Heater works fine but it causes engine temp to skyrocket?

The heater in my 1999 Chevy Cavalier works just fine, blows nice and hot, but when I run it, it causes the engine temp to shoot up really high really quick. Any advice on what's wrong? My best guess is the thermostat is stuck closed.

5 Answers

  • Anonymous
    9 months ago

    Heater core or hoses from/to the heater core is very likely leaking.  Get a pressure test done on the heater system.  But check the coolant level first.  And good mechanics do not guess.  They troubleshoot and find the failed part.

  • 9 months ago

    That's nothing to guess about. Have you taken the radiator cap off to see if there's still coolant up to the radiator neck?

    You're going to ruin the head gasket so quit driving it until it's repaired.

    Source(s): Mitsubishi Master Tech
  • marty
    Lv 7
    9 months ago

    I would check the coolant level. 

  • 9 months ago

    If the thermostat was stuck closed, the engine would be hot and running the heater  would help cool it .  Not the thermostat.

    But I do not have an answer for you.   Maybe an electrical glitch that sends the temp needle up while the actual engine temperature remains the same. (?)

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  • Joe
    Lv 7
    9 months ago

    I'm wondering if you are low on coolant, and it only flows over the temperature sensor (for the dash gauge) when the heater valve is open.  (i.e.: You may be overheating and not know it.)

    If your coolant level is OK, you can check your engine thermostat easily enough.  Just put your hand on the upper radiator hose when the car is warmed up.  If the upper radiator hose is cold, the thermostat hasn't opened.

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