1992 Jeep Cherokee 4.0 inline 6. It has 251,000 miles. It does have a very minor leak from the front of the oil pan gasket. It has required "normal" topping off of oil between oil changes. My last oil change was about a month ago using synthetic 5-30. Since then I have to add about a quart at fill-up (about 200 miles). The motor still runs well. There was a makeshift rubber hose from the ccv (rear top of valve cover) to the intake/injection that I swapped for one that I picked up at the junkyard that was original. Same with the tube from the air filter box to the valve cover (front). I'm not seeing any significant change to the color of the exhaust.
Any help or suggestions would be appreciated.
Check that leak to see if it gets worse with the engine running at cruising speed rather than idling. Do a wet vs. dry compression test to see if your piston rings are worn. Although those engines can go for hundreds of thousands of miles and keep going, that depends on how it's been treated and maintained for the last 30 years. (The 4.0 in my '99 Cherokee has similar mileage and uses no noticeable oil between changes.)
Cylinders compression
C1 115 dry, 119 wet
C2 115 dry, 118 wet
C3 120 dry, 123 wet
C4 116 dry, 120 wet
C5 120 dry, 122 wet
C6 115 dry, 119 wet
So it's not a problem with worn rings. (It is possible for oil control rings to wear when the compression rings are OK but usually they wear together.) Bad valve stem seals and/or worn valve guides can also cause oil burning. You should also double-check your PCV system is working properly. Are your spark plugs getting oil fouled? The oil has to be either burning or leaking, there are no other choices.
I'll check the CCV system today. I've found instructions to test PCV but nothing specific to CCV. Sounds like the test should be the same.
Spark plugs all look pretty much the same. They have a whitish ash like layer that doesn't come off with a wire brush. The plugs are only about 18 months old.
It's really sounding like a leak. At a quart ever 200 miles I'd expect the plugs to be covered with oily soot if that much was burning.
Check the PCV valve for proper operation and installation.
What's next?
Probably an engine overhaul.
https://www.alldata.com/sites/default/files/file-attachments/cylinder_leak-down_testing_011019.pdf
To check the leak 'at cruising speed ' do you mean to hold the RPM at around 2500 for say 10 minutes and watch the leak?
Yes. You might also want to add some fluorescent dye to help in leak spotting.
https://www.youtube.com/watch?v=LYzTyTlnsr8
Oil pans rarely leak, it's probably the front main seal, or the rear main seal. PCV, CCV, whatever you want to call it evacuates crankcase pressure to the intake manifold and when blocked, will force oil out at the weakest point where there is a seal. Did you do a leak down test to see if the valve stem seals are shot?
The CCV seems fine. Good pull from the intake and the port isn't clogged. The tube to the air filter box has just barely enough pull to hold a piece of paper to the opening. I am seeing oil at the air filter end of that tube but the filter has no oil on it yet.
I haven't done a leak down test yet.
I've had some dye in it for a week or so. I replaced the oil pan gasket and rear main seal myself 18 months ago (I'm certain that I didn't get all of the bolts back in the exact spots but all torqued appropriately). still found leaking that appeared to be either the front of the oil pan gasket or front main seal. In July of this year I had the crankshaft seal and the valve cover gaskets replaced. Still seeing a little leaking at the front of the oil pan.
I'll do a leak down test next.
I hope to get the leak down test done this week but have now watched the exhaust more closely for a longer period of time and there is quite a lot of white in the exhaust that doesn't seem to go away after the engine is up to temperature.
I'm also logging mileage and oil level.
Persistent white smoke from the exhaust is a symptom of burning coolant, a blown head gasket. Blueish/white smoke is oil burning.
The 'smoke' seems persistent but it may just be because of the cold temps. we've been in the low 30's to single digits recently. Tomorrow we'll be closer to 50f. I'll have one of my kids drive it while I follow for a little bit just so that I can see.
I finally got around to doing the leak down test. Here are the results.
Cylinder 1) 27% seems to be leaking via the oil dipstick tube.
Cylinder 2) 35%. a lot coming from the valve cover (CCV) and some from oil dipstick tube.
Cylinder 3) 35% Lots coming from valve cover (CCV) and some from the dipstick tube.
Cylinder 4) 23% Hearing it in the oil pan and dipstick tube.
Cylinder 5) 50% Leaking through to Cyl 6. Also oil pan/dipstick tube.
Cylinder 6) 25% Valve cover (CCV), oil dipstick tube.
What's next?