'96 2.2 manual COOLING PROBLEM help asap

I have a 1996 chevy cavalier 2.2 liter
i had a radiator leak, so i bought a KOYO radiator and put it in, I didnt know about "burping" the air bubbles out of the system...i drove it for about a mile, then it was extremly overheating so i turned it off and coasted about a mile (downhill) to the gas station i popped the hood and herd a sizzling sound or something like that, I was like **** theres to much pressure in the system i took the radiator cap off and that was it, it cooled down enough to make it home, the next day i tried burping the system and it just kept getting 2 hot and bubbling over no matter what, so i checked everything it seemed in order, but the car is like 10 years old and probably hasnt been that well serviced. my fan wasnt kicking in either, well i thought i got the air bubbles out so i took it around the block and wow to my suprise the fan kicked in, so i check everything and it doesnt make sense why it wont cool right and why it keeps building so much damn pressure, so i figure a water pumps only 20 bucks, i replace it, still not it, I pull the tempature gauge out and take it in the house boil some water hold the thermastat in for a few seconds it opens right up, so i put that back in i replaced the sensor right behind the engine coolant intake, still nothing i tear the whole radiator out and flush everything and replace it, still nothing, so i go buy a new radiator cap because the haynes manual says that could be one of the problems, ok so i replaced EVERYTHING execpt the heater core which i cant see being the problem, does anyone know why this would be building up so much pressure the water boiling over w/o the cap on and building 2 much pressure if i have the cap on i can't think of anything else that could be wrong someone PLEASE help me i cant afford to take it to a garage

It's possible that your first attempt at driving it with the air bubble destroyed your head gasket. This can cause air to be "injected" into your cooling system from the compression or combustion cycles of your engine. You also may just be "burping" it improperly. How are you doing it?

You can purchase a test kit that will detect exhaust gases in your coolant. I'd also do a pressure check on your cooling system It should be able to hold 15psi. A leak down test also may point to a head gasket (or other problem) due from overheating.

And the reason why it boils over without the cap is that it needs the pressure there. For every PSI, it adds an additional 3 degrees of boilover protection. So 15psi gives you an additial 45 degrees of protection.

i had my nieghbor over and he said it was my head gasket and thats a common problem on cavaliers, so i changed the oil to see if there was coolant/water in my oil, there wasnt so i finished changing the oil and took my thermostat out of the car and drove it around the block it seems to be fine FOR NOW but im going to have the head gasket checked quick but i can't afford to have that replaced.

A head gasket leak may not leak coolant into the oil. It could also leak coolant into the cylinder. It may also not be bad enough that it doesn't leak all the time, or when cool. But if it's leaking into the cylinder, it can either be burned with the combustion (tell tale sign is white smoke) or just forced out as a liquid, possibly draining out the tail pipe.