Chevrolet Repair: 98 jimmy/ chevrolet blazer heater not hot, chevrolet blazer, haynes manual

Question
ive got 98 jimmy 4x4 4.3/700r4.
recent repairs in last 60 days;
-new windshield installed
-new RS hub assembly
-L/R balljoints
-L a-arm bushings
now the heater(coincidence?) quit blowing hot.
i suspect either the rotary dash temp switch or the valve that lets water into the core.
water is full, overflow is fine, temp at normal 220 +/-.
heater hose is hot on the 'in' side and cold on the 'out' side.
it does leak enough through to heater core that on speed '1/low' that it does eventually get warm in the car but on '2-4/hi' it quickly loses all heat in the core and blows cool/cold.
what about a haynes manual?
is it heater valve or control switch?
is there a test for either?
do i have to cut my car in half to get to tht heater valve?
thanks
mark
http://customclassics.org

Answer
Hi Mark, what you need to do is flush the heater core, it's plugged! To do this just remove the hoses from the intake and the waterpump. Now run water through from the waterpump hose first and then the intake hose. This will make your heat so hot you can fry a... forget it! If your really into it, flush the cooling system too to get out all the little stuff that plugs the core.

Have a Great Day

Mark