My heater is warm at best when the truck is at normal operating temperature, using a temperature sensor on my multimeter it's only about 100 degrees. Just had a new headgasket put on at a local diesel shop and the head machined and tested with no leaks or cracks in the head. Put a new waterpump and Cummins thermostat in it, not sure if it was a 180 or 190 but it runs right at 190 according to the dash gauge. What should be the normal temperature of the air coming out of the heater? And any ideas what to check?2001 3/4 5.9 L Standard output, rebuilt engine stock.

I see about 140-150*F from my vents so I would imagine your heater core is plugged up with debris. You might try flushing the heater core out...

Ok, I'll try and flush it out with garden hose and see if that works. For what it's worth, I was outside with it earlier and it was 80 today and got it up to normal running temperature it got up to about 118 and both heater hoses were hot.

Actually if the heater core is working it would have one hose slightly cooler than the other because of heat exchange... Just like the radiator the upper hose is always hotter than the lower. But when the heater core gets plugged up then both hose are hot because there is no exchange of heat...

I'll check it when after I get home and see what they're like, I didn't pay that close of attention to it this afternoon when I checked it. Thanks for your help.