Engine runs cold, then hot, then cold again

Discussion in 'Gen 1 & 2 - Engine, Exhaust, Drive Line & AC syste' started by Kens1992mtxSHO, Jul 14, 2007.

  1. Kens1992mtxSHO

    Kens1992mtxSHO Ken is a bad, bad boy and should be banned Staff Member

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    My engine has suddenly started running much colder than normal. Under highway speeds the temp gauge sits at the "L" in NORMAL. When I get off the highway the temp gauge goes up to the "O" or the "R". One instance it went up to "N". My SHO didn't do this prior to my Y-Pipe install on Tuesday.

    I'm guessing it's the thermostat going bad. What else will cause such rapid temp fluctuations?

    Thanks guys,
    Ken

    EDIT: I have checked coolant levels and everything is OK. No oil in reservoir tank either.
     
    Last edited: Jul 14, 2007
  2. excidere

    excidere lost

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    You might also make sure that the fan is kicking on like it should, which that could be relay/motor problem, or a thermostat problem. My old '95 did that and the reason it was so low on the highway was that the air was pushing through the radiator anyways, and then at a stop or low speed there was not enough air getting through to cool efficiently.

    In my particular case the fan motor had burnt up, so I replaced it and all was good!
     
  3. sdpatt

    sdpatt Sr. SHO Engr.

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    The thermostat has the ultimate responsibility for controlling the temperature of the engine coolant. If it is running too cold when moving, the thermostat is likely not moderating the coolant flow properly. However, if the engine is running hot only when not moving, the radiator fan operation should be checked.
     
  4. 93rev2sev

    93rev2sev SHO Member

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    Co-signed thermostat problem.
    It's probably sticking in both open and closed positions...pressure gets high enough from heat and it pops open...all the way...and sticks there. Engine cools down and thermo stays open...until...it gets cool enough to pop it closed again where the cycle will repeat.
     

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