why is my acceleration great from stop to go, but poor when going from ex. 40 to 60mph?

Question

my car is mercedes 350sdl (turbo diesel).

this problem is really odd. I have great pickup from a red light to Go, but it is very poor from a moderate speed , say 40mph, to a high speed 60mph or above.

what is REALLY odd about it is that if I use the cruise control system to speed myself up (instead of pushing the gas pedal), the pickup seems good.

For example I was on the highway last night doing about 55mph, I wanted to accelerate to around 75 so I push down - nothing happens ... I floor it - nothing happens really, it does eventually speed up but it is verrry slow and there was no pickup at all.

It seems like the engine is not getting enough power?? But why would it be good at low speeds...

my mechanic is going to test drive it tomorrow so hopefully he will figure it out.

when I go to have "maintenance" done at a service station , what do I ask for if I want the engine/acceleration completely checked / gaskets make sure there are no cracks etc.
Thanks in advance.

Answer 1

It sounds like the transmission is not kicking down a gear, so it takes longer to accelerate when it is stuck in high gear. This is supposed to happen when you press down on the gas pedal, maybe the linkage needs adjustment so the tranny knows when the pedal is down and dropping down a gear is needed.