N14 Celect Plus Misfire and Gray Smoke

Almost sounds electrical, you said you replaced the injector harness, but did you also replace the pass-throughs in the rocker boxes? Best to check resistance but that means pulling the Jake's to isolate the solenoid from the internal harness.

Lastly, have you ever replaced injectors? They might just be worn out and not delivering enough fuel without the higher rail pressure that a few hundred rpms due to internal leaks.

 

That should not be the case, the ECM very carefully monitors TPS input, any misgivings will put in limp mode. The KW mentioned in this thread currently has this issue, the PO cut and used butt end splices and going over a heavy bump will immediately put the truck in limp mode. It may be possible that the sensor is loose and wiggling is just increasing commanded throttle, but a bad TPS will most likely set a fault. The N14s were notorious for injector issues, they are pricey at 300ish a piece, but a random misfire sounds more like injectors to me.

 

How many times have you replaced your fuel filter? We went through 5 fuel filters and 2 fill ups with dropping the last 10 or so gallons out of the saddles before it cleaned up for us. Still running a biocide and have 2 fuel filters in the box just in case. Have you checked your check valve as well? Those are known to get clogged with algae as they are pre fuel filter, clean it up and put it back in. Measuring your fuel suction will tell you if you have a restriction somewhere else.
The fact that it disappears on cruise has me a bit dazzled, I'm not too familiar with the N14 calibration but it has me wondering if its possible that there's a separate fueling table for cruise control and someone messed with the standard calibration. On the gassers you have a lean cruise table that's completely independent of the normal operation table that is only used when cruise is enabled. But like I said, I've got no experience with diesel ECM tuning. If you're friendly with your local cummins dealer parts person they might be willing to check your calibration ID without charging you, never hurts to ask. But assuming all the new parts were not faulty out of the box, and the overhead is within spec that's the last logical thing to check.

 

Same issues, the truck still sounds like it misses at idle on a cylinder but it makes great power and out pulls most trucks on some mountain passes according to the driver, hes got a pretty light foot too, and will religiously pull off at 900EGT.

We did everything driveway mechanic style because we dont have insite... yet. For us we ended up removing the check valve and running an extremely heavy initial dose of a biocide in both tanks and let the truck high idle for a few hours a day, and swapped fuel filters every other day till we stopped seeing jelly in the filters. Then we started doing 50 mile trips bobtail and checking the filters after every trip for about a week. Once we ran fuel level down far enough we pulled the saddle drains and drained the remaining fuel and washed the sides of the tank with alcohol biocide mix and let them air out for a day. Same day we had the saddles airing out we reran the overhead (no difference), inspected the camshaft with a camera. I'm still not 100% sure that the truck is in full working order, but it's been OTR continuously for a couple months now with no issues (knock on wood). The combination of the bad overhead, alage, and injectors had me going between electrical and mechanical issues, but the misfire was constant regardless of cruise or no. Basically, the algae was clogging the filter so it would run great until it drained the fuel in the filter and then it would only idle. The injectors fixed the smoking issue, and the push tubes fixed the miss at idle, although the miss at speed was most likely fuel starvation.


We checked for pin holes by pressurizing the saddles through the vent tubes to 40PSI and left if over night (unvented caps), came back the next morning and there weren't any puddles and it was at 33PSI so we assumed no holes. It being winter I'd doubt you'd see those kinds of results so I'd honestly just look for a puddle.
Then we checked fuel suction and pump pressure, both should be stable at idle, if you're sucking air you'll most likely see some fluctuation in the needle pressure side, you'd want to see about 115-130ish PSI pressure side at idle.

Our largest concern was cleaning the pick up tube screens but thankfully some engineer at KW but the saddle drain bolt directly below the pickup tubes so we were able to get them cleaned out easily, I'd imagine a pin hole in a pickup would be masked with a full tank though.