I have been having this issue lately and cant seem to find an answer online. I had a coolant leak two weeks ago from a hose that split. Got it fixed. Then I noticed I had no heat and coolant spit out of my reservoir so I bled it to get the air out.
Now lately I have noticed that I dont really get any heat until I get on the highway and sometimes when I get off the highway the heat seems to cool off again. I replaced the rad cap today and drove around. I got heat from city driving but when I got home, coolant was boiling in the reservoir again when I turned the car off. Its an sr, dual flex a lite fans (on all the time but I think only one is working now). Stock rad i believe.
What should I do next? I was going to get that other fan to work and change the thermostat.
I did have my heat on full when i bled. Thermostat is i think the way that i am going to go. I dont think its the head gasket because there is no oil/coolant mixing. I have seen that possibility discussed as well.
A head gasket can leak without putting oil into the antifreeze (combustion pressure from a cylinder pushing past the gasket into a water passage would cause bubbles/boiling in the system). I suggest getting a pressure test done on the system and go from there.
Here is where it gets funny… i changed the rad cap yesterday and today i drove to work and had the heat come on quick… no boiling when i got to work. 30 min drive. about 10 min road, 15 min highway, 5 min road…