I have this problem in my r53 JCW (200hp) 2004 model:
With 3rd and 4th gear and maximum acceleration, full throttle have power loss between 3000 and 4000 rpm when accelerating from 2000rpm like 2 or 3 small cut offs (stuttering)
The problem start after the inspection 2 service at 82000 km
they put wrong poly belt and wrong plugs BKR6EQUP NGKs fitted rather than the specified BKR7EQUP and start stuttering
I go the next day and they put the correct plugs and belt but the problem don't go.
The dealer check the fuel injectors and they are OK
They change the fuell filter and reprogramming the ecu but nothing
It only happens when accelerating past s out 3000/3500 rpm and feels like the power cuts for s moment.

if anyone can help me I would appreciate

 

Have you changed your spark plug wires yet? How about the coil terminals? Check for corrosion.

 

+1 on checking the coil for corrosion. That is a very common problem. If coil post is corroded clean off with brush and/or emery cloth. Put a little dielectric grease on the post and you should be good. You can get small packets of dielectric grease at most local auto parts stores.

Did they also upgrade your ECU software during the intial Inspection 2 Service? There have been yo-yo type problems reported with various versions of software. Some versions seem to be better than others. You can search for yo-yo in the forum. Usually related to the Bypass Valve (BPV) which should not have started having problems at the same time as the service.

Scary dealership work. Not sure I would trust or believe anything they said they did.

 

Dealership said that they check and change the coil the spark plug wires from another jcw (used not new parts) and check the bypass valve, and that they didnt upgrade the ECU
I check the bypass valve seems work OK. I will check the coil for corrosion.
thanks

 

I check the coil there is no corrosion.
I reset the ECU but nothing.

 

Clean your front wheel speed sensors.

Jeremy

 

I will try it, but no light is on.
The speed sensors is with the wires for brake pads? because they change them at the service.

Thanks
Apostolis

 

I check again the coil for corrosion I clean them and put dielectric spray
The car is better
No cut offs in 3d and some times 1 cut off in 4th speed

 

The dealer used USED WIRES AND COIL?
The wires are easy to check with a ohm meter...the coil not so simple....but was a way to isolate the issue a bit...
Parts are the same for a s or jcw ....
Blowoff valve can be ziptied in closed postion to see if it changes anything before you spend the $$ to replace it..pretty cheap part, but if it is fine, why spend the $$.
sounds like it is not a very good dealership...most likly not a mini dealer from the sound of it...
Some of this troubleshooting is pretty basic for any trained mechanic...an ohm meter to check plug wires is even basic home mechanic stuff....

 

If cleaning the coil contacts helped it might be worth checking the wire contacts very carefully. It is hard to see the contacts in the wire boot but try with a flashlight and maybe even a magnifying glass. The other problem is that it is hard to clean the contacts within the spark plug wire boots.

I hate to just throw money at a problem without really knowing the problem but I would tend to lean towards getting one of Way's coil, plug wires, spark plug kits:
http://www.waymotorworks.com/wmw-ignition-package.html

 

I check the coil and the sparkplugs again I clean them with metal brush and put dielectric spray again.
The problem was the corrosion . It has to be brushed to be clean the coil from corrosion

THE PROBLEM SOLVED

Thank you all

 

Glad you resolved the problem.
It is too bad that the dealer could not solve this common and simple problem.

 

Thank you for your help