1999 pontiac grand am gt is overheating i replaced the thermostat, waterpump, and the upper hose gave it a full flushing and still leaking antifreeze and now it started to overheat on me. at first it would just tell me i was low on coolant so i would fill it. but i was doing that twice a week. then i found that the upper radiator hose had a crack replaced that. still low coolant light would come on. so replaced the thermostat and the waterpump. but b4 all this it never overheated now it's overheating. i'm lost and need help.

Sounds like you had a sticking thermostat when you replaced the waterpump causing an air pocket. Replace the thermostat, and jack the front of the car so that the radiator cap is highest part of the cooling system. Then run the car and burp the air out of the coolant system.

is there a chance that one of your heater hoses are clogged? or where they connect to the engine are clogged still? sounds like a clogged system. Cracked heater housing? Outside air bypassing the heater coil while driving?

Sounds like you still have air in the system..Does the vehicle have any bleeder screws on the thermostat housing or around the waterpump area..If it does open them up until you get a good stream coming out

does the upper and lower hoses seem to be the same temp when car begins to overheat? ore the radiator cooling fans coming on before the car overheats? did you pressure test the cooling system and radiator cap? also may want to remove the lower hose and flush the radiator. is the check engine light on and does it run bad?

I would replace the rad, you've done everything else.

The Grand Marquis's have a know problem that causing overheating. The intake manifold cracks around the water inlet tubes on the intake manifold causing it to loose anti-freeze which in turn causes overheating. Mercury has a beefed up replacement intake manifold that cures the problem. It was a subject of an unannounced recall but they would replace it unless the problem occurs. The recall was for 7 years which the 1999 model wouldn't qualify for now. It happened to me and cost about $450.00 (parts and labor) to fix it. I used a local repair shop to do the work. If, after the engine is hot, you hear water/steam coming from the top of the manifold, you have the problem

if you have done all you said you have their is only two things left i know of that will cause them to overheat and the first thing is if its the 4 cyl 2.4 litre it is probably the head gasket blown really common on the 2.4 engine. If it the 3.1 litre engine they have two bleeder valves that when you flush the out or they get low on antifreeze they get air in the line and stop the flow of antifreeze getting threw the engine causing them to overheat and possibly damage your engine. Hope this helped out with your overheating problem.

never use straight antifreeze, always use a 50/50 mix, i would do a complete cooling system flush.

White smoke is a sign of burining antifreeze. When heater is not blowing warm air, that is a sign of faulty heater core.
No power, coolant loss, overheating, is sign of headgasket leak.
Double check your thermostat as well.