it may be somewhat easier to take the air box off if you have big hands like me haha..
Th best way to check for a plugged heater core in my experince is when the car is warm or hot turn your heat on medium blower speed with heat maxed out and touch both hoses if they are both hot you have flow..if one hot one cold its plugged..when i say both hoses i mean the hoses that run to the heater core itself not the radiator hoses
the northstars ive seen with bad head gaskets..they got replaced and like said never run right if they don't crap out a short time after
the cap.you mean the radiator cap? i mean to pressure test the coolant system with a pressure tester to see if there is a leak still
2000 Suburban 3" lift and some power mods..The daily
06 impala SS CAI,Exhaust,pirellies the other daily
86 camaro Far from stock......The hot rod
1992 camaro RS 25th anv 350 5 spd..The cruiser