Help needed from the cat Gurus

Seems simple - no problem before cam replacement, problem after cam replacement. Your gut also says it's something to do with the cam, so bite the bullet and replace it again. First rule of troubleshooting - identify the last thing that was touched before the problem started. No matter how unlikely you think it is, that is probably the culprit.

 

The camshaft could very easily be the culprit. I bought a used ECM from somebody and wanted to test it before I resold it. So I installed it on my own truck. Upon starting the truck it had a wild lope to it. I was surprised by this, but let the engine warm up to take it for a test drive to see that everything functioned in the ECM properly. Once I went for the test drive I was very shocked by what happened while trying to accelerate. The engine actually back fired so hard through the exhaust that it sounded like a cannon going off in the truck. The engine actually hesitated and started to slow down as I started to depress the throttle before it back fired. Th problem was the camshaft in my engine didn't match the ECM injection timing. I installed a MBN 525 HP ECM with the stock file in it. The injector lobe on the camshaft was timed differently from the programming and caused the engine to hesitate and back fire. As soon as it back fired I stopped my test drive, reflashed the ECM with a 6NZ file. The lope at idle stopped and the engine accelerated properly when I depressed the throttle.

The cam could be the problem in your truck, but I would expect it to occur all the time. But the timing tables in the programming could be different depending on engine load, air and coolant temps causing the problem to come and go depending on other factors the ECM reacts to.

The other thing is could someone have possibly left a rag covering the aftercooler inlet? The rag would now be floating in the inlet side of the cooler causing boost restrictions as it moved around inside the cooler. This would restrict air flow to the engine, causing an over fueling issue. The problem with this idea is the ECM should see the restriction as a low boost condition at the intake manifold and not give the engine fuel

 

Have you check your camshaft lash? I ask because I have had close to same problem with 2WS at Cat at years ago. Replaced short block and assembled engine with other tech on different shift. Truck did as your is doing under load only, white smoke stumble intermitantly. After a set of injectors and sensors and stuff. I checked the timing calibration again ( was done at at first start up and passed) and it failed. Found the adjustable camshaft idler had moved due the bolts were not torqued on stub shaft plate. Reset lash and problem was repaired. It was not getting a cam to crank calibration fault, but we had never left the parking lot, just loaded engine around yard.

 

Correct the ecm should use default timing and even if the timing calibration wasn't done to the ecm it shouldn't run like that, it would just throw a code.