Thanks guys for all the advice I'm just wondering how to go about
back-flushing the heater core on my 1998 GMC JIMMY. I'm willing to try this
to see if the heater core is plugged. What I don't understand is why I'm
getting good blowing heat from the dash vents and then when I turn the knob
to winsheild defrost, it blows out cold?

"Sam" <sam.mangiapane (AT) sympatico (DOT) ca> wrote

Because when it's set to defrost, the heater has to compete with the A/C.
H

"Sam" <sam.mangiapane (AT) sympatico (DOT) ca> wrote

OK, you'll have two heater hoses running thru the firewall to your heater
core. Simply remove the hose clamps (these guys are the spring-type, super
easy to remove), and use a tapered-end sprayer on your garden hose (you
know, the real cheap brass ones). Stick the tapered end in one of the hoses
and let er rip! Then, reverse the direction of the flow, spraying water
into the other heater core hose.

You likely have ok heat from the vents as the heater isn't competing with
the A/C evaporator when in defrost mode.

Doc