Assuming you're talking about the engine temp. sensor (and not the ambient temp. or intake temp. sensor):
Temp. gage reads other than what you know the engine temperature to be. Also, since the fans work off that same sensor, you may get overheating (due to the fans not working) - but your gage won't be indicating overheating even though the engine is actually overheating since the same sensor output is used to drive the gage and the fans. That's if the sensor output says it's cold when the engine is actually hot. If the opposite is true, then gage would read high when the engine is not hot, and the fans would run a lot or all the time.
'98 LXi - Later Concorde gages (black w/ chrome rings)/'99 LX - LHS gages (white) - HIR bulbs
Well with your fans, Id check the relay. If your running warm, bleed the cooling system. As for the gas mileage and black smoke, I've never heard of those symptoms. If you are overheating and you blew a headgasket A. You wouldn't go too far. B. You'd be blowing white smoke.
When the fans are on hi speed, what is the reading on your temp. gage? Read my previous post.
normal temp.. just behind the middle line .....
ok my other question do i need to flush the cooling system? one of the mechanics from payne dodge dealer told me just to wait for the engine to be cold .. n just remove n replace the coolant sensor.. is he right or not ?
coolant temperature sensor adjust the air to fuel ratio depeneding on the temperature of your engine. If it is bad it will run your too rich(to much gas, not enough air) and it will run rought at idle and definatly have black smoke, it will make your engine stall, as described in your case. In some cases you can unplug the sensor and the PCM will compensate by running a default air/fuel ratio and in some cases allowing the car to run without the sensor, I'd just replace it.