There actually aren't many good ways to tell. Most of the time the problem is something other than the radiator. Some of the things to check for are.
The condition of the coolant. If it has junk in it or if the wrong type was used it might cause this.
What the radiator looks like inside. When you look in the neck of the radiator you can usually see some of the tubes in the radiator. If they look like they have buildup on them, then that is an indication that they might be getting plugged.
If you replace the thermostat and it is still overheating then you should check out the radiator, but I would replace the thermostat first.
You also need to have a good look at your fan shroud. If it is broken it can cause this type of problem. Also check the grill and make sure nothing is stuck to it and it isn't clogged. Be sure the radiator is not clogged with bugs too.
Thursday, September 15th, 2011 AT 10:14 PM