98 xlt 4.0L, 4x4, ~185K, Manual Transmission
For about a year now, my ranger has had an intermittent misfire on cylinder 6 (sometimes sets code P0306). I have code P0455 set, but I believe this is unrelated. By intermittent, I mean that it will start rough, drive for a mile, run fine, get on the interstate, misfire for a while, and then run fine again. Happens in all weather, does not seem to matter if then engine is warm or cold, and is not speed dependent (might happen at idle or going down the road). I have not had time to seriously troubleshoot it until now, but I am at a loss. (warning, long story ahead)

About a year ago, I had a miss on cylinder 6. I had replaced a failed fuel pump a couple of months before, and the truck had seemed to be low on power. The last time this occurred, I had had a snotty injector. I pulled al 6, cleaned, and pressure tested them with 90lbs of air (forward and backward). Some junk came out, and one was slightly snotty. After cleaning, it seemed fine. I used a tube full of carb cleaner connected to an air hose as a cleaning method. The truck seemed to run better afterward. I did have to replace the rubber seals twice as I buggered them up putting them on and it leaked, but I eventually got it running correct again.

Fast forward a couple of months, and my misfire comes back. Since I could not remember when the last plug change had been (at least a couple of years), I decided to replace all 6 plugs. The problem went away a couple of days, but then came back. I used bosh plugs.

Since I could also not remember the last time I had changed plug wires, I went ahead and replaced them. When changing them, I decided to pull #6 plug to check it. When I did, it broke off at the base during removal. Due to how it broke, I believe this was a factory defect. Thinking I found the problem, I replaced the plug with a spare I had from the same lot as the original, and continued replacing the plug wires. The problem went away for a few days and came back.

Now, being annoyed that the misfire seemed the come and go, I suspected the replacement plug was bad since it was from the same lot. When I pulled it, it had some surface cracks near its base. Since I had read Bosh plugs are not the best, I got an NGK as a replacement thinking that they are hotter and might help to burn the junk that may have accumulate in the cylinder. The replacement plug helped, but did not fix the problem this time.

At the time, I had no more time to look at it, so I let it go for a couple of months since it was not bad enough to be un-drivable and the the CEL was not flashing. Got time to look into it about a month ago. Did a compression check on #5 and #6 cylinders (#5 had started misfiring slightly, but was just a plug that I'd forgotten to change the first time). I had both plugs removed and both cylinders ran up to and held slightly above 120lbs. I know this seems a bit low, but I did replace the heads with aftermarket ones right after I got the truck due to the cracked head problem. Since both matched each other and both held pressure for at least 3 minutes, I am going to assume the lower pressure is due to the aftermarket heads. This lea me to believe that maybe when I had the broken plug, it took part of the coil pack with it. In addition, the last time the coil pack was changed, a shop did it without asking me, so I did not know what kind was on it. Replaced the coil pack and gave it another round of NGK plugs to make sure all cylinders are the same. The problem went away for about 2 days and then came back. Misfire #6 cylinder (1.67% rate according to mode 6).

At this point, the injectors should be clean, the ignition system is new, and I have compression. I have attached a couple of pictures of what the #6 plug looks like when I pull it. One other thing to note is that the threads of the plug always seem to be a bit oily when I pull it. Not sure i that is related or not, but none of the others do that. I am not sure where to look next as all the normal misfire causes seem to be ruled out.

Plug Pics

 

Looking at the gross plug fouling deposits, its obvious cyl 6 has a oil control problem of some kind. Might be valve guide seals, or maybe a worn valve guide, or coked up oil control ring, How did the other plugs look?

 

The other plugs look normal. It does leak oil around the sensor, so its hard to tell what is going into a cylinder and what is leaking. A friend also suggested that maybe a valve is getting stuck a little bit. Not sure how to check those without removing the head or without an inspection camera.

 

Lets think about this some more, you could have a stethoscope listen around #6 valve train for noise that might suggest a worn guide state. Or remove the valve cover & have a look at the stem seals condition to see if any look worn, hardened, or damaged & maybe give the stem some side to side force to determine if it seems loose, or maybe give the stem a shot of something low viscosity like Marvel Oil & see if it quickly disappears around the stem & guide interface.

If you find a seal problem & fix it, maybe consider removing the #6 spark plug & giving the cylinder a good overnight soaking with a couple ounces of Marvel Oil to free up the rings.

Then add a 20oz dose of Techron Concentrate Plus at the pump before filling (so to get good mixing) with Chevron, Texaco, CalTex or now Shell, all of which already have some PEA in their fuel ad pack, so the extra 1oz/gal Techron will raise the treat rate 10X above pump gas alone, so can often tidy up the fuel injectors, intake valves, piston crowns & combustion chambers in one dose, if we'll run most of the treated tank out before refilling & do some daily rpm run up's to 3500 for a mile or two to blow out loosened deposits.

If a follow up #6 plug inspection shows a clean plug, call it fixed for now. If it looks better but not as good as you'd like, repeat the Techron dose at the pump on the next refill & repeat the daily high rpm blow it out routine & let us know how it goes.

 

Sorry for the slow reply. Unfortunately this truck is my daily driver as well, and we only have 2 vehicles between my wife and I, so I am trying to plan out when to do this work. Can't tear into the motor until after this weekend since my in-laws will be in town. Anyway, pawpaw, what makes you suspect burnt valve over malfunctioning lifter? Reason being I am not sure I understand how I would have good compression in the cylinder and also have a valve that doesn't seal properly?

