Airbox/oil
CF Veteran
Joined: Nov 2008
Posts: 6,989
Likes: 4
From: Oak Harbor, WA.
Year: 1987
Model: Cherokee
Engine: 4.0 liter RENIX I-6, DIY Cold Air Intake, 2.5 FM Exhaust, 3 Core Radiator
The 4.0 liter has a very bad Crank Case ventilation system. Basically what you have are 2 fittings on your valvecover, one forward and one rear. The rear fitting is hooked to a vacuum source on your intake manifold. By design this line is too small and often becomes clogged with sludge, dirt, oil and varnish. The forward fitting is hooked to your intake air box. The 4.0 creates massive pressure in the crank case and oiling system. Especially at highway speed and when the motor has 100K + miles on it. When the rear line becomes clogged like yours most likely is. Mine was badly clogged when I first bought my XJ. It had 185K in the ticker. All of that pressure is pushed out through the forward line into the air intake box and you guessed it all of oil sludge, dirt and varnish comes with it and gets all over your air filter. There is nothing that can be done to totally prevent this. You can put a larger fitting and hose on the rear fitting from your valve cover. This helped mine a little but, after a long road trip I still have oils in the air intake. I just wipe it out. The good news is that this doesn't mean your motor is about to blow or anything like that. It is just something you have to live with. There are several ways guys have set their Jeeps yp to keep the oil out of the intake. You can completely disconnect the front fitting from the hose going to the air intake box and just attach a small air filter to the forward fitting on the valvecover. Just get one you can clean and re-use because you will be cleaning it often. You can also rig up some kind of oil filtering system in the hose between the valvecover and the air intake box if you'd like to go that route.
Senior Member
Joined: Apr 2009
Posts: 845
Likes: 0
From: central IL
Year: 01
Model: Cherokee
Engine: 4.0
I looked online before and they sell a kit for that replaces the pvc and some gaskets/hoses it's supossed to be around 30 or 40 bucks google is where i found it
Seasoned Member
Joined: Feb 2009
Posts: 326
Likes: 0
From: Columbia SC
Year: 1991
Model: Cherokee
Engine: 4.0
CF Veteran
Joined: Feb 2009
Posts: 1,184
Likes: 4
From: Ohio
Year: 1992
Model: Cherokee
Engine: 1995 ZJ 4.0
The 4.0 liter has a very bad Crank Case ventilation system. Basically what you have are 2 fittings on your valvecover, one forward and one rear. The rear fitting is hooked to a vacuum source on your intake manifold. By design this line is too small and often becomes clogged with sludge, dirt, oil and varnish. The forward fitting is hooked to your intake air box. The 4.0 creates massive pressure in the crank case and oiling system. Especially at highway speed and when the motor has 100K + miles on it. When the rear line becomes clogged like yours most likely is. Mine was badly clogged when I first bought my XJ. It had 185K in the ticker. All of that pressure is pushed out through the forward line into the air intake box and you guessed it all of oil sludge, dirt and varnish comes with it and gets all over your air filter. There is nothing that can be done to totally prevent this. You can put a larger fitting and hose on the rear fitting from your valve cover. This helped mine a little but, after a long road trip I still have oils in the air intake. I just wipe it out. The good news is that this doesn't mean your motor is about to blow or anything like that. It is just something you have to live with. There are several ways guys have set their Jeeps yp to keep the oil out of the intake. You can completely disconnect the front fitting from the hose going to the air intake box and just attach a small air filter to the forward fitting on the valvecover. Just get one you can clean and re-use because you will be cleaning it often. You can also rig up some kind of oil filtering system in the hose between the valvecover and the air intake box if you'd like to go that route.