Odd Fluctuating Boost - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 1 (permalink) Old 03-20-2019, 01:37 PM Thread Starter
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Odd Fluctuating Boost

This is a new one on me, and I've never seen it posted on the boards, so I'm hoping someone might have some insight. I was driving down the interstate at 80MPH using cruise control. I needed to slow a bit, so I used the brake pedal to disengage the speed control. Got back on the throttle slowly like normal (I find the tranny engages the direct drive better if I gradually roll on the throttle after having let off rather than just hammering it down), and I'm still doing 80 no problem with plenty of power, but now I can hear what sounds like revving the engine followed by letting off the throttle, repeatedly. The truck is not bucking, not accelerating or decelerating, just "revvvv, drop, revvv, drop", over and over again. I do a quick scan of my gauges and the boost level is going between 5 and 10 PSI. I pull out the OBDII adapter and start checking things out on TorquePro. Nothing is amiss. Nothing! ICP and IPR are barely moving (flat, straight road and no wind that day), EBP was steady at 20.5 PSI (don't ask why I remember that lol) and even the VGT was steady with what the computer was commanding it to do. The only thing that was going on was a fluctuation in pressure.

I pulled off at the next exit and did a restart. Hey, sometimes turning it on and off fixes a lot! lol Nope, didn't work. Pulled over again and checked boots, CAC tubes, MAP sensor and hose, you name it. Everything was fine. So, I thought about anything I had done recently, and the only thing was I swapped FICM tunes. So, put it back to the Atlas 40. Nope, still doing it. Swapped out stock, economy, 80, still doing it, so I put it back to the 100. Oh, BTW, it was doing this fluctuating boost level at all engine speeds, but was just more pronounced at higher engine speeds since more exhaust flow usually equals more boost. Anyway, nothing worked. So, I decided I would try disconnecting and then reconnecting things. ICP didn't do it, but the VGT solenoid did. I disconnected the solenoid while the truck was running and it sounded like I had pulled the turbo off completely. Was truly an odd sound! lol Anyway, just resetting that connection between the CPU and the solenoid took care of the issue. Haven't had a problem in a week. Really odd.

I know sometimes it's literally just a bad/dirty connection, but the fact nothing else was fluctuating even the slightest makes no sense. I would have assumed once the CPU saw 10PSI it would have commanded the VGT to drop the pressure, or raise it when it dropped to 5, but everything stayed constant. So odd. Thoughts?

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