When attempting to accelerate, passing gear engages and turbo is working. RPMs go up, but the car lacks power with almost no acceleration.

Filters have been cleaned and diesel purged but problem still persists. So, it is very dangerous to try to pass another vehicle while driving this car.

Does anyone have any ideas as to what I can do now to try and remedy this problem?

Properly check your transmission fluid level. Engine increases output with little apparent effect could be too much slippage in transmission.

More importantly, what RPM's are you going up to? Fuel taper begins at 4000 RPM and has fuel cut at 4500RPM on these cars. When adjusted properly, unless you're at speeds <40mph, these cars should almost never drop a gear to accelerate.

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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!)