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I am trying to find the cause of a rich running engine, and I'm running out of ideas of what to do next. 

History:  Wife bought me and my son this Jeep sight unseen a few months back for us to fix up. 213,000 miles

Initial engine problems identified: Heavy exhaust smell, cracked exhaust manifold, Clogged CCV valves with oil bypass in air filter.

Overall the jeep runs great other than the strong exhaust smell and bluish smoke from exhaust pipe. No noticeable power difference from other wrangler 4.0s i've driven.  Haven't driven it enough to report on gas mileage. Smell is too harsh to drive for long periods of time, especially with top off.


Items I have tested or replaced:

1. New CCV valves, gaskets, vacuum lines, air filter - not getting any blow by since

2. New exhaust manifold

3. New NGK O2 Sensor

4. New spark plugs, oil change, oil filter

5. New vacuum line from fuel pressure regulator to intake manifold

6. Compression Test 1:150 2:155 3:160 4:155 5:160 6:160

7. MAP sensor - tested ok

8. Fuel Pressure - 31 psi

9. Fuel pressure regulator - I believe tested ok, disconnected vacuum line and fuel pressure increase 10psi

10. IAT - tested ok, but had blow by residue so ordered new one to replace (not installed) 

11. Thermostat - Engine gets up to around 205 per inside gauge and then drops 10deg or so, so thermostat is opening.  Heat is working too good for Florida

12. Injectors - Tested resistance in Ohms and they are within range (13.8-14.4), also disconnected each one individually and there was a noticeable reaction from the engine

13. Vacuum Leaks - Have checked and replaced what was cracked.  I don't feel there are any leaks.


At this point I'm looking for advice on what to do next: 

My plan: 

1. Replace IAT just because I ordered a new one (arrives tomorrow)

2. New Cap and Wires

3. Check/ replace coolant temp sensors (both)

4. Probably needs a new CAT, but wanted to get the rich running under control before replacement.  Maybe the old cat is non functioning and is the cause of this? I don't feel that this is the cause, but not enough experience to know for sure. 

5. Replace injectors?? Maybe a bad spray pattern...clog??? 


Thank you in advance for any guidance! 


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playingsnooky...The bluish smoke is the clue.  Despite the decent compression, your engine is burning oil.  You have found telltale signs in several areas.  The oil in the exhaust stream raises havoc with the A/F ratios, and the O2 sensor is trying to compensate with a richer mix.  A clogged cat is possible if oil burning has gone on for some time.

Even though the engine seems to run okay and perform like other 4.0Ls, I would check the exhaust for back-pressure and also run a CO/HC reading with an engine analyzer and tailpipe probe.  HC will be high from oil burning, CO may be normal or close to normal.

My first focus would be valve guide wear and leaking valve guide seals.  You can service valve guide seals without an engine tear-down by removing the valve cover and removing rocker arms.  Seals can be replaced with a simple overhead valve spring compressor.  With all valves closed, you can rotate the crankshaft to bring each piston to TDC before removing that cylinder's valve springs.  This eliminates the risk of a valve dropping into the cylinder. 

An air hold or leakdown tester can apply pressure through the spark plug hole, performing a cylinder leakdown test with the piston at TDC and the valves closed.  Simultaneously, the compressed air will help hold the two valves in position while removing and installing springs.

It is very easy to check valve guide wear at the same time.  You can rock the valve stems sideways to check clearance between the stems and guides.  (It may be necessary to relieve the air supply pressure to allow the valve stem to move freely.)  Worn guides require cylinder head removal and a cylinder head rebuild at the local machine shop.  Before making the call to remove the cylinder head, you have performed a leakdown test and know with reasonable certainty the degree of piston ring wear and ring sealing ability.  If there is a high percentage of leakage past the rings, it would be wise to rebuild the entire engine.

At 213,000 miles with an unknown history, the engine begs a cylinder leakdown test.  A cranking compression check is unreliable when there is cylinder taper.  A leakdown test is at TDC, the point of maximum cylinder taper.  Static at that point, the rings will show their true sealing ability. 

Let us know what you find.  Replacing guide seals would be the easiest route if that's the oil loss.  A valve seat and face grind with valve guide renewal would be more work but not unexpected at this mileage.

A simple vacuum test at the intake manifold is always revealing in these cases.  I like to run such a test as part of any diagnostic troubleshooting.


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  • Moses Ludel changed the title to 1995 YJ Wrangler 4.0L Running Rich
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playingsnooky...You're welcome...It's impossible to "tune" away a mechanical problem.  You'll be thrilled with the leakdown tester.  There are five basic engine needs:

1)  Normal oil pressure and lubrication (proper bearing clearances, etc.)

2) Normal compression and cylinder/valve seal (the leakdown test)

3) Correct valve timing (timing chain and sprocket wear or jumped teeth)

4) Correct valve lift (camshaft lobe condition, valve clearances, etc.)

5) Normal manifold vacuum (manifold gasket seal, valve lift and timing, compression, etc.)

The rest is tuning once these other needs have been confirmed...Looking forward to your findings!


