My 01 4.4 does the same thing. I notice it mostly when getting on to a highway. Partial but intentional throttle application. Sometimes it isn't there at all and is when I realize how much I love that engine! I have changed fuel filter, live streamed data looking for something, monitored fuel trims. I can't nail it down. Was thinking of sending injectors out to be cleaned. My motor is fresh with all new vanos, guides, CCV, gaskets. I have smoked it and nothing. This seems to be showing up on posts more and more. Wish there was a way to find a common denominator for all of us, other than these cars are starting to get up there in years.

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Dave S.
Northeast
2003 4.6is
2005 E53 4.4
2003 E39 Msport(a REAL ONE!)
1959 Fiat 600