I’ve been having issues with my ESOF. My 4 wheel drive does not engage.

I started checking for vacuum leaks and found there must be a leak inside driver hub. All else checks out.

I even pulled the nipple out of the hub and checked for leaks...nothing. I put in back in and it leaks. No leak around where it connects.

any ideas.

 

There are two seals in the hub and at least one of them is obviously compromised. There is a seal pressed onto the stub axle that seals the inside of the knuckle cavity and a yellow o-ring on the hub that seals the outside of the knuckle.

You have two options. Manually lock your hubs and live with 30 seconds of defrost every time you go into or out of 4wd or strip that corner down and replace the seal and o-ring.

To see what that entails I've attached a write-up I saved from another website (it's also here: https://www.superdutypsd.com/ball_joint_article.php). It's for ball joints but to replace the seals you do most of the same stuff (just ignore steps 3-6, 9-11, and 21-28 - you are not removing the knuckle so don't need to disconnect fittings, separate the tie rod, or replace the ball joints). You will need to pull the hub in order to get the stub axle out. I don't know how old this is so some of the part numbers may be superceded but the axle seal is shown in step 29 o-ring is shown in step 35. To install the seal (which must be near perfect or it won't seal) you need the special tool or you can make your own for about $20 (step 30). I made mine and it worked fine.

If you have any questions you can search or post in the 199-2003 7.3L Powerstroke sub-forum. Regardless if you have a diesel or not the truck is the same and many of us have done this a few times. You'll get more answers over there as the first generation Superduty is not very relevant over here any more so most of us keep to the 7.3L forum.