losing coolant, no visible leak

A new radiator cap is the first and easiest and cheapest thing you can start with. As mentioned above.

Pressure test the cap, and the cooling system. Then have it tested for exhaust in the coolant, that'll tell if it is a head gasket. Have you ever over heated it?
Look over the coolant hoses and clamps, and look good, with a powerful flashlight.

If your still running the original hose clamps that could be your problem they can and will break. The heater hose clamp that goes into the top of the block over one of the COP on the right side of the engine was also a hose clamp problem. Fluid would drip down into the spark plug hole.
First thing I would do if you still have the original hose clamps is change them.

Yeah, I'll second the pressure test.. or third it in this case... LOL! At least that way you can put the cooling system under pressure without getting it so hot you can't work on it. And you don't have to be driving to get it properly pressurized.

It's probably something simple like a small pin hole in a hose or as stated a broken hose clamp or leaking radiator cap.

Look along the sides of the engine block for signs of liquid running down too. It could well be a leaking head gasket. Does your engine oil look like chocolate milk on the dipstick? If so, then you probably do have a blown head gasket. It may also be a freeze plug that's leaking....or even a coolant drain plug. All worth checking.

tlaffourtit wrote:
get it pressure tested

Right answer! You can go to most chain auto parts stores and they will loan you a cooling system pressure tester. With the same device, you can also pressure test the radiator cap.

Safe travels.

I had a smaller loss of coolant on mine with no signs of leaks or puddling on the ground. One day, while changing oil, I saw a cracked hose clamp on the bottom radiator hose. Evidently, it only leaked when the engine was hot and it promptly evaporated off the hot engine. Replaced the clamp and the loss stopped. However, with the rate of loss you are experiencing, it's probably something worse------------but worth checking the clamps. A lot of them failed.

In fact, anyone with a V10 should probably just replace those clamps with a worm screw clamp.

I think Mountainaire hit the nail on the head.

Check your oil and if it is turning a brownish color you have a bad head gasket.
If you do not catch it in time, it will dilute your oil to where eventually it will ruin the engine.

If there is no change in color, check that the oil level is not going up on the oil gauge. If it is climbing up, it is your head gasket.

2005 V10 had similar problem,turned out to be cracked hose clamp on one of the hoses.

cracked coolant overflow tank. Hard to see, and may drip only when driving and radior pressure is up. Drips fall to road and are not seen.

Ever changed radiator cap? It can lose that amount as steam when under load, almost undetectable...easiest/cheapest thing to try first. :)