I have a 99 Ford F-150 Lariat. I am smelling oil in my coolant reservoir. I have looked all over the engine and do not see any leaks. I do not see any oil on the ground. The truck does not overheat so I don't know how it is getting in. My engine runs great. What can be causing this problem?

 

I'm thinking you may have a cracked head or have blown a head gasket. Look for oil leaking out between the cylinder head and the crankcase. If you don't see any, that doesn't mean it's not cracked or a blown head gasket - but it's a place to start.

Is it still running well? Aside from the oil in coolant...

 

99, you are in the wrong forum but. I see you have a 5.4, I would suspect a bad head gasket. Seems it took a while to figure out how to build the head gaskets when they went to aluminum heads.

 

I would agree with the others that it is likely a head gasket. Had a head gasket blow on a 1990 4-Runner and it had this as one of the symptoms. As it gets worst you will possibly have serious drivability issues and or overheating. Try posting this is the right section. this section is for 87-96 trucks. The guys in the other section might be able to help more. Also there is mixed opinions on this, but you could try a head gasket sealer. Check out scotty kilmers youtube video on how to use the sealer.
Leak sealer:
How to test for a blown head gasket:

 

Since you're already here, I'll be glad to chime in. I owned an expedition with the modular motor and it had a couple of blown head gaskets. I did try the leak sealants in the post with the videos, and they didn't work for me. I agree that they would be a cheaper fix than replacing the head gaskets, but oil in the coolant is pretty clear sign that you are probably headed there. I replaced them myself and fixed a few other issues along the way. It took a while, but it's about $2500 for a shop to do it vs. around $800 to do it myself, including the price of refurbished heads and cams. If your heads are ok, it's around $300 in parts.

 

It could also be the engine oil cooler or the gasket at the oil filter adapter.