1999 grand am se 3.4 v6 having overheating problems

cooling fans not coming on before coolant starts flowing out of the resevour pressure cap. I have replaced the Engine Coolant Temp sensor and the thermostat. hot wired fans to car battery and they work. when i tried to jump the relay (#12) fans did not turn on. Also engine only has about 3000miles on it since it was rebuilt. My oil isn't milky and i have no other external leaks. please help!!!!

Replace coolant reservoir cap,

you can disconnect ECT sensor and start the engine if coolant fan comes on system is good,
you can also go to radio shack and get yourself 70ohm resistor disconnect ect sensor and instead of sensor plug in resistor this will make PCM think that ect is reading high water temp about 266 deg start engine and fan should come on if not something is wrong.

i have replaced the resevour cap. i unplugged the ect and the fans came on

so if water is still coming out after replacing the reservoir cap, then you most likely had too much coolant or air in the cooling system.

Alright this is gettin crazy.. i bleed the cooling system for about 20min. and after plugging the ect back in the fans seemed to be working(they would come on when a/c was turned on and after a/c was turned off they seemed to run as needed i suppose) but after shutting the car down and driving around for a min its back to the same ole thing. do you know at what temp the fans should come on? also coolant pukes out the overflow at about 210 degrees and it takes some time for it to do it(car ran at idle in driveway for over 1/2 an hour then drove it around for 10 min before it started overheating again).

How many miles you have on the car? Sometimes clogged exhaust system can contribute to engine overheating (hot exhaust gases unable to exit the engine on time). The easiest thing to do is remove the egr. Use aluminum foil to to make plugs for egr upper plenum passages so that you do not have vacuum leak. and let that egr pipe hang loose disconnected to free up some back pressure if there is any. Then test it again to see if that will keep engine temperature down for a longer time. Yes engine will be louder it will sound like a bad muffler.

I had similar symptoms on my 2000 Alero 3.4 v6. Turned out to be the coolant reservoir itself, not the cap. It just would not hold pressure. Coolant will boil at much lower temp if it is not under pressure. Cap is supposed to hold up to 15psi before venting. I was able to figure this out by squeezing the upper radiator hose near the thermostat - when cold it should be easy to squeeze, when engine is up to temp (after 10-15 min drive) it should be much firmer (like a basketball). If its not holding pressure, your coolant will boil deep down in the engine and spew out wherever it can. Bleeding air out of system is important; make sure you turn heat on full blast and let engine get good and warm, then open the bleed valve. Hope this helps.

try putting some of that plumbing white tape around the threads of the coolant reservoir put a lot so when you tighten your new cap at least you can eliminate that posibility

Thank you for all your help. After going to the bone yard and getting a resevior, and bleeding the system again it seems to be working now. Fans kick an around 230 degrees and it cools back down to 200. thanks again