Misfire has me stumped.

Keith Browning · May 24, 2014

Its a 2000 -ish 7.3L that idles rough and smoothes out when RPMs rise. There seems to be an excessive amount of black smoke on acceleration - I expect some but this truck is a little heavy, not billowing though. It originally had a P1211 for ICP higher or lower than expected however I could find no fault with the IPR or the ICP as of yet. No injector electrical codes, Fuel pressure remains in the 60's (measured at the filter housing) cylinder contribution test flags cylinder 8, #8 perf_del shows ~7.75%. I performed a manual compression test on bank 2 and all cylinders were dead even @ 475 PSI. I replaced injector 8 and still have the misfire. I substituted a known good IDM for giggles, no change.

Need to brainstorm for ideas here.

I am thinking perhaps there is something going on in either the fuel rail or the oil rail that is hydraulically affecting number 8. We know that that cylinder is a little sensitive to fuel pressure variations and that is the basis for this concept. I did not test fuel pressure at the head however - would probably be wise to do so.

8WA Sman · May 24, 2014

I take the cylinder 8 contribution and delta readings are not because gray cam sensor? I also assume you checked for a chip/tuner on the P1211? Some techs use an injector kill box to find low volume misfires on 7.3's,not that I own a kill box.

Keith Browning · May 24, 2014

the 7+% delta is well beyond what I normally see

Bruce Amacker · May 24, 2014

Right,

the 7+% delta is well beyond what I normally see

Right, a good #8 is usually in the 1-3.5% range.

Is #8 dead at an idle? Use an IR to confirm. Is the air bled out after the inj change? Did you drive it?

BS ideas:

Swap a CMP

Did you clear adaptive strategy after swapping the inj?

Check FP at the port under the turbo and run a fuel sample from it while running looking for air (#6 washer leaking into the rail?)

Watch the inj dump ports idling and see if #8 looks different and unplug #8 idling

Swap #6 and #8 injectors to make sure you didn't get a bad #8 inj. (I know they're different)

Check for oil aeration by taking a sample

Check LP oil pressure

Obviously the buzz test was good, and you probably inspected the harness connectors and made sure they were tight

Fuel drain sample- water in rail running to the rear cyl

Crap in the fuel screen feeding the left head

Check power and ground v drops at PCM/IDM

Is MFDES really high like above 16?- starving for fuel

Run the CCT test in drive if it's an auto trans

Case ground PID should be zero

IPR 9-11% hot idle?

:grin:

blown99 · May 25, 2014

I see #8 showing in the 8-9% with a new set of injectors with the grey sensor on a regular basis. I also always swap out to the old black sensor or blue for inj. diag. Try a running compression test . I have seen issues with worn valve guides causing similar complaints. The base cranking compression would be ok and indicate no issues. The running compression test would pinpoint the cylinder with the issue.

ktmlew · May 25, 2014

Base engine issues are becoming a more common concern as engines, both gas & diesel, last several 100k miles.

batmantech · May 25, 2014

I had one similar it was a faulty "new" injector. Like Bruce said swap it with #6. I guess if they sit on the shelf too long it causes problems

lmorris · May 26, 2014

This might help. One of our other techs had a 2002 7.3L a few years back. Had a slight misfire with rough idle, kept flagging #8. He changed out the injector and it didn't fix it, thinking he had a bad injector he changed it again, same thing. He ended up trading it in for a 6L. The Used car department gave it to us to fix, after swapping in a black CMP and using the injector cut-out box we found it was another injector that was the culprit.

Brad Clayton · May 26, 2014

Had one a couple of years ago with a rough idle that smoothed out while driving.......ended up being an overfilled crankcase.

Keith Browning · May 29, 2014

I tried an old black CMP and lost 3% on the PER_DEL_8 and it's a wee bit smoother. Cant leave that sensor in obviously so I put in another new one I had laying around and back up to almost 8%. MFDES is around 0.06 gr. IPR Hot Idle is 11.3-11.5% which I sorta expect with a rough idle. No oil aeration and it does smooth out at higher RPMs if I didn't mention that. Otherwise this thing runs well.

I am now off to look at the fuel rail items: pressure, inlet screen and possible air though I would think that if I had combustion getting past injector6 I would be seeing issues of some kind on that cylinder. I am more and more thinking that there is something going on with the base engine. Nothing here points to anything "system wide" or anything related to bank 2 only... just cylinder 8.

RyanG · December 6, 2017

Keith, I know this is an old post but I was curious what you ended up finding. I have one right now with same cylinder. Came in missing on 5 and 8. Checked everything as listed above, still showing perdel pid at 6.75, compression at 450 psi for both cylinders. Replaced both injectors and miss went away on 5 but still has little bit of a shake at idle. Off idle runs great.

Keith Browning · December 9, 2017

That was a while ago but I think we were headed toward an injector or a set for some reason but what I remember is that the customer declined to repair so I don't have a conclusion. Sorry.

RyanG · December 9, 2017

No worries, right now I'm selling the remaining injectors due to hard cold start so will se if maybe another injector is causing the issue on that bank. Should of sold a set originally but at the time it was still staying in the 50's at night and I was unaware of the hard cold start.