Interesting article well explained WHAT PRESSURE RADIATOR CAP RATING SHOULD I USE? Posted on February 21, 2014 by Tom DeWitt We get asked this question daily and it seems most people misunderstand what the pressure cap does.  The cap pressure rating has nothing to do with regulating your engine coolant temperature, nor will it make your engine run cooler. Lets take a pan of water and heat it up. The water heats up and boils and 212 degrees. Now let's take a pressure cooker and do the same thing. The water will expand and create pressure. The boiling point of the water increases three degrees for every 1 psi the pressure goes up. If the pressure goes up 15 psi we add 45 degrees. We have now increased the boiling point to 257 instead of 212 degrees and we can cook much faster.   A coolant mixture of 50% water and 50% ethylene glycol has a boiling point of 223 degrees. A system with 15 psi cap will add 45 degrees for a final boiling point of 268 degrees. The real purpose of pressurizing is to give drivers a higher operating zone in case of extreme conditions. For example, lets say your classic car normally runs at 180-190 and boiling is not a concern. Then on a hot day the temperature goes to 200-210. This is not a problem either but then traffic stops and you cant move. Now the temperature starts to rise to 240 and without a pressurized system the coolant would boil and spew all over the place. The loss of coolant would eventually fill the radiator with air and the temperature would rise even higher. Remember, none of the above running temperatures have anything to do with the cap rating. They are simply conditions of the cooling systems capability, the ambient air temperature, and air flow. Often customers will report their system boiling over at temperatures of 220 or lower and as explained above this isnt possible. What is happening is the system is simply overfilled. When the coolant expands it has no place to go but exit and people confuse this with boiling. After the coolant has exhausted to the driveway and the engine cools down, some will re-fill the system and continue the cycle over and over. The simple solution is to stop adding fluid. Once the system pushes out the excess fluid it will naturally find the proper level. Feel free to post your comments by filling out the form below. If you have any questions that require a reply click here.  Note that questions posted in the comment form will NOT be replied too.  Next Post COMMENTS Posted July 12, 2018 by jasim Highly appreciate info above its very effective yet logical. i had some issues with my car cooling system it help me a lot. it let figure me out the correct problem. i recommend this a very useful info upto the professional standards. Posted October 12, 2018 by Kevin Paolinetti You mention how a classic car on a hot day can begin to heat up considerably especially when stopped in traffic. I recently had this happen with a 62 Chevy Impala. If a cooling system is working properly, then no matter what the temperature outside is doing, the system should be able to keep the cars engine from entering the danger zone. Given that the cars thermostat, hoses, fan, and radiator are all doing their job, I find it hard to believe that the psi rating of the cap would have no bearing on whether or not the car overheats or not in a hot weather environment. If you clarify this, it would be appreciated. Im trying to zero in on my own overheating issue with the Impala. Posted November 25, 2018 by Tom DeWitt Kevin, Normally I do not reply to questions here but since you asked for clarification I will try to explain. Remember pressurizing the system only changes one thing, the point at which the fluid boils. Car companies do this because sometimes we get stuck in conditions that are not optimal or desired. This could be a traffic jam on a 90 degree day. Your coolant will rise to abnormal levels, such as 230-240. The last thing you want to happen is to have the fluid start boiling over, so pressurizing moves this level 30-40 degree higher.