Heavy carbon deposits in intake manifold... causes?

I'm in the process of replacing the intake/exhaust manifold gasket on my 4.0L and was greeted by some really heavy carbon deposits inside the intake manifold, extending into the cylinder head. This stuff is wet and gunky, and I'm sure it's not doing anything good for my engine. I ran a can of Seafoam engine cleaner through the intake only 75 miles ago, so I'm surprised to see this much build-up.

What do you think caused this to happen? My immediate thought was piston rings, but the air filter is clean and the motor really doesn't use any more oil than usual (for a Jeep). Oil pressure, per the dash gauge, does seem low -- it stays at 40 psi while driving and dips down to 15 psi while idling, but I understand that's a common thing for these engines. Might even be the sending unit, who knows.

Now, I've been averaging about 12 mpg... way less than what it should be. I've been suspecting an intermittent oxygen sensor was causing the engine to run richer than necessary, which could account for the carbon I'm seeing. Would a rich condition cause this much gook?

What you see is oil, not carbon. Your valve guides are shot. It is sucking oil down the valve stems. That is the only way you get oil in the intake side. The fix is to pull the head, do a valve job including sleeving, not knurling, the valve guides. A cheap temporary fix is just to replace the valve seals, but it won't last all that long.

Here is how to tell you're valve guides are shot. You will not notice blue exhaust driving, but drive a while, then go to get a burger at BurgerKing and wait in the drive through line for a couple of minutes idling. It will be blowing blue smoke by the time you get to the pickup window.

Some of that oil can come from the CCV system, probably not all of it. Also some dirt can get by the air filter hit the oil and gunk everything up. How many miles on the engine?

Katarn

And I don't know if I'd worry too much about worn valve guides if I wasn't bothered by the oil consumption or an occasional puff of blue smoke. If you decide to scrape that stuff out try to avoid getting any of that, or that old gasket material, into the engine.

Looks like a good time to service the crankcase ventilation system too. Clean out the hoses, and the fittings where the small hose connects to the intake plenum and the rocker arm cover. Replace any that might be ready to break and start sucking unfiltered air into the engine. Getting the crankcase pressure to negative (may be impossible if you have too much blowby) will help slow future buildup.

What year?

Going from the port shape, I'd be inclined to say RENIX (1987-1990,) and trace oil from the CCV system, coupled with carbon contamination from the EGR system, will make deposits much like you see (although mine are a bit dryer.)

I really wish I could do away with the EGR setup and go with water or methanol fogging - it would work better and cleaner, but it wouldn't pass smog out here (and they're a real pain about the smog thing here in CA...)

If you're not using any oil, there's not a lot that can be done about it. Try cleaning your CCV tubing, orifice, and baffles in the valve cover, and that should reduce the oil getting into the intake. Not much you can do about the EGR, I'm afraid...

5-90

5-90, it's a '98 (96,000 miles), so I fortunately don't have to deal with an EGR system. I'll check out the crankcase ventilation system, though, as Dave, katarn, and yourself have mentioned. I think I did that already, but I'll try again... .

You know, I wouldn't be surprised if this engine wasn't beat on by the previous owners. I picked up the XJ at the junkyard and fixed the body damage, but judging by the condition of the interior, this poor Jeep was driven hard. And disrespected.

I never thought about valve guides, old_man. I replaced the valve springs while I was restoring the body (call it preventive maintenace; I had a bad experience with my Wrangler), and I also did valve stem seals while I was in there. I can see how bad guides would destroy seals, but we're only talking about 4,000 miles -- that's all I've put on since it's been done. Could they wear out that quickly? The old seals were rock-hard, so maybe this gunk (not necessarily the wetness) is from years gone by?

On another note, I found the usual "Jeep crack" in my exhaust manifold. At least now I won't have to worry about when it'll happen!

I cleaned out the intake passages, brazed the cracks in the exhaust manifold (though it still ticks, ugh), and cleaned off the oxygen sensor. Everything went together fine.

As for the oil buildup, I pulled the rear CCV line while the engine was running and discovered an oily residue on the intake manifold-end of the hose... so I guess it's drawing oil in from there. I replaced the intake manifold gasket a few months ago, and I recall unbolting the CCV screens from the inside of the cover to clean everything out. One of those gaskets might have fallen apart upon reassembly, causing oil to be drawn into the ventilation lines. When I have time I'll pull the valve cover and find out.

Thanks for all of the help, guys.