Truck Repair: 1998 jimmy blazer heaternot working, flow control valve, radiator tank

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
Hello Marc,
I happen to have a Haynes manual for that truck, and as I suspected, there is no coolant flow control valve in the heater hose system.
For one of your hoses to remain cold would indicate that there is a restriction.
One possible restriction would be an air pocket trapped in the heater core.
The other possibility is the core is plugged with sludge.
I would remove the return hose, probably from the radiator tank if that is where it connects, and then run the engine to see if it pumps coolant out of the hose.
You don't need to wait for the thermostat to open, because coolant should circulate through the heater core even with the thermostat closed.....better actually.
If coolant does not flow, then remove the supply hose from the heater core tube.....carefully, so as not to pull the tube loose from the core inside the heater housing. I would actually probably pull it loose from the intake manifold, unless I intended to replace the hose, in which case, I would slice it off of the heater core tube.
I say that, because if you damage the heater core, it is supposed to require removal of the dash to replace it.

When the hoses are both loose, use a garden hose, and flush in both directions through the heater core to make sure it is clear.
If restriction is felt while flushing, I would not try to force city water pressure to build inside the core. Some city pressures can be much higher than the 14 to 18 PSI normally found in the vehicle radiator cap, and you could burst the heater core, or cause it to leak.

The rotary control valve in the dash should control vacuum delivery to the different air control doors in the heater housing, and if a vacuum supply line was disturbed, or broken during some of the work you had done, then the doors might not work properly, but since you checked deep enough to know that one heater hose is hot, and the other cold, I am convinced the coolant flow is where I would look.
But if the right ball joints were replaced, they were in the area of the vacuum canister, and some of those lines are plastic, and break easily, so that would sure be worth a check.

Van