I have a 2004 TL that seems to be using a lot of coolant. I have probably added 2 gallons of coolant to system over last 2 or 3 years. There is no noticeable leak under car when it is parked and no coolant smell when I drive it. When I change the oil, I have never noticed any fluid under the car or on any of the parts. Any ideas as to the cause?

 

When cold, use a pressure tester on the radiator. Let it sit for a while, check the pressure and if a small drop in pressure is noticed, pump it up. If there is a leak it will be found.

 

I had similar issue about a month ago and noticed that the leak was coming from the Heater Valve on the firewall, it was a pretty small leak until one day the plastic center piece melted off and started to spill everything out... My guess is start there and give that a try.. There is a DIY somewhere in here that helped me change mines.. Good Luck...

 

could be a leaky overflow tank cap.

 

check under your throttle body area as well. I have a minor leak in that area too, seems like the prev owner replaced the hose and its leaking again (screw type hose clamps vs. the factory honda tension ones).

If you havent had your water pump replaced yet, that may also be leaking a little. All these little leaks might add up to a gallon over a year i suppose.