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boil over at "low" temp

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    boil over at "low" temp

    our '89 hawk has developed the nasty habit of pissing coolant out of the drain tube even though the temp gauge registers nothing higher than a mid-level engine temperature. the coolant flow stops after we turn off the bike to let it cool. you can hear it burbling somewhere within the bike.

    i've checked the fan by shunting the fan switch and the fan works and the fuse is good. we recently swapped out what we presume was a 50/50 glycol mix for a water wetter solution (this was done just to change out the existing coolant which appeared to not have been changed in eons, not to drop the engine temp). the bike temp gauge registers temperature, though it could be bad and out of calibration.

    barring the situation where the thermostat AND the fan switch have given up the ghost, it seems that the general conclusion on this board is that the radiator cap is bad and can no longer hold pressure in the cooling system.

    i have seen mention on BARF that people have swapped out this cap with one that vents at a higher pressure. is this possible? recommended? should i go through a dealer to get this part? should i replace the fan switch or thermostat just to safe?

    thanks for your help.
    jeremy and traci

    swap the thermostat with a new one, sounds like it is not opening to let the 'open' circuit flow.


      good suggestion - i didn't think of that. thanks.

      from what i recall from the manual (don't have it in front of me) i believe you can check if the thermostat works by heating it in water and looking for a certain valve to open.

      obviously this is not quite right but has anyone followed the recommended steps to trouble shoot a thermostat? any other tricks?

      thanks for your help,


        Napa small radiator cap part number 703-2443
        interchangable with 703-1443

        Stant 180 part number 35788 or BT 378 180

        The photos show the Stant number you need. I had to grind a little off the
        pin to clear the top half of the housing, and grind one millimeter off the
        radius to seat it in the lower housing. Final mod is to drill the by-pass
        hole (no typo this time). Final photo shows the saucepan test with an ASTM
        certified thermometer (thank you Weyerhaeuser lab). It starts to open at
        180 deg and is fully open 190 deg

        Hope this helps!
        1\2 Liter Duc Eater


          This seems to be a common issue with Hawks. The thermostatic switch (LH side of the rad) that turns the fan on, is tripped at or very close to the boiling point of the coolant. This happens when the gauge reads what appears to be very uncomfortably hot. (if the switch is working at all) A lot of people also wire a over-ride switch to ground that simply turns on the fan. I kick mine on when i'm coming up on a traffic jam, or whenever the needle is 1/4" above the lower white line. Normal temp (for me) is about 1/8" above the lower white line, using a stock themostat.
          -I'm sorry....I did not know she was your sister.
          -If Buckleys cold mixture went would anybody know????
          -Dont piss off the quiet guy with the chain saw.


            I had a similar problem with mine. It would pee after shut down, the temp gauge was reading slightly higher than normal. After getting used to how the bike sounds under normal operation I began to notice a whirling sound, I believe something is making the water pump cavitate, like an air bubble in the lines. I flushed the coolant system a few times but I still hear the cavitating noise from time to time, I just give the bike a little shake and it stops. If I let the cavitating noise continue it begins to heat up.

            Anyone else experienced this?