[DANTIP]

I helped diagnose a cooling problem that only occurred at idle with the A/C on. This truck had all new cooling parts (radiator, hoses, water pump, thermostat, clutch fan, etc.). The truck would overheat in traffic.

I received a picture like this one (I chose this one because the gap is so evident) and was able to help the owner diagnose the problem. Notice the poor fit of the shroud to the radiator (you can see the fan clutch through the gap). At idle, the only air going through the radiator was being pulled by the fan. The air is going to follow the path of least resistance. The gap shown is an example of how an aftermarket radiator may not fit the shroud properly. The truck would do fine on the highway with the A/C on but at low speed, when the fan was supposed to be pulling air across the condenser then through the radiator, too much air was sneaking through the unintended gaps between the radiator and shroud. Another cause of this would be incorrect radiator mounts (4-core mounts with a 3-core radiator for example).

Long-Story-Short, make sure this gap is very narrow or find some way of sealing it up. Otherwise you can expect low-speed cooling problems.