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I was able to perform the leakdown test this morning.  I did perform the test on a cold engine as this was the first time I had done this test and wasn't sure about burning myself.  I used the screwdriver method to determine TDC. I removed the dipstick, radiator cap, and hose to from the valve cover to the airbox determine leak points. Below are the results:

Cylinder 1: 30% Air flowing out of hose from valve cover to air box

Cylinder 2: 27% Air flowing out of hose from valve cover to air box

Cylinder 3: 17% Air flowing out of hose from valve cover to air box

Cylinder 4: 23% Air flowing out of hose from valve cover to air box

Cylinder 5: 30% Air flowing out of hose from valve cover to air box

Cylinder 6: 28% Air flowing out of hose from valve cover to air box


Based on these results I'm planning on moving forward with the valve seal replacement and testing the valve guides by the wiggle method you described in your first post.  Does this sound like the best course of action? 

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playingsnooky...Your findings are not frightening.  From a drivability standpoint, you should have adequate seal.  To be clear, however, this engine has wear.  My rule for normal leakdown is 10%-12% on an engine with proper break-in.  20% raises eyebrows (maybe more service life left but distinct wear).  A new engine will typically have 8%-10% leakage.

30% leak is significant.  This is usually a hint that valves and possibly rings are near the end of their service life.  An engine like this can often run for some time if cranking compression is still normal and oil consumption not excessive.  Cranking compression "grabs" air/fuel mix at the bottom of the cylinder and thrusts it up, which nearly always will give the false impression of an engine in better condition than it actually is.  Your leakdown test is much more accurate for pinpointing wear.

So the bigger concern is where did the extra 15%-18% leakage go during the leakdown test?  Out the tailpipe?  Up through the intake?  Into the crankcase?  If through the intake, you have intake valve leakage.  Out the tailpipe is exhaust valve leakage (common).  Through the crankcase and out the dipstick tube, or out the oil filler opening, you have piston ring blowby.  You have leakage, it's a matter of what leaks.

Your screw driver method for TDC is okay.  The main idea is to get the piston/rings near the maximum cylinder wall wear/taper point in the cylinder.  This is just below the ridge at the top of the piston ring travel.  If you need to run the leakdown test again to pinpoint where the leakage occurs, do so.

Regarding blue smoke, if you do not have noticeable leaks where I describe, you likely have guide seal and guide wear issues.  Rebuilding the head would enable an inspection of cylinder walls, piston to wall clearance and such.  If you decide at that point to re-ring the engine, and if the cylinder walls will hone within specification, my exchange with "Wayne" will prove useful.  The rules and findings for his 2.5L apply to your 4.0L...Read through the exchange before removing the cylinder head.

Keep us posted...


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Thank you Moses,

For all cylinders the air leak was flowing through the front CCV valve breather tube.  I could place my finger halfway across the opening and feel the air flow on my face if I placed it close enough.  Is that a sign of piston ring blow by or more of the valve guide/seal scenario.  I did not notice any air flow out of the exhaust, dip stick, or bubbling coolant in the radiator when performing the test.

My main goal is to get rid of the blue smoke knowing full well a rebuild is in the future... hopefully be able to have a little fun with it before that point. 

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playingsnooky...That's piston ring blowby.  Two ways to further confirm this:

1) Remove #1 spark plug (more accessible and a cylinder with 30% leak).  Bring that piston to just before TDC.  With a clean oil spray can or other means, put about a tablespoon of clean motor oil in the cylinder and carefully bring the piston to TDC.  The oil will run down around the piston crown and temporarily seal the piston/rings.  Since you're not check compression with a gauge, any increase in compression will not impact a leakdown test.  Run the leakdown test with the CCV breather tube open as before.  You will momentarily have a much improved piston ring seal (until the oil presses past the rings).  If the percentage of leak drops dramatically to no more than 20%, you have leaking rings.  With air still applied, the leak percentage will increase as the oil blows downward.  

2)  Optional:  Same test using a traditional compression gauge:  1)  run a compression gauge test of #1 cylinder dry, 2) put a tablespoon of oil in the spark plug hole, 3) run another cranking compression test.  If compression jumps way up, you have a ring seal issue.  After cranking a bit, the compression gauge pressure will drop down as oil leaves the upper cylinder.  Note:  Always hold the throttle open when running a cranking compression test with a compression gauge.  Otherwise, you will compromise the air flow and manifold pressure, reducing the compression PSI reading.

If the seal or compression improve from the oil-in-the-cylinder, it's because the valves were/are sealing relatively well.  If the compression does not change in either test with the oil added, there is a valve leak.  I'm betting on the rings, but you need to confirm.  Guides and seals may also be worn, though your overarching concern is to determine whether there is a ring seal problem. 

Should you decide to rebuild the engine or do rings, pistons, bearings and a cylinder head rebuild with the engine in the chassis, make sure the standard bores can honed like in Wayne's case.  Otherwise, the engine will need a complete teardown, cleaning and machining, fitting with new oversized pistons, a new camshaft and lifters, timing chain and sprockets, cam bearings, crankshaft work as needed and bearings, new oil pump, screen, etc., etc.  A rebuilt exchange engine would be available.  The 4.6L stroker build with injectors upgrade is an option.


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