I have a 2003 Monte Carlo, at 150,000 miles, and I'm leaking coolant. My car has been overheating for some time now and I've done several things to try and resolve this problem. (This is a "must know" I am almost completely ignorant when it comes to cars.) I've changed the thermostat and the fans all within six months. I've had a leak of coolant for a while and I've had to "bleed" the air out and add more coolant every once in a while to temporarily cover up the problem. I've added stop leak as well. Also, when it begins to overheat I usually turn on my heat on full blast to cool it down, but recently my heat doesn't work and only blows cold air. Since the heat didn't work i just had to pull over and wait for it to cool. When I pulled over I saw what appeared to be steam or smoke coming from under my hood. When looking under the hood I see coolant all over the place. There are small puddles of it on the floor as well. I've asked around and I've been told that I've likely clogged my heating core with stop leak. I feel that I need a new radiator. Any thoughts on the rest of the problems? Thank you.

 

Pulling over one time is not good, let alone multiple time. Neither is stop leak, which is likely why your heater doesn't work. You most likely need LIM gaskets, head gaskets, and/or heads at this point.

 

Can you see where the coolant is leaking, there are two verry commom points on the 3800 lower intake manifold gaskets and the coolan elbows,

If it it's a 3100/3400 it is likely the lower intake manifold gaskets,

But it is all specualtion unless you can see where it is comming out of/get it pressure tested and you pin point the leak,

You are doing more harm than good by continuing to drive it with the leak, and it getting hot

 

the coolant systems in either the 3.4 or the 3.8 are very finicky systems. they are likely to get air pockets. THis may be another reason besides using too much stop leak to why your heater isnt working. air get trapped in your coolant system and wont allow it to circulate properly. you could also be masking more serious problems. your temp sensors work off coolant heat. meaning that if there is no coolant in the system, your engine temp is not accurate. you need to find out where the water is going, whether internally or externally. you also may be doing more damage to your engine by having it over heat so often. these motors can be messed up very easily by over heating, and can have have head gasket problems. intakes seem to leak at the out side corners of the lower gaskets, some 3.8 will leak inside the large plastic upper intake. the coolant elbows are known to leak, also the good ol water pump leaks are common as well. as with all engines, coolant is important. when you are having coolant leaks, you need to have them check out by a qualified tech, or someone who is familiar with the procedures of getting the air out of these systems.