87 cherokee 4.0l oil entering air filter box thru rear of box from valve cover (Crankcase filtered fresh air supply ?). What would cause?

Get a new PCV valve.

87 cherokee 4.0l oil entering air filter box thru rear of box from valve cover (Crankcase filtered fresh air supply ?). What would cause?

Ok, here goes. Very simple. For this year of Jeep, they didn't use a PCV valve. Instead, they tried this silly system called a CrankCaseVentilation System. In effect, it sucked all the old air out through that small hose (from the back of your crankcase) and supplied fresh air in the big hose (in the front of your crankcase). Now, as the miles add up, the dirty air builds up oil and junk in the small hose, blocking it. After this is done, the dirty air has no where to go, so it goes back up the bigger air hose into the air filter box, giving you a second oil filter you never wanted. :-) This also means you drop your hp, since you're not getting as much air. Also, all of the little air hoses in the system get oil in them over time, which in mine dropped my idol rate and everything. Now, solution:

Go to your local auto store. NAPA is where I go. Ask for the complete air hose package for your rig. It used to come all as one, but they've since split it up into three packages. Here are basically what they are:
+ The CCV air hose, w/ both end fittings (this is the small hose)
+ The left half of your ait hose harness, with all of the cute little hoses attached.
+ The right half of your air hose harness, with all of the cute little hoses attached.

Your next job is to install all of said hoses, which should replace all of the oil-clogged lines, giving your rig the air it needs to run all of the little stuff. A new filter, and a cleaning of the filter box, and you should be back in business.

Now, to keep things clean, pick up a can of Carb Cleaner or some other cleaner than disolves oil without hurting plastic. Then, every few weeks, remove the small hose, and spray the cleaner through, cleaning out any buildup BEFORE it blocks the hose. If you keep up on this, and regular oil changes, you should have no problems. I learned the hard way, and forgot for a few months. Hello oil filter #2! :-)

Cost: right around $50 for everything listed, including cleaner / new filter

Note: Once I used some of that engine restore stuff, and it seems that it sealed up that little line just like it was supposed to do to my engine. I had a foamy goooo coming through all my air hoses, and my Jeep barely ran. I hope it helped my engine more than it helped my air system. :-P So, it might have been a one-off, or maybe something to avoid.

Good luck! Email me with any questions!