I bought my 96 F150 4x4 5.0L back in the fall. When i bought it i noticed that there wasn't any coolant in the reservoir but the radiator was completely full. It was driven for about 50 miles this way and the radiator never became low at all but i filled the reservoir to the cold fill line. i drove it about 15 miles and got back home and noticed that the coolant seemed to be at the same level that i filled it too. It has never over heated either, runs great. I drove it another time and now all the coolant is gone out of the reservior but radiator is still full and has good pressure and everything. ive let it sit in the drive and warm up and have never seen any coolant drips after 15 or 20 minutes or long drives. the reservoir doesnt leak anywhere. is there any way to tell if its leaking into the pistons or anything? i'm kinda confused because its disappearing but radiator is always full even after those 50 miles and it stays cool. any ideas?

 

Take it to a radiator shop & have it pressure tested. Or it might have just needed burping, and just sucked up the reserve. People tend to use their Caps long past their intended lifespan &/or a leak less than a pinhole will also cause it.

 

My 4.9 does the exact same thing and I'm worried it's going through the head gasket as it has 171,000 miles on it.

 

my 4.9 does it also and it has only 102k

 

Thanks for your input guys, i was also hoping it just needed burped or something. I was hesitant to start this thread before i could fill the reservoir again and drive it a little more to see if it kept disappearing. I'm at college and its hard to get home to work on her all the time. Just couldn't stop worrying about maybe a gasket leak. It runs great and only has a 104k on it tho. I'll test it again this weekend. Maybe new radiator cap, also have a new thermostat to install.

 

My 94' 5.0L did that too when I first got it, turns out it was just low on coolant! The previous owner must of not topped it off or 'burped' it. I filled the radiator and overflow initially, then topped off the overflow twice. Since then, all is well and I don't hear the water circulating through the heater core like I used too! If you hear "water" sounds behind your glovebox, then you certianly have air in the system and like a baby, needs to be burped.

 

Thanks for all of your help guys. When i get home this weekend I'll finish up the under drive pulley install and top off the radiator and reserve tank again and take a drive to see if it continues to disappear.

 

yea i have the same truck and same engine and same problem and thats all it was. after i did the timing chain it hasn't done it since

 

I've always heard water sounds from my truck from underneath as soon as it gets back to normal idle, and right after I shut it off. Any clue what that could mean? There's no thermostat issues, and it's definately not overheating