Use 4.0 valve cover on 90 4.2l engine omit pcv valve use breather

PCV bad. (or its hose pinched, clogged or even backward)

1. PCV is failing fix this first for $5 or remove it and see how bad blow by is. visually. (remove only valve covers side to test )
2. or cracked or collapsed piston.
3. or cracked head gasket, compression tests pass?

yes one of the best jeeps are YJ, not full 10 stinking married computers , wish every day I never sold my 2002,
2002 is the last good jeep IMO

blow by is most mostly past bad piston rings, even warped cylinder from past overheated engine,

The YJ used a 2.5 L AMC 150 I4 or optional 4.2 L AMC 258 I6 until 1990

which one, 4 or 6 cylinders.???

YJ jeep Jurassic Park

1992 Jeep Wrangler YJ Sahara models
all are great.

Oil is coming from pcv or breather pipe. You probably have excessive piston ring wear causing blowby and forcing combustion gasses up the engine and into the breather and pcv system. Oil will come with the gasses if not filtered out. Check the pcv valve for proper operation and check any breather cap for baffles to keep the oil in the valve covers. Also consider getting an engine rebuild, if you want to keep the car.

I can only think there is a fault somewhere in the pcv system that has somehow been overlooked.

The vacuum part of the pvc system provides the motive power for the flow of clean fresh air through the crankcase and into the inlet manifold via two routes, taking with it the various crankcase emissions to be burned up during the combustion process.

At idle and low revs and other times when manifold depression is high, such as during overrun, crankcase emissions tend to be drawn through the pcv valve directly into the manifold and at other times, when engine speed and load increases the venting increasingly takes place through a second vent into the air cleaner or intake. At low speed, etc. the second vent becomes the supply of clean fresh air for the crankcase.

In the early days of the pcv system some engines had fresh air supplied via a filter fitted to the crankcase and others used a vented oil filler cap. A few engines had no air inlet at all and the manifold was simply connected directly through a length of half-inch hose to the valve cover. Those engines almost never leaked oil...
For a few years BMC used a complicated vacuum regulator valve mounted on the crankcase and again a large bore hose to the manifold.

Since those days most manufacturers have settled on one of two systems using either a pcv valve or a small air bleed into the manifold to provide the motive power for the ventilation of the crankcase. Mostly there is a flame or spark arrestor/oil mist trap in the breather that sometimes becomes blocked and causes trouble, though the trouble is usually the opposite of too much vacuum.

I suggest you go back to the drawing board and recheck everything. During a service I usually test the breather by putting the compressed air blow-gun down the dipstick tube and sealing it with a cloth. If the crankcase breather is clear and working properly no pressure will build up inside the engine.
Obviously some caution is needed to avoid blowing out seals and gaskets if the breather is found to be inadequate.

Follow the breather hose(s) to where they meet the engine and that is where the pcv valve is usually to be found if your engine has one; valve cover, crankcase or inlet manifold are the likely locations.

Not all engines use a pcv valve. Many engines just have a tiny hole into the inlet manifold which typically soon blocks and needs cleaning.

Ford was one of the manufacturers who in the early days of a pcv system being required, used an oil filler cap to attach the breather system to and the tiny hole was inside the cap along with the flame trap and oil mist filter.
That like many of the plastic pcv valves was intended to be a throw-away item.

I hope I have helped you a little.

You can disconnect the PCV from the valve cover and install it there or buy new valve covers. Essentially a breather does exactly what the PCV does but not as efficiently and it doesn't re-burn the ventilated gases. Please rate me on this answer.

Remove PCV valve from rocker cover. Run engine at i
dle. Place thumb over open end of PCV valve to
check for vacuum. If there is no vacuum at valve, c
heck for obstruction in manifold port, hoses or PCV
valve. Repair or replace as necessary.

Turn engine off. Remove PCV valve. Shake valve and
listen for rattle of check valve inside. If a clear
rattle is not heard, replace PCV valve.

Visually inspect valve for varnish or deposits whic
h may make PCV valve operation sticky, restricted o
r
cause incomplete seating of valve. Replace if neces
sary. An engine must be sealed for the PCV system t
o
function as designed. If leakage, sludging or dilut
ion of oil is noted AND the PCV system is functioni
ng
properly, check engine for cause and repair as requ
ired to insure PCV system will continue to function
properly. An engine operating without any crankcase
ventilation can be damaged so it is important to
replace PCV valve and air cleaner breather at regul
ar intervals (at least every 30,000 miles). Check a
ll hoses
and clamps for failure or deterioration.

Look for leaks high on the engine, like valve cover gaskets...

High oil pressure, if you actually had that, would not necessarily exit the engine....Crankcase pressures are not related to system pressure, per se.

Get a mirror and inspect very carefully.. Sometimes you have to clean the engine and test again, as air blows leaks all over (while driving...)

the motoor is having trouble breathing pcv may be clogged check all breathers clean as needed or its over filled

cornejo1971: There is no crank case oil breather per say. There are two PCV valves on the engine. There is one located on the intake plenum on each side. Typically there are hoses which come off the tops of the valve covers that go to the PCV valves.

I had the same problem. I changed the pcv valve and checked the hose. It still did the same thing. Then I checked the metal tubing that the hose connects to and found it was full of hardened gunk. I sprayed PB Blaster into the tube let it soak for awhile. Then took an old choke cable, put it on a drill and ran it into the tube like a drain snake. It took a while of doing this over and over but eventually got it cleared out. Now, no more oil all over the engine and hood compartment.