GMC Sonoma

I've got a 2000 GMC Sonoma with a heater that works when it feels like it.
The heater controls change the air flow ok, ie floor; dash; defrost.
At times, the heater seems to have water flow through it but maybe
restricted, because when heat is turned on, it's hot but cools quickly like
it has reduced water flow.
Other times the heater works like gang-busters

I've checked the water level
I've checked the thermistat
Both hoses to/from the heater get hot

Is there a control for the water flow to the heater core on this model?

What am I missing?

Bill wrote:
> I've got a 2000 GMC Sonoma with a heater that works when it feels like it.
> The heater controls change the air flow ok, ie floor; dash; defrost.
> At times, the heater seems to have water flow through it but maybe
> restricted, because when heat is turned on, it's hot but cools quickly like
> it has reduced water flow.
> Other times the heater works like gang-busters
>
> I've checked the water level
> I've checked the thermistat
> Both hoses to/from the heater get hot
>
> Is there a control for the water flow to the heater core on this model?
>
> What am I missing?
>
>
Check a repair manual, Chilton's Total Car Care or Haynes, for vacuum
hose routing and control of those louver doors under your dash. Make
sure the vacuum hoses are not cracked under your hood. That is, if
yours is operated by vacuum. On some vehicles the doors are
electro-mechanically operated. Are you sure the heater core is not
clogged? Is your radiator properly filled? Is the thermostat OK, not
stuck open? Have you ever flushed your radiator system after removing
the thermostat? This is for a thorough system flush backwards and forwards.