[fredjacksonsan]

The temperature seems to be ok, my 4.0 liter runs at about 210-220 all the time.

Is the AC compressor coming on and off quickly, or every few seconds? Every few seconds is normal, but if it is cycling quickly then you are probably low on coolant.

As far as the check engine light, turn the key on-off-on-off-on within 5 seconds, and read the codes that come up on the odometer. Post them here and we'll be able to help out.