Heater blows cool - what else on the checklist?

[DeadheadNM]

The underdash guts of my 72 Blazer with A/C are exposed as I added an am/fm radio recently and need to replace a leaking heater core. This winter I had lukewarm air blowing from the vents and want to fix this problem when I replace the heater core. So far here's what I've checked to trouble shoot the lack of hot air:

- coolant level is fine
- have hot coolant flowing to the heater core past the heater control valve
- vacuum lines seem routed appropriately with no apparent leak
- A/C control lines seem appropriately routed

I'm concerned about some broken/stuck flapper valve or bad vacuum actuator somewhere. Which of these duct work components should I inspect? Should there be a noticeable vacuum coming from the vac ball affixed to the A/C condenser box on the firewall with the K5 running?

I'd appreciate any and all suggestions.

Thanks, Pete

[67chevy1]

The heater core wouldn't be about half plugged would it?

[DeadheadNM]

Originally Posted by 67chevy1

The heater core wouldn't be about half plugged would it?

Haven't torn into it yet - it's possible for sure. I'm trying to come up with any other possibilities to check in case the core isn't the problem. I want to fix it the first time and quickly so I can drive the roads in Glacier Park now that spring is here - with the top off the Blazer I may need a working heater up on Logan Pass!

[SS Tim]

I'd start with a factory manual review then a vacuum check. Use a hand pump and check your actuators and switches. The ball (reservoir) just stores vacuum but it would be worth leak checking. Cap the ports and pull a vacuum on it.
Might consider a set of seat heaters they can make a world of difference.

[DeadheadNM]

Originally Posted by SS Tim

I'd start with a factory manual review then a vacuum check. Use a hand pump and check your actuators and switches. The ball (reservoir) just stores vacuum but it would be worth leak checking. Cap the ports and pull a vacuum on it.
Might consider a set of seat heaters they can make a world of difference.

Thanks Tim - good suggestions!