Customer Concern: No heat. Checked the heater water valve, found hot on one side and not on the other. Made sure that temperature control was on hot. Checked for vacuum to the water valve, found none. Removed the control panel and did a smoke test, found a yellow vacuum line under cover for dog house, was not connected to anything, the end was a Y connection. Applied vacuum to the Yellow hose and found vacuum at the water valve. According to vacuum diagram the White vacuum line is the water valve feed. But apparently this is incorrect. Tests/Procedures: 1. Test for stuck water valve.
2. Locate the Orange hose from the temperature control to the recirculation vacuum valve. Connect the water valve Y into the vacuum line. Potential Causes: Connection — Hose connection. Water Valve
Tech Tips: Vacuum line is from the recirculation vacuum actuator control hose. The water valve gets vacuum only when maximum A/C is selected.
I have this problem with my Blazer, found that the heater core gets plugged up on the inlet side from the garbage in the coolant system. Quick check if this is the problem: Have warm air on low speed with the temp all the way up then turns cold after turning the speed up to full on the fan. Quick Repair: 1) while the coolant is cold, pull off the inlet and outlet hoses on the core, located on the fire wall above the distributer. Take a garden hose and back flush the core on full pressure on the smaller pipe. Little messy, but effective. I usually do it both ways a couple of times. Watch close and may see a slug of brown garbage come out. Put back together and check if the heat comes back. Check your coolant levels and temp rating after.