When driving my truck this morning ('97 Ram 2500, 125k) the temp gauge didn't seem to go into the normal operating temperature, but stayed as shown in the pic. I drew a red line to show where it normally sits at when warm. I replaced the thermostat a while back, so I don't think it is a stuck t-stat. Any ideas? Also, it is making a rattling noise in the engine, sounds like a pulley or bearing noise. I don't know if the temp gauge issue is related to the noise or not.

 

The temp gauges on these trucks are notorious for being inaccurate. The best way to gauge it is if you connect an old school analog gauge.

Also, where do you live?
If you're cooling fan is messed up and you have cooler temperatures it will read cooler since the fan is not functioning.

 

Burp the cooling system again and make sure there is the correct amount of coolant. And +1 on the fan clutch. It should be fairly hard to turn the fan when cold and easier to spin when the motor is warmed up. Another sign it's bad is if you hear it roaring when warmed up. As far as the rattling noise goes, remove the belt and check ALL the pulley's on ALL of the accessories and idler pulley and tensioner pulley.

And what brand tstat did you install? And did you test it before installing? It may be defective and stuck open .

 

Don't know where you live...but is it perhaps colder outside now?

 

I just read my previous post and sorry but I typed/wrote it backwards. The clutch should be easier to turn when cold, not vice versa . Sorry for the confusion.