Usually when coolant is leaking but you can't find any liquid on engine parts or on the ground,its most likely leaking from a blown head gasket or leaking lower intake gasket.A lower intake gasket leak won't have any of the typical symptoms that a blown head gasket does.The only symptom it will have is constantly needing to refill the radiator up with coolant.It can display the coolant mixed with the oil symptom if its been leaking longer than a certain amount of time.My Mustang had several leaking intake gaskets in just a few months time.I finally got the lower intake checked at a machine shop and it was warped 0.50 thousandths.If you change the gasket,it wouldn't hurt to get it checked while you've got it off the truck.Its also a good idea to replace the lower intake bolts too.ARP,s are a good upgraded bolt which will give you peace of mind.Make sure you check underneath the water pump for a leak out of the pumps weep hole.Check your passenger side floorboard too.
If escaped coolant is hitting your hot engine block then it is evaporating and leaving no trace. You aren't going to be able to find it visually. You will need to have a shop put the cooling system under high pressure to force liquid through any weaknesses in the system.
Look for any accumulations or puddles under the hood.
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No water in the oil, no smoke from exhaust, not running hot. Runs GREAT! Any ideas where to start?
