Just food for thought: When I bought my 260E I had a massive lack of power at anything below 3000rpm. Soon as it hit ~3k it would accelerate VERY hard. I did a complete tuneup and problem still persisted. I checked fuel pressures and everything was to spec. Your description sounds almost exactly what I experienced except my 2.6 never 'sputtered'. It just pretty much just stopped accelerating entirely for about 3-4 seconds then slowly spin faster and faster until it would open up fine at above 3k.

I swapped out the EHA for the hell of it and suddenly it was a completely different animal. No more bottom end power issues and it passed smog too. Fuel pressures were still the same as well showing it was not a simple condition of the new EHA providing a richer/leaner condition.

The EHA can influence some very strange running conditions that don't make sense according to K-Jet diagnosis and spec. Remember these engines are old and may not always act as suspected.

I would suggest going to a local junkyard and grabbing 2-3 of them so you can rule them off. Seeing as it is only 2 bolts, it is easy to swap.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
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2003 Gray Mercedes ML350 - 122k