Car getting hot?

i have a 98 olds. aurora and i have replaced the thermostat 3 times. i have done a complete flush twice. the problem is when the car is running in the city it gets hot,but on the freeway its okay. i was told by someone working in a part store that my fans are supposed to kick on at the temperature of 160, but they arent. the fans work, but it seems they are not coming on till the car reaches 200- 220 degress and then the coolant backs up on comes out from the reserve tank. there for i dont have a radiator cap for this car, there isnt one, you have to fill it through the over flow tank. i have replaced the hoses, and i am stuck on this. i work on my cars, and this one i am not sure of.....please can someone tell me whats wrong. im not sure if its a fan relay, or a sensor. but my car is all electrical. anyone have any suggestens?

Car getting hot?hijack this

try replacing your radiator cap or make sure your fans kick on at 160 degrees or below also check and make sure you dont have a blown head gasket how to check is you take off your oil cap and see if it has something like a pudding color on the part of the inside of the oil cap that goes in the valve cover that would also make it run hot. if any more questions e-mail me at hays_keenan@yahoo.com Ive been working on cars since i was 8 thanks alot for letting me lend you my advise i wont be able to get to my email on week ends so do it during the week