I am sure my heater core is plugged up. My truck spent most of its life in Texas. I replaced the thermostat last year and helped with the engine getting warm but I still have little heat. I have heard stories of some older Cummins trucks having their heater hoses reversed to help with heat. I think I need to clean my heater core. Any ideas? What about CLR? I was thinking if I take the two hoses off and flush and fill the core with CLR it may help. Flushing with clean water before reattaching of course. Has anyone had luck with flushing before? What did you use? Help please. My wife is freezing. BTW the driver side set heater is not working either.

Reverse flush the heater core. Drain the coolant or pinch off the lines, flush the heater core with a garden hose, blow the heater core out with air, and refill. If you just pinched the lines, top off the system with pure anti-freeze to make up for the water in the heater core.

I have done this to multiple vehicles, 99% of the time it makes a world of difference.

I run the garden hose through last year both ways but I was in a rush. I was looking to bump it up a notch. How much PSI can it handle? I think I may need Chemicals.

The rad cap is set to pop at 15psi, so at least that much pressure. Doing it with hot water may help as well. You could reverse the heater hoses, and run rad flush through the entire system. Only issue I see with that is the debris plugging the heater core has to go somewhere. Other side of the core, into the rad, somewhere like that.

Maybe it is just me but every time i have flushed a radiator it will spring a leak. IMHO flushing is for newer units. If your unit is getting to the point where time and mileage is working against it just replace it. I just replaced my heater core on my 99 2500 and it took about 4 hours. Half of the time was discovering some hidden bolts. I could do it now in under two hours. Probable about 20 bolts total.

had the same problem. took out the glove box there's a vacuum valve behind there i took the rod out of the clip and wired the outside air vent shut lots of heat in the cab now at -40c

this may be a dumb question but did you check the temps of intake and discaharge hoses, if the dicharge is considerably cooler then the core is fouled but if they are close then you have a air dist. issue, temp mode door

flushing does work but yes it does sometimes lead to leaking, flushing the crap out also can mean flsuhing the last bit of corrosion that was holding the leak in

im not quite sure how reversing intake and discharge can improve heat output, if anyone can enlighten me on this I would appreciate it

2 hours for R&R heater core is smokin fast, without destoying things my best time is about 4.5

good luck
scott