My newly acquired 2.3l (5-spd.) runs 50% "in the temp. range" 98% of the time - for many miles down the highway. Heat on/off does seem to really be a factor. Then 2% of the time, temp. moves upward to "high end" of OK operating range (makes me nervous....). Then just when I panic, it lowers back down to the middle of the range which I consider NORMAL. Folks say - "air bubble". OK, I have a "non radiator cap" radiator and I've driven this truck many times - How is the best way to burp these trucks if that's not done automatically as you drive, stop, re-start engine over few days? Any ideas of my "intermediate over-heat issue" would be greatly appreciated.....I might "buy a pointer-temp. gauge to say how hot she's actually running when at the higher-end of the op. range (but gauge seems "right on" most times.....)....Larry

 

No, not an air bubble
Yes, you do have a Degas cooling system

Could be heater hose bypass system
The heater hoses have TWO functions, first is the obvious, warm up heater core in the cab.
Second is not so obvious, they are the water pump/thermostat bypass
Coolant needs to flow out from one heater hose and then back into the engine on the other one, ALL THE TIME
If there is a blockage you will get random heat up cool down cycling, no rhyme or reason to it, and comes and goes

Could be heater core itself is old and clogging up, if heat in cab is not super hot then that could be it.
There is a bypass valve that cuts flow to heater core for MAX AC, it could be broken, so valve is opening and closing