L39 overheating

[Don87]

Man I'm impressed (thankful) with the quick responses. Yes the radiator and overflow bottle is full. The system is clean. I put 100% antifreeze in when changed. No I have not run it with the cap off. I didn't know this is necessary. Will it help to do it now? I have noticed if I run it for a short time and remove the cap it has back pressure in the radiator (not from steam). Should I change the oil to check for water?
Steve

When changing the antifreeze what I do is fill the system, then with the cap off, run the engine until the thermostat opens, then refill the system(too eliminate air). then I'll wait til the thermostat opens a second time, and refill if needed. I increase the throttle to get the thermostat to open quicker.