 

Where did you surmise I was thinking of a burned valve? Because of the #6 spark plug deposits, I was thinking along the lines of a damaged, or worn, or hardened valve stem seal, or maybe worn valve guide, or because of all the deposits on the #6 spark plug, perhaps a stuck oil control ring, or some combination there-of. EDIT: Since this is a 4.0L, what viscosity & service grade engine oil have you been using?

 

Pawpaw, I apologize. I misread your earlier post. I've been using 5W-30 conventional in it, walmart brand. Have not been very good at keeping regular service intervals. Its been a while. The reason I am thinking it is not my heads is that they only have 40K on them, and I was good doing oil changes every 3000K until about a year ago. When I moved 1.5 years ago, I had a local company change the oil and that's when it started leaking again. I also had them do my wife's car. It had never used oil before then. After, it used some and the dipstick smelled like kerosene. Never took it back, but I have no idea what got put in there. I have done change myself since then.

 

5W--30 is the correct viscosity for the 4.0L as its oil pump never got up graded to be able to deliver enough volume of the lower viscosity 20wt at operating temp. Any idea what the shop you had changing the oil was putting in there? What viscosity did the receipts say they were using?
Doesn't matter now I suppose, as something is going on with cylinder #6 if only its spark plug is fouled, so question is What is the root cause?

After looking at the plug deposit photos again & noticing they look kinda Dry, not really oily, maybe #6 fuel injector is defective, or is dirty & has a really bad runny nose & #6 cyl is running really Rich. So hook up your ELM scan tool & running FORScan or the like diagnostic software on the viewing device of your choice & have it take a look at the short & long term fuel trim on the drivers side cyl bank,(Bank-2), compared to Bank-1 passenger side & see if it'll show any fuel trim corruption, or control problems with Bank-2 in which cyl-6 resides? Post up the fuel trim Numbers & any trouble code clues the scan tool finds.

If you don't have a ELM, or can't borrow one, hook up a fuel pressure gauge to the under hood fuel rail Schrader valve test port & see if fuel pressure is in spec & that it doesn't quickly bleed off after the engine is shut down.

If you can't do that, if its been a while since you changed the oil, pull the dipstick & give it a sniff to see if it reeks of gasoline. Or maybe after it sits for a spell after driving & cools down some, remove #6 spark plug & give it a sniff to see if it reeks or is damp with fuel. If so, remove the #6 fuel injector for inspection & or cleaning, or replacement if it looks to need it.

A bunch more thoughts for consideration, let us know what you find.

 

Finally got to get numbers from the truck a few days ago. I ran it from costco home. This includes some stop and go as well as interstate driving. Here are the plots of the data. The second bank trim does seem odd (trying to figure out how I have negative percentages).

 

I pulled the driver's side valve cover this morning. There is oil around all valves, and everything appears normal. There is no gas smell (other than from the ERG tube). I tried to move everything by hand. Nothing budged. I also tried to lift the rockers with a screw driver. Again, nothing moved. There are no indications of exhaust gas leaking into the valves. I will try adding some light oil as suggested, but preliminary inspection doesn't seem to show anything. Is there anything else I should check with the top end apart?

 

While the valve cover is off, check closely around #6 intake valve seal & guide. Rotate the engine so the valve is unloaded / closed (no additional spring compression) & see if you can spot anything suspicious with the seal, guide, or valve stem.
If ok, pull the fuel injector & inspect for signs of a runny nose.
If no joy there, maybe put the #6 cylinder piston oil control ring on the suspect list as suggested previously. Maybe consider warming the engine a little & performing a overnight piston soak with something like a couple table spoons of Marvel Oil or the like for #6.cylinder & see if you can get a positive result.
If you have, or can come by a fiber optic probe, remove #6 spark plug & have a look around the #6 combustion chamber for clues & let us know what you find.
More thoughts for consideration.

 

I rolled the motor over by hand this morning, checking the valves as I went. The #6 intake rocker arm has a little bit of play in it compared to the other rockers which I cannot wiggle by hand at all. Once I freed it up some it was easier to get it to move. It does not move much, but it does move. Being the intake valve explains why the plug is wet. I suspect this would be caused by a lifter since a bad valve or spring would occur constantly instead of intermittently. I also suspect a lifter as it is not very loose moving it by hand, but when the engine is running, if the lifter valve was stuck open, it could bleed down quickly instead of slowly. Does this logic make sense?

 

Does the Unloaded #6 Valve Stem have any play when wiggled in its guide that you can see or feel? How does the #6 valve stem Seal feel & look? Any cracks, chips & does it feel hard, or split, or crumble when squeezed?

 

Got the rocker arms off. Both valves feel tight. Was not able to move the intake side at all. Could barely move the exhaust side. It appeared to be just the spring turning a little bit, though. The valve stem did not appear to move at all. The valves all look the same. The seal looks just like the rest on both of the #6 valves and I see no cracks or signs of gases coming out (all shiny metal). Didn't realize I would have to take the head off to take the lifter out, so if it could be a lifter, I will have to pull the head anyway. If I do need to do that, should I just pull both heads and clean all the lifters while in there?

 

if the #6 cyl valve stem seals, guides look & feel ok, I'd move the #6 cyl piston rings higher up on the suspect list. A mucked up or broken oil control ring could be causing problems in #6 combustion chamber if it's stuck, or broken & allowing too much oil to remain on the cylinder wall. As suggested earlier, you could remove #6 spark plug & insert a fiber optic camera probe for a look-see.
Lifters aren't going to cause too much oil in #6 cylinder. Worn valve guides, leaking stem seals or a mucked up piston oil control ring can.