Check your ALDA. If it is good, it should be able to hold vacuum. If it doesn't, you probably need a reseal. My SDL was behaving this way before I tuned the IP and resealed the ALDA. Post-tuneup the difference is incredible. I wound up making the IP leaner, but with the now-functional ALDA the power pickup is very smooth and the power delivery is more or less "effortless" when driving around in town.

Prior to the tuning and ALDA fixing, it was DOG until the turbo spooled up, then had decent power. The problem came in city traffic where it was in a high gear and the turbo just wasn't spooling. GUTLESS.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)