97 gmc Sierra 1500 5.7 heat problems

[killswitch]

Well to start off I have Luke warm air I've replaced the thermostat 3 times plus the water pump and the clutch fan. Someone already by passed the heater control valve. I have flushed the system too I can not figure this out I unhooked my ac unit just in case that was kicking on. I took the heater blower apart to see if there could have been a mouse nest at one time but it was clear. I did how ever notice the the panel for the heat is touchie I have to mess with the knob for it to kick on and I just noticed I moved the other knob that controls the hot and cold and the air did not change is that my problem? I will be so mad if it is I never would have guess it I always replaced the thermostat and it fixed the problem.

 

[Bill325]

I have a 1989 1500 with the 5.7 liter engine. The coolant return line for the heater empties back into the top radiator tank right under the radiator cap.
If you run the engine without the cap, you can see if coolant is circulating through the heater core. ( Your coolant level might have to be down an inch or two in order to see the heater core coolant flowing). If there's just a weak trickle of coolant, your heater core might be clogged, or there may be some other restriction in the heater coolant circuit. The cylinder head gasket has some coolant holes that are sized to control the coolant flow through the heater core. If the head gasket is badly eaten away around the coolant holes, it may not be directing enough coolant through the heater core. I never did figure out exactly why my heater output was so weak , but the engine eventually blew a head gasket on the driver side . I removed the cylinder head, and found a broken head bolt . I also found that the head gasket coolant holes were badly eaten away . I suspect the same problem on the passenger side caused the heater to work poorly. Due to several such problems, the truck is now a lawn ornament. I hope your problem is more simple to repair .