Ok, so I am dealing with an issue I have not experienced before. I'll start from the beginning with everything to help figure out what's going on. I have a 2003 sedan (auto), last May I had the motor replaced with a used motor with 40k miles and went ahead and replaced the radiator as well. Literally the day after I got the car back from the shop the car died on me in the middle of the road picking up my kid, so it sat for a while until I could get to the bottom of what it was. It wound up being the cam sensor, so I had all sensors replaced. Well, buying a new car I ended up letting the car set for a while driving it a hand full of times until 2 weeks ago. I took it to Firestone to get new tires put on (biggest mistake) and when I leave on the highway I start smelling this burning smell. At first I thought it was my brakes from how long it sat, because it was similar to a hot brake smell. Well, I get to my dads house and I see smoke coming up from the drivers side fender. When I pop the hood it is hot AF under the hood, complete oil burning smell and I look around and see the dipstick isn't in all the way. Well when Firestone did their complimentary fluid checks, the tech told the manager they had trouble getting the dipstick back in and did not tell me so I could check it out. I cleaned up what I could not being home and put the dipstick back in and drove back to my house (about a 45 minute drive on the highway) and at the stop light before I get to my house I see smoke coming from under the car and smelling hot af again. Now, the temps on the dash never even hit the middle, but when I parked and popped my hood it was ridiculously hot under there and my overflow was boiling.

After this I had the car towed to have an engine degreasing done and oil topped back off, drove it home about 10 miles and could already smell the burning oil again. This I assumed was because of possible drip down on my exhaust, so the next morning I let the car idle and get up to temp setting for about 30 minutes. Everything was fine at this point, no smoke no abnormal heat and coolant was fine. Today I let my wife drive the car since I took hers to get some work done and she called me saying that the temp shot up to the H and for some weird reason turned the AC on by itself. This was about 15 minutes of driving on the highway at 60-65. I told her to turn the AC off and just take the side road to the hospital she works at and she said the temps dropped back down below the half way mark and stayed there the rest of the way to work.

At this point, I'm confused at what's going on that would make that reservoir boil like that but not show overheating. Then now the car wants to overheat on the highway. I love the car, but holy money sink lately lol. Oh and the water pump was replaced before I had the new motor put in and the mechanic put that new water pump on the new motor when he replaced my radiator.

 

First off, these cars are notorious for getting air pockets in the cooling system. The heater hoses are routed up and around the valve covers, thus susceptible to air pockets. Do you have any coolant leaks to your knowledge? If you have a leak, this can lead to air getting trapped.
Second off, it could be your thermostat is sticking. When this happens, coolant is only circulating around the engine, and not through the radiator. If you replace it, make sure to use OEM or Nismo only!!!
Third, since you have an 03 sedan, do you know if you have a single mechanical puller fan (drives off a belt pulley) or dual electric puller fans?
-If you have a mechanical puller, you also should have an electric pusher fan on the drivers side front of the condenser. Make sure both these fans are running with the A/C turned on. I've heard the front pusher can become jammed or turn slowly.
-If you have dual electric pullers, make sure both fans are turning on with the A/C turned on. They run at 2 speeds, for my 2004 sedan, they turn on low speed when the water temps reach ~202-204*, and high speed at ~208* OR when the A/C is turned on.
For the oil smell, check your valve covers for leaks, as well as the cam sensor o-rings.

 

Thank you for the info! I've been seeing this about the air pockets with these cars in the reading I've been doing today and since I've only put 3k miles on the car in short commutes that does have me wondering if the shop that replaced my engine did it right since they specialize in domestic drag cars. I have not seen any leaks since the engine I replaced finally went and cracked the radiator leaving a lake in my work parking lot. I will have to do some closer looking when she gets home with it this evening to make sure there are no leaks and I've never actually paid attention to the fans. It's the first production of 03 that doesn't even have the trunk button on the dash. I just ordered a new oil cooler o-ring and dipstick, I'll add a new thermostat to the order since they are cheap and see where I go from there.