04 grand Cherokee 4.0 boils over don't show hot
 

Brother just got a new to him 04 grand Cherokee 4.0L with 146k. Unfortunately we know nothing on the history of the vehicle. Been running good for the 500 miles he has had it up until today. Started running rough and threw code 1281, engine temp to cold for to long. However when this happened it boiled water out of the coolant reservoir. Gauge never showed quite 210 (center of gauge). I have a scanner so I put it on after I got off work for him and it never indicated hot. However once it was obviously getting hot the scanner showed the temps rapidly bouncing from 180 to 196. Hottest the scanner indicated before it boiled was 204. Function test was able to run the electric fan. Coolant looks clean and full. No mixing in oil. Trying to figure out why the temp is not reading correct which I assume is why fan doesn't run and causes the overheating. He did put a new rad cap on with no change

Sounds like a bad temperature sending unit. I would start there. If it's malfunctioning it will read too cold or bottom of gauge. What kind of driving was he doing when it overheated? If he was steadily over 25-30 mph something else may be wrong, since it shouldn't need the fan above these speeds unless he's working the engine hard.

Yeah it seems to only get hot while idling and it dont take long. We replaced the thermostat yesterday with no change. Like I said the gauge never reads more than 190 but the reservoir boils like mad. Temp sensor is going to be next. We did find a large piece of 1/4 inch thick black plastic wedged under the temp sensor inside the housing and I was convinced that it would be the problem but no difference once removed. The fan dont come on when it's hot but I believe its cause the computer dont realize the true temp. The fan runs with the ac on.

I think you probably have a bad head gasket allowing exhaust gases to enter the cooling system and build up too much pressure. You can verify this by going to a radiator shop and have them test the coolant for exhaust gases

Agree with that last post. Bubbles in the overflow bottle ..not good. Easy test is a radiator pressure tester. Most autozones etc will rent you one for free. Just put it on the radiator and pump it up to 15, should hold there. If not there is a leak. If you see no leaks then ...start thinking head gasket.