While trying to figure out my non-engaging front axle I noticed my main vacuum line coming off the pcv assembly that runs along the firewall to the check valve was covered with oil. I was terrified it was ATF but no smell-just engine oil. I replaced the pcv valve last week and noticed yesterday I still have the problem. Is a vacuum leak causing this? I never noticed that little quarter-turn twist off diverter before. The orange seal on it felt rather hard. Maybe it's time for a new assembly and new vacuum line that runs off it.

 

I once seen a vacuum line disconnected from the reserve ball in the fender , it had enough vacuum to pull the under battery vacuum diaphragm but not quite enough to engage the front differential . That being said ...yeah a vacuum leak could cause the front axle to not engage

 

These vacuum hoses can look ok but still leak due to rot. I replaced all of mine. If you have the 4-button transfer case, it is pretty easy. In my signature below I have a link to my thread on it for my 1999 LT.

 

I fought this issue for a year convinced it wasn't tranny fluid because it didn't smell like it and was not red. Replaced the transfer case switch and the "oil" in my vacuum lines stopped. Just sayin

 

I replaced the transfer switch a year ago when I was fighting the axle not pulling in all the way. Replaced PCV valve two weeks ago, same problem. It was the main vacuum line coming off of the manifold going into the check valve. Replaced that line last week and tried 4x4-couldn't move front drive shaft as it was finally completely locked in. Switched back to 2x4 for confirmation and the drive shaft turned free. I've driven it a week and need to pop the hood and double check the line for oil. I'm fairly sure the old line had become porous which caused my weak vacuum as well as making the line suck engine oil through the PCV valve. It probably explained the real crappy mileage I've noticed for the last year. Luckily it's not my daily driver as I use my Blazer only to get to work during winter storms. My old '93 Escort is my daily driver and the best damn $700 I ever spent on a used car nearly 8 years ago.