My 1999 Chevy Malibu runs hot? It started when it overheated. We changed the thermostat, then water pump, then temp sensor gauge and the temp gauge goes high but only when the heater stops blowing hot air. What is causing the heater to go from blowing hot to cool and then hot again? What do I do now? Im at a loss. One minute it is in the center of the temp gauge and then it shoots up and then it will come down real fast. Engine doesn't seem to be overheating. I noticed it goes up when the heater starts to blow cool air instead of hot. Please help!

Are you sure the radiator is full of coolant and has no air pockets ?
How long does it take for the temp gauge to go up to hot ?
Do you know for a fact the engine really is hot ?

replace thermostat did bleed coolant system check coolant level.because if not engine could have overheated and blowed headgaskets warp or cracked the heads check engine compression.sound like engine got too hot.

Change the coolant sending unit for the gauge. Usually this is different than the Coolant Temp. Sensor which is for the pcm. Maybe the sending unit is no longer functioning correctly after overheating. Of course, a scanner could show the temperature as recorded by the CTS, or you could use a hand held infrared temperature device to get a better idea of the true temperature.

Sounds like you have air in the system, try burbing the system , they make a special funnail for this procedure

well, a lot potentially COULD be wrong. here is my list of what to check.

hoses for deterioration and sponginess, its unlikely but possible your hoses are collapsing due to poor condition and not letting water flow through it

check thermostat for proper operation ( remove thermostat, compress spring to open it insert string then suspend it in a pan of boiling water. it will fall off the spring at the aprox. temp its set at. if it is stuck closed then its your culprit)

check water pump ( hard to diagnose, some cars have a weep hole at the bottom of the water pump that drips water while running at any temp. )

check antifreeze dilution and condition, water cools better that anti-freeze 50-50 is recommended but in hotter climates 25-75 is more appropriate. also the dirtier it is the worse it cools

ensure your engine fan is operating by running the car and observing it with the hood open. at a specified temp ( 200-230 F neighborhood) it should turn on. If it doesn't then check your fuses and wiring and repair/replace as needed

finally check for a clogged radiator/ flush it. If the car was bought used or you know for a fact the radiator stop-leak pellets were used in it. you will most likely have to replace the radiator

other factors are engine timing (shouldn't be an issue for your year of car)

I hope this helps a bit

try the thermostat, located on the top radiator hose. top side of engine. also does it steam?? does it lose water but no leak, does the heat blow hot? if your heat isnt hot and it steams and loses water you may have a cracked head or head gasket. if you have heat, doesnt lose water and doesnt steam it may also be your water pump. most pumps will leak visibly from the weep hole when running

Last time I had this problem I had a cracked head. With the cap off the radiator, run the engine at idle to operating temperature. When your looking in the radiator you should see the water circulating if the water pump is working. When you say the motor is over heating, if you have fluid coming out of the over flow hose then yes you are most likely overheating, if you are just seeing the gauge indicate you are over heating you may just have a bad engine temp sensor.

Sounds like you have a faulty head gasket or maybe a cracked head.This can cause overheating and the check engine light to come on.The code for the o2 sensor is probably caused by coolant getting into the exhaust system and coating the sensor.The coolant fan may work but it cannot remove enough heat from the rad to make the engine cool off.Try some engine coolant stop leak to see if it will help.This maybe only a temperary fix.Let me know if you need more.MOE

is it acually getting hot or is it just the gauge on the dash saying so? a quality scan tool can show you what the coolant temp sensor is telling the pcm it may differ from the gauge.

coolant sensor maybe 19 mm key its near the upper radiator hose in the engine thanks pierre