One thing that can cause an overheat that likely would not show up as a leak is a sticking thermostat. These can be tricky to detect. You can often feel the radiator hoses. If the top hose is cool or cold to the touch and you know that you’re at operating temp, you might have a stuck thermostat. It’s a common misconception that both hoses should be hot to the touch when the engine is running. This isn’t necessarily true; the upper hose normally contains the hot coolant exiting the engine to be cooled by the radiator. The lower hose is the return and should be considerably cooler than the upper hose. This means the radiator is doing its job. I get a lot of questions about this very thing and I’d like to set the record straight: With the engine at operating temperature, upper hose hot, lower hose cooler than the upper. This is normal.
Getting back to the thermostat: If you suspect it’s bad, you can prove that. Just remove it from the engine and insert it into boiling water. If it’s working properly, it should open up when it reaches its threshold temperature, usually between 180º F and 220º F; if not, it’s bad, and you should replace it.
Many times you’ll see a temperature rating stamped on the thermostat. As long as the water is at that temperature, the thermostat should open up. As for what to replace it with, I always recommend OE (original equipment) for thermostats. Engine temperature is very important in modern fuel-injected engines, and the best thermostats are the OE ones in my experience. I’m not saying don’t use aftermarket thermostats, as I know many of you are trying to save money and in some cases the OE thermostat might not be available; I just recommend an OE thermostat for best results. The takeaway is to use OE when possible when it comes to thermostats. I’ve always had good results with original equipment thermostats, but not so much with some aftermarket brands. Remember that I mentioned earlier that it can be difficult to bleed some cooling systems because the thermostat didn’t open? Nine times out of 10 it’s an aftermarket thermostat (and sometimes a aftermarket radiator).
Lastly, do not alter the temperature range of your thermostat. As stated, engine temperature is critical to a fuel-injected engine; change the thermostat’s temperature rating and you’re asking for trouble. Put in what the manufacturer calls for; if you don’t, it can compromise fuel economy and performance—so don’t mess around here.