So my 98 Ram 1500 was heating up a little. Long story short, I had crud in the cooling system, so I flushed the system till everything looked clean coming out. The truck no longer gets hot and it seems the thermostat is opening and closing correctly, BUT I am not getting much heat in the cab of the truck when I turn the heater on? If I turn the heat off for a sec, then turn it back on it will blow some warm air for a min, but then back to cool air. I am hoping my heater core is not bad, so I am checking to see if I am missing something?
Sounds like a clogged heater core have to me but first verify the coolant level and water pump operation. If you have an infared thermometer, check your heater hoses. The inlet should be close to vehicle operating temp and the outlet should be around 25 degrees cooler. You can try back flushing the core but don't be surprised if you have to replace the core.
Had the same problem, flushed the core...still no heat. Had pressure test done came back good. Started to overheat after a while, verified thermostat was working and water pump was good. I fiinally found a small leak on the side of the radiator so I repalced it along with the cap. Still no heat and had to keep adding coolant but no visible leak. Long story short I foung out head gaskets were bad. Replaced both sides and viola....heat and no more issues. Start with the basics and then move on to the extreme. Hope this helps.