This sounds like the radiator fan is not coming on. You can see if the fan will come on with the A/C. When the A/C is switch on the rad. fan should be forced on. If the fan does not come on then you may have. 1) blown fuse. 2) bad fan relay. 3) bad fan motor. If the fan comes on with the A/C Your engine coolant sensor is bad. You can by pass the sensor and force the fan on all the time. Or find an after market fan thermostat. 
See Hyden Automotive
Try and find a good used one at a local salvage yard. Be sure your rad coil is clean of bugs and road debris. If yours has the GM 2.8L V6 make sure you do not have a bad head gasket, these engines have a tendency to leak coolant into one of the cylinders. If you are loosing coolant with out seeing it drip onto the ground, it may be going into a cylinder. If you think this may be happening, have the coolant checked for the presence of exhaust gas. There is a test to see. If you do not want to cover that cost, you can keep an eye on the coolant level, if it seams to be disappearing, you can add a coolant system sealant. The best ones look like metal flakes in copper or alum. colors.
Thank you for you help with this you seem like you know these cars. I just bought this 1998 chevy monza 3 days ago used and I Feel like maybe i got took. The AC does not work which I knew when I got the car. It seems like there are two fans by the radiator. one in front right and the other in back left. The one in the back left is running great and it keeps the left side ( driver) of the radiator cool however the fan on the right front does not come on and that side is very hot. Could this be the problem?
The system uses both fans when the A/C is on. The pusher fan, out front is for when the A/C is on. You can wire this fan to stay on with the other to help with this problem. During the summer this may be a good idea, but come colder weather you may need to separate them again. Last thought, have you checked the coolant thermostat? The short of it is, to replace it, if it is suspect. Look for the top rad. hose where it connects to the engine. This neck is the T-stat housing. if it looks like it has been off and has a new gasket under it, it has been replaced, so that would not be the problem.
Thanks for your help. We took the thermostat off to see if that was the problem and the car is not overheating. in fact it does not heat up much at all so we thing that that was the problem. I am going to put a new one on tomorrow. Thanks for your help
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Thanks for your help. Well I went and got the fan looked at and they said that it was running slow so they fixed it and now it is running faster, but my car is still overheating. I drained all of the water out and replaced it with coolant but still it is running hot. I am wondering what my next steps should be.
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