Starting information: Car has 220k miles, temp hangs around the 3/4 mark on the gauge (normally it’s slightly below the middle mark). I test it by driving to work everyday, which is a 40 min drive on the highway going 65-70mph. It does reach the red mark or the H at times. Sometimes it stays normal temperature the whole drive, but that is very rare. The A/C does not work when it gets hot, it just blows warm air. Having no A/C on at all does not effect the temperature. Reducing speed when getting off the highway does reduce the temperature from H to 3/4, but it never goes to the middle mark or below. Driving non-highway speeds, it still does overheat. By overheating, I mean the gauge shows the temperature at the 3/4 mark or higher, it never smokes or breaks down.
Overheating first started about a month ago, mechanic said right fan was not working and to replace thermostat and coolant. I replaced the thermostat and added new coolant, the fan seemed to work properly, (would turn on when it started to get hot and stayed on), that did not fix it. I noticed the car was leaking coolant from the radiator and the reservoir, when I did an inspection the reservoir hose was spitting coolant back out, so I figured the radiator cap was faulty and couldn’t hold the pressure. Turns out it was, the cap was completely broken, so I replaced that and it seemed to work fine for a day. Now the car stays at the red line or H the whole way, even reducing speed to 40mph does not change the temperature. Checking the reservoir when the car gets this hot shows it is completely full (normally at the half way mark), so I’m going to replace the cap again since maybe it’s also broken and can’t hold the pressure. If that doesn’t work, I’m at a loss as what the problem/fix is and could use help.