I get asked this question daily and most people stress out over the decision when they really shouldn't. A coolant mixture of 50/50 has a boiling point of 223 degrees. Pressurizing the system will raise the boiling point by 3 degrees for every 1 psi on the pressure cap. A system with only a 7 psi cap will have a boiling point of 244 degrees and that should be a temperature that no system should ever reach. Typical pressure caps are either 13# (262 F) and 15# (268 F) and six degrees difference is nothing to lose any sleep over. Sometimes people will overfill the system and mistake expanding fluid for boiling fluid and think they need a higher pressure cap to prevent the coolant from escaping. After the coolant has exhausted to the driveway and the engine cools down, they will re-fill the system and continue the cycle over and over.