Chrysler Repair: venting coolant from overflow but temp gauge normal, head gasket leak, shaft bearings

Question
hi ,
   I have a 2000,chrysler,300m,3.5 litre v6,
I hope you can help when I drive for 30 mins or so the temp gauge starts to climb with air con on or not ,temp will rise to just below half ,when I stop steam and coolant is venting from the expansion tank vent .I have had the water pump checked ,new radiator and pressure caps The engine has recently had new cam shaft bearings , piston rings and head gasket Here in Kuwait I was advised to remove the thermostat can you help ive taken it to a few mechanics who all have different ideas

Answer
Hi Rick,
If the temp gauge only reaches to half-scale, and it seems as though the temp of the engine/expansion tank fluid is not really so high as to boil the coolant (which I assume has antifreeze mixed in at about 50% concentration), then I would wonder if you might have a head gasket leak which results in the cooling system being pressurized from the combustion chamber of one of the cylinders. You could go to a radiator shop and have them use a meter to see if there is exhaust gas detected as coming out of the expansion tank/cap area. That would be the  test to do for that possibility. If that were the case then you could have the spark plugs removed from the side of the engine where the head gasket was replaced and look either for wet spark plugs or lower that normal compression reading with a compression gauge on each of the three cylinders on that side of the engine. If that was found, then it would be clear that whoever replaced the head gasket was not successful in doing that job and should be asked to do it again (assuming it is still under warranty for that repair job). If that side seems OK then do the same tests on the other bank of cylinders for the possibility that there is a leak on the side that presumably wasn't touched previously.
So that would be the most likely reason for over-pressurization of the cooling system when the temp gauge is not reading H.
Sorry for the delay in responding but I just found your question in the "pool" to which it had been referred by the other expert, Kevin.
Please read the PS below.
Thanks,
Roland