I can lose a gallon of coolant in a few hundred miles, no overheating, no visible leaks, nothing on the ground. Coolant smell with hot engine.

 

Blown head gasket between water passage and cylinder. Pull the plugs and you will find one or more steam cleaned.

 

What are you working on ?

 

I have an F250 1990 with the plastic tank radiator and only 65000 miles. The engine and transmission fluids are clear. After a short but hot trip the reservoir is usually empty while still hot.

 

Do a cooling system pressure test. I'd remove all 8 spark plugs, leave system pressurized for 15 minutes or longer, and then crank engine over and see if any coolant shoots out of the spark plug holes. If you have a leak into a cylinder, you don't want to hydrolock your engine during this test.

The leak could be in lots of places. My plastic radiator side tank has a pretty good leak on it - it doesn't leak as much coolant as yours but in a month's worth of driving it empties the overflow reservoir.

 

I had a mechanic do a pressure test for 24 hours which it passed. He also recommended looking the spark plugs. I will do it when the heat wave is over.

 

Hmmm. This raises the possibility that your leak is only present when the engine and cooling system is hot. You may have to do the pressure test with the engine hot (a bit tough due to not wanting to take the radiator cap off with the system pressurized).

The other possibility is that you have a loose hose somewhere that only sprays coolant when hot, but my money is on an internal leak in the engine (head gasket, crack, etc).

 

Now I have had a radiator specialist check my coolant system and inspect my spark plugs. He found nothing but the bypass hose I had tried to replace myself. The spark plug were all uniform (and replaced at 70k). So now I am going to pack extra coolant jugs and head off on my 5th wheel trip. Thanks for the advice.

 

Try a new radiator cap of sufficent pressure, I had that problem once, its cheap and easy to try, and fixed mine.

 

A gallon is a lot of coolant to lose in that amount of time. I have been aware of fairly minor leaks that are hard to detect because the coolant leaks so slowly that they just evaporate. What size engine do you have? It is possible that you have a crack or issue with coolant getting just into an exhaust port, but does not usually happen on Ford motors.