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Message: [QUOTE="CurtisF, post: 1273090, member: 829"] To explain what happens a bit more.... The issue occurs only during regen, and only at very low RPM. A slight change in RPM, or a slight change in engine load and the problem is not there. If it were in fact improper valve guide clearance, this issue would be more prominent other than these very specific scenarios. It would be duplicated at somewhat varying RPM's, and at many different engine loads. It would also present itself outside of a regen cycle. Furthermore, if they were sticking that bad and making contact with the pistons, there would be marks, damage, etc. But that's not happening. Tolerances are easy to measure. Ford has not found this to be the issue or the cause. They have done repairs simply to try and appease customers because they had a hard time locating the specific issue. It's hard to duplicate and you have to be in the right place at the right time for it to happen. If you have customers screaming and you don't know how to fix it, throw parts at it and hope for the best, keep testing until you find the solution. So let's dive further into what happens, because there are a few interesting quirks that many folks here might not be aware of.... Ok, so we've established that when you vary engine load and RPM, the problem disappears. Also interesting to note is that this only occurs early in the regen cycle, and not happening later in the regen. I've tried over and over again on my truck, I can't duplicate it more than halfway through a regen. Same is being reported across various forums and by many owners. This is also why dealers have difficulty duplicating when they do a forced regen, especially when the exhaust filter is well under being full. The problem is far more prominent in the 15 and up model year. There were significant changes. It has been discovered by a few tuners that they can simply make a few tweaks to the calibration, and the problem disappears. So when you look at that, what other significant factor could cause the issue? The theory I heard is.... pressure. Pressure changes with RPM and engine load. Pressure changes as you get further into the regen cycle, even at the same load and RPM that cause the problem earlier in the cycle. Pressure is different on the 15+ model years, making the issue easier to duplicate and far more prominent. Tweak the calibration a bit, and you change pressure and the problem goes away. With that said, how do you guys think pressure plays a role here? [/QUOTE]