Blowby

Hello, I have a 91 4.0 with 150,000 miles we just got it a few months ago intended for my girlfiends first trail rig.... It was running great untill the other day. I am getting alot of crankcase pressure now, I know you are all going to suggest checking the ccv system etc etc.... it's all good, I've had the valve cover off replaced ccv tubes and done everything I can think of... the compression checks were 135,145,145,165,160,150 and with about 10ML. of oil in each cylinder 150,175,165,185,180,175.... This engine is running great! I have no complaints but the blowby is pushing oil up out of the dipstick and spraying the engine compartment.... it is also forcing it out the rear main seal ( been replaced not long ago), I'm sincerely worried about having a fire on the way to the trails, this all started very suddenly and it is only a problem at highway speeds around town everything is fine.... Any body? please I'm at a point where I'm ready to pull the head off to take a look but I feel that I must be missing something... I mean this thing runs without so much as a tick Thanks in Advance.... Chris

Is it actual blow-by, when you put your hand over the oil filler hole (1/4 throttle), does it want to blow your hand up. If the CVC system is working correctly, there will be very little pressure at the oil filler hole.
If there is some pressure at the the oil filler hole, pull one plug wire at a time (carefully) and see if the pressure is reduced, when one cylinder isn´t fireing.
The CCV, system is actually two systems, one with the throttle open and one with the throttle closed. When the throttle closed portion of the system isn´t working right, suddenly closing the throttle builds a lot of pressure momentarily.
Check and see if the pressure, if there is any, is most when the throttle is open (1/4-1/2 constant throttle, don´t blip it) or it gets worse when the throttle is suddenly closed.

Rare but possible. Mostly on motors that see a lot of high speed driving. Run your pistons to the top. one at a time, use a small mirror and flashlight, look for a V shaped, flash mark, or notch in the top of the piston. Running too hot of a plug, can notch a piston and cause a lot of blow-by without messing with your compression readings, a lot. Sometimes scratches a piston wall, acts like a cracked ring. Excessive pressure, mostly at higher RPM´s, with no excessive compression loss.
Wrong dipstick, too much oil. Mine I leave at half full, if I fill it too the full mark, it throws the top half someplace, pretty quick. It runs from half full to add for months. I just concider it a quirk and live with it.

Thank you very much for the Info. I'd like to know more about the ccv system that is supposed to work when the throttle is open... What am I looking for? I haven't run any tests with the filler cap off, but I have checked when I pull the dipstick at idle, there is no pressure, but when I hit the throttle I get a "blast" of vapor out of the dipstick tube at first and then followed by a steady vapor though not as much as long as I hold the throttle open.
Right know I have the steering column apart to repair some wiring when I get that put back together this afternoon I'll run down the list you gave me. Thanks a bunch

The part of the CCV system that hooks below the throttle body (or in the intake), is for throttle closed, the part that hooks above the throttle body (air cleaner box) is for throttle open (mostly). Though there is overlap of function. Most of the Jeep systems Ive looked at, are the pretty much the same engineering, in a sometimes different configuration, depending on year and model run.
` Have seen a severly overheated motor, have the rings relax, blow-by, loss of compression. Things usually get worse fairly fast. Wife did it to a FSJ, with a V-8. "What´s it mean when the little needle is in the red and there is smoke coming from under the hood?" My question, when did this happen, wifes answer "A couple of hours ago, but the smoke stopped about half way home, little needle is still in the red and there is a little light in the dash, that is glowing".