Originally Posted by sv41878

*Do you have access to a coolant system pressure tester? If not, go to the auto parts store and do the loaner program on one. Seal the system back up tight, fill it (with just water cuz I'm gonna tell you to flush the system good after you verify there are no leaks) and pump it up. Make sure the tester is sealed tight in the radiator filler neck so its not leaking around it *and pinch the line that runs to the overflow bottle off tight. When I change water pumps and t stats in the shop, I usually pump it up to 18-20 lbs of pressure and watch for leaks. It's more pressure than you would run at operating temp and will show you any leaks.If no leaks are revealed (pressure stays where you put it for 15 minutes or so), take the t stat and water pump off, pull the bottom hose from the radiator and shove a garden hose in the block, spraying it in to everywhere you can reach, then letting it run through. If it looses pressure and you can't see a leak, go back to the parts store and borrow a compression tester set to check the head and cylinders for cracks. I personally have never pulled the plug from the side of the block to flush the system, but there was a post about it and if he has had good luck with it and you feel you wanna do it, I'm not gonna challenge it since they put it there for a reason. When you get the block clean, flush the radiator and heater core (I use the air hose for the heater core, fill it with water then use air, low pressure by keeping the blow gun loosely in the end (do not seal it tight or you'll blow the heater core out) to push it, repeat this few times as this is typically the least circulated part of the system). If the PO did not do maintenance, you'll be shocked at the stuff that comes out.*Once it's flushed, spend the $35 at Pepboy's for the lifetime warrantied Bosch water pump and get a new t stat (I would say 180 t stat because of the warm climate there). When everything is sealed up tight, fill it with antifreeze (50/50 mix should be good since you don't get the bitter cold we do here) and use the pressure tester again to verify there are no leaks.
Then burp the system by letting it get to operating temp and slowly topping it off again. You'll loose some coolant as the temp rises due to the air pockets in the block expanding, but that's normal. 4.0's have a few spots in the block that seem to stubbornly hold air pockets, so after the initial burp, check it daily to see if the coolant level drops for a week or so. Usually you only need to burp it once, but I have seen a few that it took a week of regular driving to get all the air out of the system. You should be golden after this.