As far as I know, the misfire check/cylinder contribution test only runs and only means anything with respect to idle condition, but I could be wrong on that. The engine runs perfectly at idle and at 35psi on the boost gauge and everywhere in between so that is my struggle with understanding what the PCM is seeing. I feel like it idles smoother than my old '02 Corvette Z06. When I get on the go pedal I can surprise some quick cars on the interstate and not smoke out the population too.

 

Misfire codes are generated when the crankshaft speed drops more than expected during a certain period of time during engine operation. If there's a weak combustion event, or no combustion event, the crankshaft speed is read by the sensor in the space of a few milliseconds and logs it as a misfire. Whatever is happeing inside the engine during operation is creating that drop in crankshaft speed to make the ECU think there is a misfire. Only other thing that I can think of beyond what I just described is the very remote possibility that the camshaft gear is bent somehow and the space between the holes in the cam gear and the sensor are not equal and there's a wobble while engine is underway. Again. Misfre codes are generated by measuring the speed of rotation of the engine and if there's a drop in speed due to a misfire, the computer can read that and then it knows that one of the cylinders isn't contributing to the work load and logs the misfire.

 

In general I understand well the concept of misfire detection and I could still be wrong on this so if you have some documentation about 7.3 misfire detection I would like to read more. But again as far as I know 7.3 CCT and misfire detection only runs at idle. Every time the code sets its after I've been sitting idling for a few mins at operating temp.

 

I don't know much more beyond what I said. When I was still working the repair shop, I would take classes to get updated on my education for this trade and one of the subjects was misfire detection. It's really the best and only way for the computer to know that one or more cylinders are not contributing as much as the rest of the engine and that was to measure crankshaft speed. The processing power these days are able to keep up with single degrees of rotation of the crankshaft and whether it's spinning as it should. If you were to graph the speed it would be like a sine wave and a misfire will make one of the waves larger than the rest. If you have a dead miss, the code is stored in memory until cleared. The check engine light will illuminate. If you do two drive cycles in a row and no more misfires are detected, the check engine light is turned off, but the code will still be stored until it's cleared out via the scan tool.
Whatever the cause of the misfire could be any number of things. Low compression, marginal injectors are at the top of my list due to the simplicity of the combustion environment and the operating principles of how a diesel engine achieves ignition and generates power. Could be loose valve guides making the valves not seat perfectly every time and a skip happens because compression bleeds off and then the combustion event fails to happen.

 

I took PERDELs this morning once I got to the office. When i stopped at the light after my interstate ride, i saw some low 4's on #8 and once I got to the office it was down in the high 2's and then back into the 3s after a couple of minutes. The DTC did not trigger. I also took a video of my cold start and idling just for reference but my cell signal at my desk is not good enough to upload it so I will do it later. Not sure if it'll get the message across but might be clear enough by the sound that it's running well.

 

https://www.ford-trucks.com/forums/1...l#post14673387

Some info about how the misfire detection works at idle only in the post linked above. I would also like to know more about the 5% threshold mentioned in post 4 here in my thread. I will keep watching the values and see if they ever trend over 5% and see if I can correlate with the DTC coming back.

 

I guess the 5% threshold to set the P0308 is something from the rumor mill. I had to run an errand so I left work and did that and drove home to work the rest of the day from home. I watched the PERDELs at pretty much every stop along the way and mostly saw low 4's on #8 but some 3's. Once I got home I let it idle in the driveway for a few minutes. I never saw anything on the gauge but 4s and the misfire DTC came back out.

Would love to be able to scheme a way to make this thing just work like a non cali spec emissions truck and NOT run this misfire check unless specifically commanded...

 

I'm not sure the exact threshold that triggers the DTC for these. I get 5% occasionally on #8 and do not set a code. Guessing the algorithm is a little more complicated. Like you said, it's running well, so no need to deal with it, other than it's annoying.

Was thinking about idle quality and what affects it - that might contribute to a false balance code. The ICP sensor and the IPR valve can affect idle quality.

You could try running for several days with the ICP sensor unplugged, and see if the imbalance code returns. Also consider cleaning/resealing the IPR if it has not been done in several years.

 

Thanks for sharing some ideas! I will think about that. I have never touched anything related to IPR* in all my years (7) and miles (73k) of ownership so maybe there is something I can improve. My main hangup on this topic is I have thought about maybe letting the Ex go to get into something different. But I don't want to tell someone "yeah it runs great but the CEL is gonna pester you forever". Hoping I can just make it go away.

*Although I did install a new Motorcraft ICP sensor a year or so ago when the thing broke down internally and oil was coming out of the connector seal and wiring...

 

I'll bet you are fine on the ICP sensor then. Agree that it would suck to have to sell it with this SES coming on, even though it's really a ghost code on these older rigs.

Wondering if the characteristics of single shot 160/30's affect this balance reading as well. I'm told stock injectors tend to run quieter/smoother due to their pilot shot. Could be that singles need a bit more margin in the balance algorithm before the flag goes up. Going to experience this firsthand shortly when my 160/30's are installed.

 

No real development on this but I wanted to share something I've been keeping an eye on for several months. Bitterroot diesel recommends OptiLube XL which I used to use somewhat consistently but fell off the wagon. Since this code started popping up I have been trying to be consistent in treating it via the standard treatment on the label. Every time I forget to use it and drive it a couple days, the code comes back. When I fill the tank and treat it with XL, the code goes away. It's like clockwork. I can't really explain it, despite my better-than-most understanding of what is actually going on inside the injector when it fires... Lucky for me XL is pretty cheap when you buy it by the gallon.

 

Literally the next day it made a liar out of me. Since then, it has come and go twice on its own... Oh well... I pulled a 7k trailer from Charlottesville to my home in the CSRA this weekend and it performed flawlessly even on long steep Virginia hills. I was very pleased!