Hard misfire at idle on 06 5.9

[ChrisAZ]

So I've been chasing my tail for some time now trying to figure out a misfire at speed issue with my 06, and every time I replace or diag the potential problem, it changes or goes away briefly. But now the pesky misfire is back with a vengeance, this time at idle, and it's finally giving me some clues as to the problem. Here's what it's doing...

Hard misfire while idling (in gear and in park), maybe 1 misfire every 2 to 5 seconds, sometimes multiple in a row, then a delay with normal idle before the next misfire.

Rail pressure, when viewed on my scanner, drops each time it misfires, usually down to around 2k to 5k.

No excessive smoke (it hazes lightly with 50HP Bosch injectors from Exergy, injectors and connector tubes are new with only 4K miles on them)

No codes, no WTS light.

When rev'd up in park, rail pressure goes up accordingly, misfire goes away above about 1K to 1.4K RPM. Comes back as soon as I let it idle.

I can regularly hit rail pressures of ~23K at WOT on the street, and it doesn't feel like it's lacking power by any means.

Prior to this misfire at idle, it would misfire while cruising down the interstate at around 65 MPH and above. Would typically be OK in town, and not do it during city driving speeds.

I have a modified FCA and 30K fuel rail pressure relief valve from T&C diesel, modded gearotor pump & high flow fitting from Industrial Injection, and I just yesterday replaced the rail pressure sensor (it is the proper sensor for an 06, and it wasn't doing this except once or twice prior to me replacing said sensor.)

What could it be? FPRV? FCA? CP3 dying? Low supply pressure? (Airdog is also new with only about 6k miles, and pressures were set when it was new, 16PSI on a hot day).

 

[ChrisAZ]

Would like to add a new development...

Tried running the truck again and watched some more parameters with my scanner.

When I rev the truck, I can sometimes see voltage spikes of 15.1 to 15.2 volts, not sure how normal that is. The cabin lights brighten with the revs/voltage increase, I assume that goes hand in hand, but again, not sure if those spikes are normal. Normal idle voltage typically runs between 14.4 and 14.6.

While it was idling and misfiring hard, I went under the hood to feel the injector lines to see if one line felt like it was "knocking" harder than any of the others. When I reached to the rearward lines and moved the injector harness slightly out of the way, it suddenly stopped misfiring. It was also getting up to temperature at this time, about 180* to 185*, so this could be a coincidence, but it was a little odd that it stopped when I moved the harness. I did move it back to see if it would start misfiring again, and it didn't.

I unplugged the FCA while it was running very briefly, and RP spiked to 26,000 and the engine got very knocky, and everything I've read says that's exactly what it should do.

 

[ranger 393]

If it stopped misfiring when you moved the harness, have you check the harness to see if a chafed wire was rubbing something/ maybe a short?

 

[sisupower]

Sounds like electrical issue in harness. I would start there.

 

[Tuesdak]

You moved the harness: This is known as a wiggle test. Bad connections, shorts, bad wires, etc. show up with this test. You just got lucky and you don't exactly know it. You see when chasing electrical that's intermittent the technicians sometimes have to "wiggle" the harness to simulate vibration and potholes in the road. This wiggle makes or breaks a bad connection and/or opens a short. So you found the problem of a bad connection and the general area of the harness it's in. Even if you hire out electronics this information is pure gold to the people who will work on it. DIY you are looking for a rubbed through wire, oil in connectors, or corroded bad connection of some sort. What did you move exactly when the problem "cured" and what connectors are at the end of it? Pic?