Having a problem with 4x4 engaging when I flip the switch when hubs are set to auto, works fine when I turn the dial to lock.

I believe I have a vacuum line problem.

Disconnected the vacuum line from the solenoid that runs to the front hubs (left the other line that runs to the reservoir connected to the solenoid) and placed a vacuum gauge on the solenoid. I get a reading of about 20 inches when I turn the switch to <nobr style="font-weight: normal; font-size: 100%; color: darkgreen;" id="itxt_nobr_3_0">4x4</nobr> and get about 5-8 inches when I turn the switch to 2wd, zero the rest of time.

I then placed a vacuum pump on the line I disconnected from the solenoid that runs to the front hubs and was able to pump it up to 20 inches and it stayed there until I released it. Here's the problem. If I disconnect the vacuum line at the knuckle, the pressure stills stays at 20 inches. Shouldn't the vacuum have gotten released when I pull the line at the knuckle? I pull the line on both front knuckles and the same result. Sounds to me like I have a clogged vacuum line?

 

You might want to check out these threads.

https://www.ford-trucks.com/forums/9...n-7-3-4x4.html

https://www.ford-trucks.com/forums/9...4-problem.html

 

If you have both knuckles disconnected, you should not have any vacuum. It sounds like blockage. You can rule out the rubber lines by disconnecting them at the frame rail and checking the lines clear. You can disconnect the plastic line off of the rubber hose coming from the solenoid and try to blow some compressed air in the plastic line to clear it. The vacuum line in the engine compartment is one plastic hose assembly. If not, you can try to snake something down there to free it also (be careful not to puncture the line). I know Guzzle (Bob) uses weed wacker line in his drill to clean the EBV tube, and it might work for this too. If that does not do it, you might be able to find a donor from a junkyard or just buy another OEM line and install it. I don't remember what it cost me, but the line goes from the solenoid down to the wheel wells ( it crosses on the front crossmember to get to the driver's wheel well). It's a simple replacement (as long as it's not too greasy and oily under there).

Good luck!