1998 gmc sierra 2500 4wd 5.7li.

engine runs cool 70-100, mostly70.
have changed thermostat 3 times.
Any ideas?

5 thoughts on “1998 gmc sierra 2500 4wd 5.7li.”

  1. Temp sending unit may be bad and giving you false reading…. If you have or can borrow an inferred temp reader, double check temp.

  2. The heat blowing warm air instead of cool air could be a Hot/Cold Blend door actuator issue. But First i would make sure the engine is getting to proper temp by using the inferred gauge before tearing the dash apart.

    Also a clogged heater-core can cause cool air, but shouldn’t cause the temp gauge to read differently.

  3. The temp gage goes up the heater air temp goes up.The gage goes down the heater air temp goes down. I will check the engine temp.
    What happens is that the engine will warm up to 100 degrees normally then suddenly cool to 70. Then if the engine is under any strain (like going up a hill) the temp will go up to 100 (normal), then when under no real strain the temp will go back down to 70.The air temp from the heater will go up and down accordingly.
    Is there something else that regulates the engine temp?

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