Acceleration Mystery

[Opa's 240D]

Owner of 1981 240D (112K Miles). Beautiful car and it has sentimental value, i.e. "Opa's 240D". It's slow as a turtle but I've never minded that. First learned how to drive on an '82 240D and just thought it came with the territory. I've held that belief until today.

Here's what I've obsevered. (1) Car does not accelerate past 70 MPH from pressing the acceleration pedal (when pushed so hard I almost get a cramp in my calf). (2) Here's the zinger. . .Played around with my cruise control on my way to work and just for kicks I pushed up on the lever to accelerate (once the maximum speed had been achieved by pushing on accelerator pedal). LOW AND BEHOLD SHE GOT UP AND WENT AND ACCELERATED PAST 70MPH!!!

It may sound ridiculous to those of your with fast MB's but going 80 MPH felt like she was about to lift off the ground!

The short of is is this: Why does the acceleration pedal (when pressed to the ground) achieve only a speed of 70MPH and the cruise control achieve higher speeds?

Second quesion: When I release the acceleration lever on the cruise control (acceleration pedal still engaged all the way) why does the speed decrease back to top speed achieved with pedal fully engaged?

Any and all help appreciated.