2017 Peterbilt 579 Heat not properly working

If you had a faulty water pump, you’d see that on your temp gauge.

I believe, you just have a water flow problem right in the area you’re messing in.

 

As the diagram shows, there are restrictors and valves. I’d disassemble everything and see if one of those pieces is plugged. While that’s all apart, I’d put a garden hose to the heater core, just make sure it still flows free and there isn’t fresh debris plugging it, somehow. Put water to it, in both directions.

 

If that gunk plugged the heater core, you can bet those flow restrictors and this new heater core are probably plugged.

You need to find out what the gunk is.

 

It’s hard to say. Could be some sort of stop leak, something degrading, old coolant having never been flushed, I can’t say for sure. There should be a coolant filter, somewhere. It’s probably packed full, too.