Coolant Leak
 

I have a 2004 with just shy of 100K miles. I started to notice a coolant smell and had the coolant light come on. I filled the bottle and all was fine for a couple of months but then one morning I came out and found a large puddle of coolant under the car. I put the car up on jacks and found that the bottom and rear of the cross member was wet with coolant. I pulled the bottom covers and can not find the source of the leak. The radiator, hoses, front of engine, even the top of the cross member seem to be dry. Thought I would check on here for suggestions before I fill it up and drive it again to see if I can find the leak. I probably need to buy a set of ramps to make this easier.

Check the overflow bottle. They're prone to cracking, which would drip on the undertray and spread out from there.

Go to an auto parts store and see if they have a coolant system tester. It will let you pressurize the system without it actually being hot enough to burn you. Then you can poke around and see where it's coming from.

as suggested get a pressure tester or get a UV light and glasses then hunt for your leak. Most coolant shows up under UV light even without leak detector added to it. If you are leaking that much its going to be a sizable hole. few places to check

1. Coolant line next to the fuel lines in the rear right of the engine bay.
2. Radiator drain plug, this will sometimes get loose with seasonal changes.
3. Any hoses secured by worm gear clamps instead of spring clamps.
4. Lines going to/from the throttle body.
5. Thermostat housing, check that nuts/bolts are tight and secure.
6. Its possible you have a freeze plug thats made its way loose allowing coolant to escape once it gets up to pressure.

I had a leak that only presented itself after getting up to temperature, it was a loose gear clamp on a silicone hose that loosened up just enough to drip coolant out then would self seal when it got cold. Could be worth running UV dye and getting the enigne nice and toasty then wait for it to leak out.

Also could be a bad cap... Also you could have a blockage causing boil overs.