1998 Dodge 1500 Ram 4x4. Heater cool at idle, warmer under load, but still never gets hot. Replaced T-stat, temp guage reads same as before: approx 195. Cardboard in front of radiator, covering 75%, doesn't make a bit of difference. Any thoughts?

You can try replacing the water pump, but your heater core isn't fine. Unless you just replaced it. In that case you haven't purged all the air out of it.

Whats your temp guage reading?

Kind of sounds to me like you have an o2 sensor problem.

You have a blown heater core my friend. I have a quick fix and a permanent one. Quick fix is to unhook the hear lines from the fire wall and couple them together. That will stop it from over heating. 2 requires a friend and a full 6 hours. Remove the dash. I recommend getting a haynes manual for the dash removal. Hope this will point you in the right direction.

check to see if both heater hoses are same temp when engine is at normal running temp if not heater core not flowing coolant may need to flush also make sure it does reach normal temp around 200

I just bought a 98 with a 360 and it has the same problem. If I rev mine up or drive in Drive not Overdrive the heat gets alot better. I talked to a service manager at a Dodge dealer and he told me the most likely problem is the heater core is plugged with casting sand that comes off the engine block over time. I flushed it twice with some improvement but I still think it needs another one or two times to make it better yet. There was alot of gunk that came out, and the second flush I caught a sample earlier then I did on the first flush and once it settled you can see there is actual sand in it.

You might have air in the lines, go ahead and find out if there is a bleeder valve. If so then bleed it after warmed up but watch out that steam is hot.

Is the engine coolant temp also low (if you have a gauge)? Your symptoms suggest the engine coolant thermostat is missing or stuck open. With unrestricted coolant flow through the radiator higher speeds mean more airflow and cooler coolant temps.

Replacing the thermostat is a simple, inexpensive fix. Hope that's your problem.

change the themostat when you remove the old themostat you should see it stuck in the open postion . before i install the new one i fill the the block with coolant install the new stat run the mtr and heater core until hot iwould also replace the cap

you need to replace the panel...