1996 Ford F-250 7.5l efi with approx 175,000 miles. "Uses" coolant. No green spots underneath. Exhaust is not weird color or odor. Its not going into the crankcase and mixing with oil. Doesn't run hot, but every couple of days, I have to add to the plastic tank. I only fill it to the cold line when it's cold. It will be empty after 3-4 days of normal driving. What could it be? I'm stumped.

 

It's leaking somewhere. If you can get a pressure tester and pump it up to 15 psi or so for an hour or so, then crawl under it with a flashlight and look for a leak. sometimes you need to warm it up and repeat the process as it may not leak when cold. Don't warm it up with the pressure tester on it, it will build too much pressure and blow something out.

 

the green ninja unicorns are sipping it out with their invisible straws at night.






























your overflow tank probably has a pinhole in it.
none of my vehicles will hold coolant in the overflow tanks for more than 2-3 days.
i just leave them empty and check the radiator every once in a while.
as long as the radiator stays full i call it good.

my 02 is the only one with a leak in the system i can not find. it uses a quart every 4 months or so.

i think it is the radiator, but it is small enough i am not going to bother with it until it gets worse.

 

Could be the tank, radiator, hose/connection, water pump, intake gasket, or heater core.

 

I was having a similar problem until I found my leak - it was right at the thermostat housing. Electrolysis had eaten away at the base metal until a small pinhole appeared. It was hard to spot and only appeared when the system was pressurized. it would spray towards the fan and the blow back on the hot engine - thus evaporating leaving no tel tale sign of leakage. when the pressure was relieved, the leak went away. I finally found it while replacing the belt. it got really dark and my flashlight glanced upon the leak as I was visually checking the belt. Fixed the hole and problem solved for me

 

Scott, sounds like you did some good diagnostic work. Glad you found it. I find the screws that hold the stat housing down have a tendency to come loose.

I like to find all known coolant leaks even though I may not fix them depending on where/what they are. 3 years ago I had a pinhole in a heater hose, such that it shot coolant at the xhaust manifold and it evaporated before I noticed. Always saw the coolant was low, figured it was the radiator or tank and didn't bother to find it because it was "small".

Driving through the Pinelands Reserve 15 miles from anywhere the hose blew up. I was able to cut the end off and put it back, but I had to make 3 one-mile walks into the woods with the coolant tank and an old soda bottle to get water. Walking that far through chest-high blueberries that you can't see 6 feet through, I resolved to never again ignore a coolant leak, no matter how small.