Welcome to the Cummins ECM repairs page. Cummins ECMs are generally very reliable but we still get to fix them every once in a while. We don't have the ECM Dyno ready to run a Cummins on it yet so I have to test each circuit individually until it's ready.
If you have an N14 you can do the easiest injector cut out test in the world. Just walk up engine while it's running and unplug the injector. Easy. ISX uses adaptive fueling to trim out each of its injectors....I'll talk about how that works at another time because that’s going to be a big one.
If you have a pre common rail ISX and you're getting misfires from three cylinders at a time check the fueling and timing solenoids first. The pre common rail ISX fueling system is weird. I have never had anyone argue with me on that. The six injectors are supplied fuel from four fuel supply galleys each fed by a solenoid....but get this...only two of the four solenoids are actually supplying fuel to the cylinders. The other two are only supplying fuel to the timing ports of the injectors. The more fuel the timing solenoids supply the more advanced the injection timing gets. So how could a control system like this control each injector independently? Well…you have to look at the injector camshaft to understand it. The injector lobes are basically inverted...Instead of having a lobe (bump on a shaft) rise up and come back down. The ISX injector camshaft “lobe” is more of a ring with a dip in it. So…..only one injector per bank can be in fueling position at any given time. This is how the ISX controls the start of injection and the injection duration of six injectors with four solenoids.
So yeah... it's weird.
But they do run...They can make almost as much horsepower as a comparable Cat but they can't complete with a Cat on torque. On the other hand the ISX will almost always do better on fuel mileage.
Anyways if your getting three misfires at a time that's a big hint that it might be a solenoid problem or possibly an ECM problem.