I do not know if it would make to much of a difference, but if you put a vlave on the inlet and outlet and turn them both off you should help at least to cut off any abient heat that would follow the coolant back to the heatercore. I noticed on my montero even with the heat off that the outlet hose was very warm and the inlet was even warmer. Just an idea.

91 Montero LS "Sandstorm", Auto, Dual Bouncy Seats, 2" Suspension lift, 2" Body Lift, GENII front IFS, 32" MT tires

89 Raider "Trailbreaker", 3.0, Auto, Dual Bouncy Seats