couple of pretty cheap fixes!


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