LONGBOX55 is right-on with the fact that you need some restriction in the flow. On all vehicles the cooling system was designed around a certain volume of coolant passing through the system at a certain rate. If the rate of flow through the system is increased then the coolant might not have time cool enough; especially under certain condition. I have seen this many times where the driver complains about overheating when stuck in slow traffic on hot days. The coolant passes through the system too fast to cool down and gets hotter and hotter. Almost every time I find that the guy has removed the thermostat and if I convince him to put a thermostat back in then the problem goes away. I have had this argument many times and still have lots of people who will not believe. Under some driving conditions the problem is minimized but stuck in traffic on a hot day is the worst.
