Hi guys, my 2012 S Max Sport (2.0 TDCi 120KW 163HP) is having trouble with hard acceleration. When I step on the gas within a second the revs drop by about 500 and if I keep my foot down they're going up and down like a yoyo! This doesn't happen in normal city driving, only if I'm trying to accelerate very quickly, or on a hill or motorway (especially in gears 5 and 6). There're no warning lights and I've checked OBS and there are no error codes. If I'm using cruise control and this happens than it automatically disengages. Any ideas what it could be? Thanks!

If car speed drops too far from that demanded by cruise control, it will drop out?
No fault codes (What is OBS, Forscan is recommended on here)?

Are revs going up and down and assume road speed is consistent with that (i.e. no clutch problems)?
My standard answer, change the fuel filter as easiest/cheapest way to rule that out (check for air leaks  in fuel line and sparkles in the housing at the same time)
Beyond that, I'll defer to others at the moment...

Sorry it was a typo, not OBS but OBD. The cruise control disengages only if it tries to accelerate or maintain speed going uphill, then the revs drop and immediately it switches off, even though the speed is still the same.

Yes, the speed is kind of consistent. When I step on the gas, revs go up, car starts to accelerate and then within a second revs drop and so the acceleration stops, meaning the speed is now slightly higher than before. So if I keep my foot down I can eventually accelerate to the desired speed as the revs keep going up and down, but overall slightly increase each time.

All filters have been changed a few months back, but I haven't checked for any air leaks. Thanks

16 hours ago, AndyV said:

Any ideas what it could be?

The clutch is slipping.

Thanks for the response. However, wouldn't slipping clutch make the revs just go really high when I step on it? I'm struggling to get the revs up as they keep dropping by about 500 every second or so. Also, I generally have no problems in gear 1 and 2. Even uphill starts are fine. Cheers

On 2/12/2025 at 1:19 AM, AndyV said:

I step on the gas within a second the revs drop by about 500 and if I keep my foot down they're going up and down like a yoyo! This doesn't happen in normal city driving, only if I'm trying to accelerate very quickly, or on a hill or motorway (especially in gears 5 and 6).

Still think you have a clutch problem.

As said, if no problems under hard acceleration in the lower gears, clutch does look like it could be the issue.

Torque applied  likely to be higher when higher gears are used?

Any slip will likely get worse pretty quickly and depending on the treatment it has had since new (history, owners?) then, if engine problems are ruled out i.e functions well throughout speed range (high speed demands more power) then I agree clutch is the most likely cause.

Option is either be gentle with it and monitor how it is progressing (may let you down?) or plan to change at a good time (is there ever one?)

Thanks guys, appreciate the advice and will have the clutch checked out. Local Halfords do free clutch inspection.

14 minutes ago, AndyV said:

 Local Halfords do free clutch inspection.

Expect them to recommend a new clutch... 😂

You can always get the "inspection", and show them the symptoms then get them to explain any reasoning and say you'll think about it or get other estimates and decide to wait if it's not too bad or consider getting it done at a time that suits you.
You haven't given any history, number of previous owners and if you are gentle with it or have regularly been driving it hard?