How many miles (km) on the car?  Did the symptoms begin suddenly, or was there any changes to the car prior to the symptoms?

If the dealer is unable to find the problem, that makes it very difficult for us to make a diagnosis remotely....   

Has the throttle body been tested?  A defective T-body will usually throw a code, but it might be worth a look.
Bad tank of gas?  On rare occasions, we have seen a car develop issues because of bad fuel.

Might be worth connecting a boost / vacuum gauge to check the pressures when the car stumbles. (even if the individual components tested okay).

Let us know what you find out.

Hello everybody , 

I have a MINI COOPER S R53 , pre - facelift (04/2004) with full JCW kit fitted in 2008 ( 210 HP) .

   Long story short : the car has low power and stalling on hard acceleration / for ex : in 6th gear the car is not able to pass over 4000 rpm . It feels like it has a power of a simple Cooper not even a Cooper S , like something is clogged ...

The start and the idle is perfect ! No error codes / no Check engine / no MIL etc onboard.

 

Until now we have checked / replaced everything on this car :

• new coil/ spark plugs 
• Bypass valve ( working perfect )
• Vacuum lines in good  order
• Fuel filter replaced / fuel pump and fuel manifold pressure tested and found k
• Compression test performed and found aprox same values on all 4 cylinders : 10 bars
• Catalyzer has been removed ( suspected to be clogged) and car has been tested without it - same bad running ...
• Supercharger overhauled ( replace oil and visual check ) - supercharged pressure is more than perfect 
• All sensors ( MAP, TMAP , ECT , Lambda etc) shown good working / good values on tester 
• The only malfunction discovered was on MAF readings ( as u know there is no MAF sensor on this car - the values are calculated according to the above sensors readings) :the values are much lower than normal. Therefore we even replace the ECU with an updated one ...without any improvement : same bad running as before ...

Presently we are out of ideas / the car is left since one week ago at BMW Dealer ...they are out of ideas too...

 

We are open to any ideas / advice ...