I am getting frustrated as all hell. Here is what I have and done to this point. 98 Ram Sport, 5.9L. w/AC.
It all started with pin holes in my radiator. Replaced the radiator and from that point I have not had sufficient heat. Prior to installing the radiator I did a complete flush. I flushed the entire system until I had perfectly clear water. It took roughly 100 gallons of water to get this. Installed the new radiator (single core) and then I had low heat on fan speed setting 1&2. When fan speed turned to 3&4 had no heat. Upper rad hose hot and lower cold with no pressure. Thermostat gauge reading in between 130 and 210 (~170). Heater lines (core) were equal but low temp. Suspected thermostat and changed it(195 thermostat). Higher heat on both heater lines, still same heat. Thermostat reading the same. Parked truck on a hill to burp it.
Drove the truck with cardboard to restrict air flow. Finally got thermo up to 210. Upper and lower rad hoses hot and pressurized.Finally started to get some heat, but still not hot. Then I back flushed heater core until I received clear water. Great clear flow. Heat increased worked fine for a couple weeks. Then back to the same scenerio. This time low heat on return heater line. Upper rad hose hot, lower ice cold with no pressure.

With cap off it looks as though it is circulating, but judging by the thermostat gauge the truck never seems to actually get to operating temperature unless I cover the radiator to restrict flow. But I still do not get any high heat when I do get the temperature up.

No visible leaks anywhere in engine compartment, never overheats, no leaks inside truck by heater core, no smell inside truck. Blend door working perfectly (actually looks brand spanking new still). Here are my questions, Could it be the rad cap not allowing pressure to build in the system (lower hose no pressure)? Could it be the water pump? Could new thermostat be stuck open? Or am I stuck with the unfortunate burden of replacing the heater core?!
Good lord help me its cold as hell in SW PA this time of year!!!

 

One of the main problems is the heater core is up higher than the rest of the cooling system. That makes it VERY good at trapping air in there.... (I have the same issue, barely adequate heat when its cold out....) My idea was to get one of the flush kits, that have the hose fittings..... and install one at the highest point I can get, on the return side from the heater core, so I can open it up, and hopefully let the air out......

You should build pressure relatively slowly.. if the upper hose isn't getting hard, first place I would look is the radiator cap. They are cheap, so, if it's original, just replace it.

 

Well.... I just logged in here to look about this same issue on my wife's truck. She has a vetbox with the heater hoses running to it to heat the water tank. I've insulated lines everywhere but the more I think about it the truck never had hot heat before the box was installed. It was done by an rv place that had done several of them by the manufacturer's specs. They installed valves so the box can be isolated and even with them off, no heat in the cab. I've even blamed crappy heat on the extended cab. I have heard a trickling noise from the dash and knew it was air. So stand the truck on it's end to get the air out of the heater core.... I'll fool with it this weekend and make sure it's building pressure.

 

I just logged on for the same reasons. My 97 Ram 1500 with a 5.9 gas motor. The colder it get's out side, the worse the heater works. I'm also getting that trickling noise under the dash. On top of that, I can smell coolant out side the truck when I park, after driving on the highway. But nothing obviously leaking.

A little back ground. Last year about this time I replaced the plenum cover and gasket with a kit from Huges. They also recommended installing a 180 degree thermostat. I'm sure that's part of the problem, but could there still be air in the system after a year? I also tried cardboard over the radiator. That helped the first cold snap, but hasn't done much the on the second one. Advice/help please?

 

The heater core likes to trap air as Heyyou stated.
Try parking the truck on a steep hill (nose up). and run it for a few minutes, to help bleed the air out.

 

The 'angle the nose up' trick didn't work for me.... I parked on a hill with a pretty serious Pucker Factor.... and I still have issues. Gonna take it into the shop tomorrow (hopefully) and try the experiment I detailed above.... sure can't hurt huh?

 

I have tried everything short of replacing the heater core which I don't think will render any success. I have flushed, and flushed, tried the funnel, tried nearly verticle, tried drawing a vacuum before filling, you name the trick I have tried it. The result still very little heat. I am still not 100% sure that the issue isn't the blend door not fully closing, but have no way to check.

 

I think you can take out the blower motor, and kinda/sorta see up in there..... While you are there, clean out as much of the stuff that tends to accumulate in there as well.

 

I'm pretty sure my doors are closing. Wife's working and truck is gone today. I got to do something about it or I'm in the dog house. I'll keep checking back.

 

The heat doesn't really work, and she drove it anyway??? Really? My wife always looked at me with 'that' look, and said: "This one's broke, YOU drive it....."