Does anyone have light exhaust fumes coming out of the EGR/PVC valve(or whatever tube that is that go's from the valve cover to the filter box) while running. My buddy has two XJ's also('90 & '91) and he says both of his do this? Is this a sign of age or that it needs a good flush? My '93 does not do it and the '92 has a '95 ZJ motor in it that only has 90,000 miles on it and it does not do this so I do not understand it?

I don't know about exhaust fumes, but blow-by is drawn off through that tube into the air box while the engine is running as this engine has no pcv system from the factory. JMO

The HO motors do not have EGR. Mopar changed the cam timing to allow for more lobe overlap in order to eliminate the EGR. By allowing the intake and exhaust to remain open (together) for a trifle bit longer, the fresh air charge pulls back a tiny bit of the spent gases. Effectively functioning the same was that the EGR does.



The 4.0 does have a crankcase ventilation. Instead of using the check valve like most other pushrod motors do ... they use a metered orifice on the back of the valve cover. Try replacing that back unit with just an open elbow or traditional PCV and watch your RPMs jump.



Joe
EndlessMtnFab

Won't removing that orifice give the engine an air-bleed of sorts? Your suggested rpm increase would indicate this. Also wouldn't this cause a lean condition? I don't know the 4.0L, so I'm curious.

Yes I have done that. I took it off to see if it was clogged up and to see if that was the reason for the amount of (I guess) blow-by or whatever was coming out? I have just owned three 4.0L I-6 Jeep motors that ran one in a 1999 wrangler one in my '92 XJ which is a '95 ZJ 4.0 and a '93 which does not blow out any smoke at all from that tube. It also comes out of the oil fill hole when you open it while it is running. I guess What I am asking is why would it get worse (nore coming out, the gas coming out is thicker, more fume like smells,ETC....)

Metered orifice would be the word. The grommet is big .. but the hole is quite small. Less than an 1/8 inch in diameter.


Would it be considered an air bleed? I dunno .. never thought of it that way. The back grommet/valve pulls air from the intake (trace the plastic hoses). If you remove it from the assembly from the valve cover, your RPMs should climb up. Cover the hole in the valve cover with your palm and it should go right back down.


I guess loosely describing it .. the sensors in the TB see the increase of air flow thru the manifold (because of the vacuum from the CCV).



Joe
EndlessMtnFab

Now I'm confused. Bear with me. Since the intake manifold is under vacuum, I think the rear hose is pulling air from the valve cover through that line to the intake and not vice versa. Then the front line is supposed to draw air from the air box to the valve cover, drawing out fumes as it leaves via the rear line, (with the metered orifice) to the intake. With that metered orifice removed there is an open air bleed, (granted a small one), allowing an increased amount of unmeasured air directly into the intake that is not being regulated by the TB, or seen by the Map sensor. That's why I questioned the rise in RPMs and was concerned about a possible lean condition. Just curious.

Blue....


Nice catch. I did describe it wrong. Been a long day here.


The intake is indeed pulling a vacuum .. hence the restriction in the rear grommet/valve. You describe the CCV perfectly. That said ....if you disconnect the rear hose, the RPM's do rise. Kinda like creating an open loop. Especially when you are going from dinky hole to open atmosphere.

Try it and see.


I don't think you would really run into a lean condition ... fuel pressure regulator would presumably try to compensate up to a certain point. But it's too late for me to really think that one thoroughly.

Can't believe I had that vacuum backwards ... wasn't thinking too clearly that time.


Nice work !


Joe

That's cool Joe, my brain gets stuck in closed loop sometimes after I think about something and it won't let it go until it's explained fully.
I was concerned about the unmetered air getting into the intake. I thought it would be like pulling a small hose/nipple connection off of an intake connection, without the resulting affect of the part/system that was associated with that hose/nipple connection being dis-connected. I.E. similar to an open air bleed, or vacuum leak.

O I understand all that now , but it has been taken apart and cleaned and I still have light exhaust/smoke coming from the front elbow? What else would cause this?

The only thing I think it could be is blow-by. Could just be a stuck ring, or severely worn valve guides.JMO

Well the valve cover was loose, some back bolts were a full turn and a half from being finger tight and after a good oil change with some decent oil and some Lucas and it is not doing it now. Thanks

That's great news.