What is a car overheating temperature?

What intermittently causes the heater to abruptly blow cold air at the same time the temperature gage indicates hot but the car is not actually overheating in a 2000 Chevy impala 3.4 v6?

• Answer:

  QuestionI have a 1099 Olds Silloette Van instead of the 2000 Chevy..... Same engine I changed the thermostat twice and believe me its not easy...... I even check them in hot water before installing the second one and they both openned okay..... I still have the same condition..... I replaced the radiator cap...... I bled the water valves on both the upper hose fitting and heater hose.... The radiator fans both work, but seem to come at a very high guage temperature.....just short of the gauge red over heat zone...... The temperature gauge will remain steady at the first quarter mark for a few miles. Then the temperature will zoom to the gauge Red zone.... No indications of Coolant getting into the engine oil...... The exhaust does not have excessive vapor. I have also noticed the reservoir tank will back fill to to almost over flowing...... This leads me to believe why the heater become cold intermittently (no hot water in heater hose)....... The water pump works.... The upper radiator hose gets real stiff as it feels it is under extreme pressure.... When the engine is cool and radiator cap is off, squeezing the upper radiator hose cause the coolant to move up and down at the fill port..... Could the temperature sensor or circuitry that tells the radiator fans when to come on be intermittent or defective? I have a 1099 Olds Silloette Van instead of the 2000 Chevy..... Same engine I changed the thermostat twice and believe me its not easy...... I even check them in hot water before installing the second one and they both openned okay..... I still have the same condition..... I replaced the radiator cap...... I bled the water valves on both the upper hose fitting and heater hose.... The radiator fans both work, but seem to come at a very high guage temperature.....just short of the gauge red over heat zone...... The temperature gauge will remain steady at the first quarter mark for a few miles. Then the temperature will zoom to the gauge Red zone.... No indications of Coolant getting into the engine oil...... The exhaust does not have excessive vapor. I have also noticed the reservoir tank will back fill to to almost over flowing...... This leads me to believe why the heater become cold intermittently (no hot water in heater hose)....... The water pump works.... The upper radiator hose gets real stiff as it feels it is under extreme pressure.... When the engine is cool and radiator cap is off, squeezing the upper radiator hose cause the coolant to move up and down at the fill port..... Could the temperature sensor or circuitry that tells the radiator fans when to come on be intermittent or defective? The Body Control Module on some GM vehicles shuts down all unnecessary electrical accessories if it detects an alternator or battery problem so the car can travel a few extra miles to get help..... Refer to GM PI (No. PIC3045B).......