Your cooling fan is not coming on when engine is warmed up. Could be the fan motor is broken. You can check this by checking the fan operation when the a/c is on and engine is warmed up. Look at the fans and see if one or two are on. If both are on then your temperature relay is unplugged or broken. If just one fan is on then most likely the fan that's not on is broken.
SOURCE: 1999 Malibu Overheating
i would take the radiator to a radiator shop and have it flow tested sometimes they are hard to bleed the air out i usually hook my coolant pressure tester up and push the air out
SOURCE: No Heat and Overheating in 1999 Chevy Silverado
After replacing the thermostat, you must bleed the air from the cooling system.
To bleed air from the 2.2L and 2.5L engines, remove the plug or sensor
on the top of the thermostat housing. Fill the radiator with coolant
until the coolant comes out the hole. Since the plug is made out of
steel and the thermostat housing is aluminum, it is a good idea to
apply an anti-seizing compound or Teflon® tape on the plug threads
prior to installation. Install the plug and continue to fill the
radiator. This will vent all trapped air from the engine.
Any trapped air in the heating system will have to be displaced by coolant. Once the cooling system is filled, with the radiator cap off, turn of the heater at it's highest setting. Start the engine and allow it to reach operating temp. You should see a drop in the coolant level as the air in the heating system is displaced by coolant. Add coolant to the proper level and replace the radiator cap.
Keep a close eye on the coolant level for at least the next couple of weeks. The cooling system is a "closed" system. Any significant decrease in coolant level indicates a problem.
If you have any questions, let me know.
Please take the time to rate this solution.
Drive safe and be warm.
clarkco
SOURCE: 99 Chevy Silverado is overheating after maintance on radiator
check radiator, could b blocked, check hoses, mayb they soft, while vehicle running c if hoses r compressing, if they r would b iether radiator or hoses
SOURCE: antifreeze ts boiling back into reservoir.
it sounds like a possible head gasket problem,you can get the fluctuation from combustion gases getting pumped into your cooling system and creating gas pockets,i would suggest taking it to a shop and have them do a block test to look for combustion in the cooling system to confirm the problem,that way you will know for sure,if it comes up negative they can also perform a flow test to make sure there is no restriction in the cooling system such as a plugged radiator or a bad thermostat,hope this sheds a little light on it
Testimonial: "I bet you've hit on the problem ! Car has been "using " antifreeze for a couple years now, with no leaking anywhere. Lost through the head gasket ?"
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