Here goes,

I get a cylinder 6 misfire nearly every time I let the truck idle for an extended period of time. The misfire does not happen right away but does sometime trigger in as little as a minute or two of idling. I can start the truck and drive without idling and never throw a misfire code all day. This can go on for days but if I let it warm up in the morning I come out to a cylinder 6 misfire code.

I have replaced all plugs, wires, distributor cap and rotor. I ohmed out all injectors and they are all within spec. Compression test is within spec aside from #8 being about 15lbs higher from what I believe is buildup on the piston crown from oil ingestions due to a blown plenum a few years back that was repaired.

The "miss" is not discernible when listening to it and the truck idles smooth.

My next plan is to move the cylinder 6 injector to an adjacent cylinder to see if it is hanging up. Aside from that, my next step would to do a leak down to see if I am dealing with a mechanical issue as opposed to fuel or electrical. I do believe a mechanical issue would be more consistent though.

I do have a code reader that can read live data. I guess I am unfamiliar with "what" triggers a misfire code and perhaps can approach it from that regard.

Also, perhaps unrelated......at full throttle shifting from 3rd into overdrive, the truck has a hesitation/miss of sorts similar to hitting the rev limiter when it drops down to around 3500 rpm after it has shifted into overdrive. If I release even just a small amount of pedal pressure after it shifts into overdrive I don't get this hesitation and the transition is smooth. Curious what this might be too. I just noticed it merging the other day on the interstate.

Thanks in advance for the help and advice!

Have a great weekend and Merry Christmas!

Thomas

 

It will actually shift into O/D at WOT? Neat. (never tested that theory.....) The electronics are notorious for doing odd and strange things at WOT throttle though, usually its a 'hung' shift, and it won't go to the next gear till you back off the gas a bit....... That *might* be related. (no real fix for it either)

As for the miss, check out your long, and short term fuel trims at idle. Should be in single digits, hopefully. Might be a lean-burn miss...... Might also wanna inspect the plug, see if it is gapped properly, and doesn't have any serious buildup on it.

 

Some good info here on what triggers the code and possible causes: https://repairpal.com/obd-ii-code-p0306
I'd agree moving the injector around is a good next step.

 

Ironically, exactly one year since I posted this the check engine light popped up again after running MONTHS without any issues. Never changed the injector around because I mistakenly thought the problem fixed itself when I replaced the computer after I had a failed 5V trace.

My hypothesis at this point is a lean injector on number 6 because it happens at idle only and more often on COLD mornings.

I do plan on switching injectors as the next logical step now. I think HeyYou was right on the money a year ago referencing the fuel trims at idle. Will pay closer attention to those.

Will continue to update until resolution is posted.

 

Further diagnostics has indicated that #6 is down on compression. Most other cylinders are 150-170 (#8 is 177).

Leakdown indicates loss through the exhaust valve. Borescope showed HEAVY carbon on #6 and #8 (assuming reason for high compression). #6 appeared to have large chunks missing.

I assume a piece damaged or hung the valve open causing it to burn.

Suspect that my blown plenum (2013ish) caused oil ingestion and carbon buildup that has gotten worst over the years. Perhaps running the 160* thermostat hasn't helped matters either. Wouldn't be surprised to have a few valve seals leaking making matters worst.

Moving forward I plan to pull both cylinder heads to inspect and replacie what I need to. While unlikely, I hope I can salvage the heads to keep cost down. While I am in there I plan to make sure my plenum repair has maintained its integrity and will also address the timing chain.

Will need to research the best method of carbon removal from the piston crowns. Would like to hear any suggestions.

Thanks and will keep y'all posted!

 

There are chemicals for just such an occasion. (de-carboning the pistons.) Local parts store should have it.

I wouldn't spend any money on the original heads. Just replace them. Given your description, they are going to need a fair bit of work in any event, and they are likely cracked. (a common problem on the stock heads.) You can get a set of NEW castings, with thicker decks, for under 1000 bucks from Odessa, or Clearwater cylinder head service. (same company, not sure why they use two different names.....)

 

Called Odessa this morning and sounds like that is the direction I will go.

On a side note, any way to drain the coolant from the heads before removing them without popping a freeze plug?

Lastly, must I remove the rocker arms prior to head removal to take the tension off the push rods?

 

There are drain plugs at the bottom of the block, just above the oil pan rail, but, those are notoriously difficult to get out. Usually, just draining from the radiator will get the coolant level low enough, that you won't spill much when you pull the heads..... be sure to clean out the cylinders before reassembly, and change the oil before starting it.

Yep, pull the rockers before you pull the heads. Keep them in order, with their respective pushrods as well.

 

Thank you for all the help thus far. Really nice to have a knowledgeable resource on these boards to help out people like me who like to tackle projects like this.

Pulled the the passenger side cylinder head today. Exhaust manifolds were a bear. Had to cut the flange bolts....spent too much time wrestling those. Well, 6 exhaust valve didn’t look awful but had evidence of leaking in the valve seat. Extensive pitting in the seat and the valve on that cylinder. Could blow compressed air through the port and feel leaking past the valve. Going to pull the lifter and ensure it’s not boogered up and check the cam making sure there isn’t a burr that’s keeping everything open a hair.

Also noticed some brown discoloration on the cylinder walls on the passenger side. Not sure if this was a result of overheating some point before I owned the truck or what would have caused this. No scaring, still an obvious crosshatch and appears normal otherwise.

Hopefully will know some more tomorrow.

 

Aside from the thicker decks, are these similar castings as the Engine Quest heads a lot of people have spoken highly of in comparison to the OEM Magnum heads? If not, would it be worth the extra money to seek out Engine Quest heads in lieu of the New Odessa heads?

Also, should I spec anything in regards to spring pressure beyond whats factory? Only modifications are Airgap and Cold air intake. Maybe 1.7 RR in the future.

Looking at ordering heads next week. Thanks!