The fans go on max for the simplest of demands

Did you actually remove the motherboard? There is a few screws that hold the heat pipes and fin array to the motherboard. these have to be just tight enough to not allow it to back off. Also if it did come off somehow you probably just need to replace the thermal paste after you clean off the old paste. the old paste dry’s out and if the “seal?” is broken almost never reseats well enough without replacing it. clean old stuff off with alcohol and qtips. Also check the fins around the outside of the fan. These can get a layer of dust that you can’t just blow out and sometimes requires manual removal.

also when you put on the heat sink each screw should be about the same tightness. if even one screw is to tight you will only have a partial connection. You should also tighten in a cross or star pattern so you have the same tightness on each screw.

My guess is that you have one side to tight so the other side of the heat plate doesn’t get down enough.