Can't find 1996 3.0L coolant leak - help or ideas needed

[98v70dad]

My '96 ranger has been parked for a while due to a coolant leak until I could get to it. The disappearing coolant has been going on for several years. For a long time I figured it was just a slow leak and I just topped it off every few months. One day about six months ago the valve that send hot water to the heater core literally blew up. I was 2 or three blocks from home when the steam cloud erupted and I basically coasted down the hill to my house. The engine temp gauge never made it into the hot range although it did go up to about halfway between normal and hot. I ordered the valve replaced it, serviced the coolant level and drove on for a while. After a while the slow leak got worse. The problem is that i can't find it. I have no coolant puddle on the driveway. The engine passes a glove and combustion gas test at the radiator cap. There is no oil in the coolant and no coolant in the oil. I don't see any cracks or holes in the overflow tank. However I fill the tank to the full line and if I let the truck sit for a month without starting it or driving it the fluid level drops to zero. The radiator remains full. So clearly the fluid is leaking out somewhere below the level of the bottom of the plastic tank. Yet, I have no puddle. The other piece of info is that the car has about 100,000 miles on it. I bought a UV light and some dye and there is clearly a leak at the typical place on the upper timing chain cover passenger side. That said its a dribble and its above the bottom of the overflow tank. It also only dribbles from that spot if the engine is running.

I could really use some help from someone smarter than me in troubleshooting this. I'm especially confused since it leaks dow even if its not running - it just takes a while.

 

[pjtoledo]

if the truck is inside try some cardboard underneath.
it will show stains after it dries.

also check the weep hole in the water pump for dried residue.

 

[SenorNoob]

Wait. You fill the expansion tank and it goes dry without running the engine? If so that has to be a very small hole in the tank or the line.

That tank should hold fluid forever unless there is a vacuum on the radiator. That only happens after the engine has warmed to operating temperature and cools back down.

I had to deal with the same problem awhile back. Mine was dry rotted. It didn't leak enough to find evidence of it. Was just always low... My wife's car had a similar problem. It would get low when you drove it. Ended up being a small leak at the nipple that the hose from the expansion tank to the radiator. Would push fluid into the tank (and leak a little) but couldn't get enough vacuum to suck it back in.

 

[98v70dad]

Wait. You fill the expansion tank and it goes dry without running the engine? If so that has to be a very small hole in the tank or the line.

That tank should hold fluid forever unless there is a vacuum on the radiator. That only happens after the engine has warmed to operating temperature and cools back down.

I had to deal with the same problem awhile back. Mine was dry rotted. It didn't leak enough to find evidence of it. Was just always low... My wife's car had a similar problem. It would get low when you drove it. Ended up being a small leak at the nipple that the hose from the expansion tank to the radiator. Would push fluid into the tank (and leak a little) but couldn't get enough vacuum to suck it back in.

Thanks, the expansion tank is my best guess at this point too. When I replaced the heater valve I took the tank out to have some more room. I took it out and cleaned it thoroughly and didn't see any obvious leaks but the hose to the radiator was a little loose when I put it back on. So, it could be leaking from there.

I haven't ruled out a blown head gasket. I get a huge blast of white smoke when I start the truck after its sat for months. I think that's condensation since I'm in the very humid deep south and the smoke clears up after a minute or so. The truck passes a glove test and a combustion gas test. I did the combustion gas test at the radiator cap twice on separate days with the same result. There's no oil in the coolant and no coolant in the oil. The truck runs fine. Anyhow, maybe its just wishful thinking but so far I'm thinking its not the head gasket.