I have an 01 with a 2.7 and 131,xxx. Car wouldn't heat up at all after hours of driving so I put a thermostat in. It didn't chance anything so I figured it was bad, I put a second one in there. Both from oreillys and the second one didn't work either. I went to the dealer and got one and not it heats up to the bottom single line mark but that's it and the cabin heat isn't hot, just mediocre to luke warm. I did bleed the system every time. I don't know what else it could be. I don't think it's a heater core because both of the hoses going into the firewall are the same temperature. They seem warm or hot but the car takes like 40 minutes for the gauge to get up to the bottom single line. What else could it be?

 

My 02 Ram did the same thing. The heater core was plugged. I used a garden hose repair kit and tapped into the exit hose of the heater core with a garden hose. I then flushed the heater core with the garden hose. All kinds of crud came out. I now have heat. By the way, both heater hoses were warm or hot and the same temperature before I flushed the core. You may also have a problem with your water temperature gauge.

 

There's a fault code for "Engine is cold for too long": It's P1281. The set condition is as follows:
"The engine does not warm to 160° while driving (throttle off idle) greater than 20 mph for 27 minutes after start."

I don't believe this code will turn on the "check engine light", but a good code reader should find it if it's there.

Aftermarket temperature sending units seem to read a little lower than OEM ones, so if you have replaced your coolant outlet manifold with one with a sender, that may be part of your problem.

You can always test your thermostat in a pot of water on the stove. It should open before the water boils (assuming you are not located way up in the mountains.)

As suggested, back-flushing the heater core might help. Be careful if you attach a hose to the system because city water pressure can be WAY higher than what the system was designed to withstand. Don't open the tap more than enough to get a gentle flow, and if there's no flow, shut it off immediately.

The heater control panel on this model is another notorious source of problems with heater performance.

And if you've been paying attention to this forum you should know by now that you should NEVER bleed the system with the bleed screw. You are only asking for trouble when the bleed screw seat starts spinning in the plastic housing, which it will. The system will bleed itself after a few driving cycles, and then you can add coolant as needed.

 

I am going to refrain from flushing the system. After I put this new thermostat in it heats up a little hotter but still takes the same amount of time. And I did out a new coolant air bleeder thing on the top of the motor. It came with the bleeder, the new sensor, and everything(sorry I'm drawing a blank on the actual name, it's too early!). The other day I tried to change the sensor to see what it would do and I couldn't get a wrench to fit it. Not an 18, 19, or 20 and not anything SAE so I just left it alone. I don't see how it could be the sensor or the heater core though because if those were wrong I would think it would overheat but it seems like the cars thermostat is stuck open and not heating up. I also tried cardboard in front of the engine and radiator and it didn't do anything(the fans don't run when it's this cold anyway but I figured it was worth a shot).