Took my first 600 mile run this weekend. About halfway in, I saw the temp rise, but not overheat. I have Touchscan, and an ELM327 OBDII adaptor, so I could monitor the temps pretty accurately. I never drove over 63mph for the whole trip, but once it started heating up, it was pretty consistent:
45 mph around 215
55 mph around 224
62 mph around 235

Moving the temp lever from A/C to Heat would produce a bit of heat for 30 seconds or so, and then room temps. I have verified that the heater core control valve is operating by looking at the position of the arm on the side when changing from heat to a/c and back.

I have changed the water pump a couple of months ago, and did not "burp" the system, so initially I'm thinking that I might have air in the system, causing the hot spot. Also, the van has not been used for nearly 2 years prior to my purchase in February, so I suspect that the rear air/heat, and the front heater core may have air pockets as well. I have since rigged up the equivalent of a Lisle no spill funnel, and have run the engine for a couple of hours over 2 or 3 sessions of heating and cool down, noting a couple of occasions of bubbling through the system.
I have shot the heater core in the rear of the van with a digital temp gauge at 167 degrees, so I think I am getting flow to there. I have shot the front heater core hoses, especially at the y fittings with around 160 and 135 on the other hose. definitely not cold, but could be conducting heat from the engine compartment.
A test run today appears to have gotten somewhat better, but I have not driven more than 5 miles at 55-60 mph. Got up to 198, but no higher. I can live with that in Florida.

Sorry for the ramble, but I have to do another 600 miles in a few days. wondering is there is something obvious I'm missing, or any other suggestions you might have from experience with running hot, but not running away to overheat.
thanks for looking.

 

You don't mention which engine but based on these symptoms I'd say either your thermostat is sticking and not opening fully, otherwise your fan clutch is shot.

 

I would also look at flushing the cooling system. With low heat coming from the heater core I suspect your core and very possibly your radiator are partially clogged.

On my van I had to replace the radiator though it looked good and clean from inside the opening of the cap it had plugged up cores.

Do not use a flush kit, just remove your heater core hoses and flush with a garden hose both directions a few times to make sure all the debris is removed. If unable to get good water flow I would replace the cores.

 

When you changed out the water pump, did you use pre-mixed 50/50 green coolant, or mixed it yourself or long life coolant.

If the answer is pre-mixed 50/50 from the jug, you are safe there.

If you mixed it yourself to 50/50, did you use ONLY distilled water or hose/faucet water? Distilled water is the answer, else internal corrosion happens within a few weeks.

If you used long life (non-green color) coolant, you will have all sorts of troubles since the Ram Vans do not have coolant systems that can support this. The long life coolants take longer to react to high temp conditions and has a weaker ability to protect against galvanic corrosion.

 

Thanks everyone for the suggestions. It has gotten somewhat better, but I haven't driven for long periods, so I believe that I will still encounter the issue on the way back.
Plan is to drain, and backflush the systems independently so I don't move gunk from one heater core to another or the radiator. Then I will be sure to use the 50/50 premix. I did do my own mixing of Prestone and yup, hose water.

 

I also have a 1996 Ram 2500 & live in Florida. Mine has a 5.2L and is my work vehicle. As the weather started getting hotter, I found my temp gauge climbing to about 2/3rds, then it would drop once the t-stat opened, but then would creep back up around 2/3rds. The fan clutch was leaking out the front so I replaced it but it didn't change it. Then a week ago, I discovered a leak but it only dripped when shut off. Coolant level was fine, but after checking it with a pressure tester, I found a leak at the upper left cores in the radiator. I replaced it and now the Old Girl runs nice & cool (below half) like she did before the heat came. I found a radiator online at Advance Auto for $185. Use promo code HUGE50 and you only pay $135. Hope this helps.

 

Funny this thread would pick back up. The van was running ok around town until I went up a substantial hill last week. by the time I got to the top, the temp was up to 238 degrees. I shut it off and went to a 2 hour meeting. When I came back out, I had leaked about a gallon and a half of coolant. Still couldn't tell where. going back home, I had to run the heater full blast to control the temp. Once I arrived, I opened the hood, and saw that coolant was everywhere,and there was a very tiny pinhole stream coming from the bypass pipe, near the base that plugs into the water pump. Of course, the stream was aimed at the radiator fan, which explains why it was everywhere! The pinhole was from corrosion, I think.
I replaced the pipe, and so far, it appears solved once and for all.
I can only figure that my daily driving doesn't get me to high rpms, so there wasn't enough pressure to significantly leak. On the interstate, and climbing significant hills was another story. The part is available at Autozone for 10 bucks. Most online stores were 22-36.00 How's that for a switch?

 

Sounds like you have it solved. I would still keep an eye on your radiator, as mine appeared to be circulating well but was causing the van to run warm. What a difference...runs cool even though it's 90 outside. Good Luck