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Leak?

 

You mean, like oily fumes creeping out from under the flange? The problem isn't the oil filler cap, the problem is excessive blow-by pressurizing the crankcase more than the PCV system can suck out.

 

Mine does this also. Is something wrong with my truck?

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Leak?

 

You mean, like oily fumes creeping out from under the flange? The problem isn't the oil filler cap, the problem is excessive blow-by pressurizing the crankcase more than the PCV system can suck out.

 

Mine does this also. Is something wrong with my truck?

Sadly, yes.

 

Since sometime in the late 1960s, automobile engines have not had the crankcases open to the atmosphere. The 1950s engines we had when I was a young sprout had an open tube coming out the side of the engine and turned down toward the ground. That was ventilation for the crankcase -- if the crankcase needed air, it sucked it in through that tube, and (more often) if there was blowby, the excess pressure in the crancase was vented to the atmosphere through that tube.

 

Then we started getting concerned about air pollution, so eliminating that oily smoke that vented from all the crankcases became an issue. So the manufacturers eliminated the open tube, and created the Positive Crankcase Ventilation (PCV) system. Instead of an open tube, they put a check valve in the valve cover, and a vacuum line to the intake manifold, along with another line from the air cleaner to the valve cover. The idea was to create a closed system. The PCV line sucked the oily fumes out of the crankcase and into the intake manifold, where they mixed with the incoming air-fuel mixture and were burned up in the engine. The inlet tube from the air cleaner provided a path for fresh air to replace the oily fumes the PCV system sucked out, and the equalize the atmospheric pressure in the crankcase.

 

The Jeep 4.0L engine uses essentially the same system, except there's no PCV valve -- the very small tube coming off the back end of the valve cover is the suction tube, and because it's so small they decided a physical valve wasn't needed.

 

What happens is that, as the engine gets old and the piston rings wear, they lose compression and more blowby gets past the rings. This builds up pressure in the crankcase. At the same time, sludge tends to restrict that little tube, leaving no avenue for the excess pressure to vent. So two things happen: First, the front inlet tube becomes a pressure vent tube, and oily fumes get pushed back into the airbox. Open your airbox, and you'll probably see oil soaking the air cleaner element. That's where the oil comes from. If the blowby is severe enough, even the inlet tube doesn't provide enough relief, so the pressure goes up the oil drain return passages, pressurizes the valve chamber, and oily fumes start to seep out around the filler cap.

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Well did compression check compression still in spec but on the low end......got a new pcv and got a new breather filter. And seemed to help at the moment but I will start saving some money to rebuild the old four banger, but hopefully she will last a few more years.

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