Welcome to jeepz.com. Whether it be the I6 258 or the V8 304 the same condition may exist . Yes , trapped may very well be causing this issue and likely to be the cause since there wasn't an issue before the water pump swap. With the engine off and cool and radiator cap removed , vigorously squeeze the upper radiator hose repeatedly until no air bubbles are withnessed escaping the filler neck port of the radiator . You'll want to keep the radiator topped off as you are performing this task of squeezing ( the overflow bottle as well) so that gravity will assist . The act of squeezing helps to distribute fluid mixture into the engine and through the heater core and back to radiator to help expel air pockets . Keep the level up and monitor frequently so that gravity continues to assist as you continue to squeeze the upper hose. Elevating the front of the jeep may be helpful for a V8 but seems to be essential for an I6 . Squeezing a lower hose helps as well but you'll want to start with the top simply because the manual transfer process is actually easier from the top at the beginning of this procedure than to start with the bottom . Once the level plains out and level change is no longer noticed ( and your forearm is numb) it is assumed no more air is present . You may try the lower hose to see if any effect is noted and effective . Be certain to maintain the proper filler neck level throughout the process and upon completion and overflow bottle as well. With radiator and overflow capped , now start the engine . You'll want the radiator capped because you want the cooling system to pull coolant from the overflow / reserve so that future air pockets are avoided . It will take several warming / cooling cycles at least to any small amounts of air that cannot easily be removed which usually is introduced from the radiator tank when the cap is open. The act of the overflow / reserve tank coolant flowing to radiator should fill and contain any air pockets and air should circulate back to the overflow tank , theoretically , if there are no system leaks . All hoses must be clamped tight ( but not over tightened ) and radiator cap holding correct pressure for cooling system to function properly. Make certain overflow tube as well as all hoses ( upper , lower and heater core lines in and out ) don't kink and cause restriction for system to function properly. If all previous conditions conditions are met , cooling system should maintain proper circulation and air will work its way from system soonest that cannot be expelled by squeezing hoses . Let engine run and observe temperature gauge throughout process . I should ask first that the correct water pump is in place . Does your system require a reverse flow water pump ? This is important and an incorrect pump installed will cause overheating conditions . Do not wish to doubt anyone's ability but only wish to advise for future readers not to commit any blunders as it is enough work to swap parts and no one wants to do it twice. Hope this helps and your cooling systems woes vanish. Best of luck . Thank you for your post and especially your membership here at jeepz.com .
