the only way to see if the radiator is leaking is to get it up to temp fully with the coolant level all the way up...you will see the leak if it is leaking, or at least evidence of the leak. It is normal for the exhaust to have steam come out of it for a little bit on start up. It is condensation built up in the exhaust which heats up and turns to steam. If it continues to steam while up to temp or running during the day or it smells sweet, then that is a problem. I still dont understand how you could have a radiator that went bad in the same spot twice. Are you positive there was a leak in both of them in the same area? I dont know how much the LIM replacement would help. Do you have any coolant which puddles on the intake on top of the motor while it is running? Does your oil look normal and is at proper levels? I would be more concerned about the radiator and what is damaging it. Please answer these questions and we will go from there.