Recently purchased a manual 2004 Cooper S from Chicago and brought it up to Minnesota. While sorting out some issues I finally got it on the road however I noticed around 2800 rpm when the car is warm I will get the slight hesitation, bucking, yo-yo. I replaces the spark plugs and wires along with a new bypass valve and serpentine belt that didn't solve the problem. I did zip tie the stock BPV and I could feel the always on boost but as soon as the car warned up the car began having the same issues. At WOT I can't feel it at all but right at 2800 rpm the engine falls flat or feels like it hits a wall. I have no codes or any other issues associated with the engine (aside from a supercharger that is slowly going to poo).

Does anyone have any ideas? Could it possibly be a vac leak?

 

These cars have a flat spot between 2750-3200 RPM and it is different per car, mods, altitude, climate, etc. It's something in the factory tuning, if you have verified there isn't any mechanical faults I would brush it off for the time being.

 

I can feel the flat spot bit the car almost feels like it's misfiring as soon as the car warms up bit not throwing any codes. I can feel it very slightly in first gear and itbecomes more prominent in higher gears at roughly the same rpm.

 

I'm still dealing with the same problem. After the DT BPV, the flutter went away, but I don't like the feeling. The car still doesn't feel right when accelerating. It feels like it's not getting enough fuel...it struggles and then gets going.

I have no vacuum leaks. Do you have a boost gauge?

By any chance, do you have longer cranks every morning?

I'm wondering, because I have that problem, and the car also kinda stumbles at start-up, but once it warms up it idles fine.

I created my own fuel pressure gauge and adapted it to the end of the rail, and noticed that my fuel pressure seems to disappear next morning. Thus, I have longer crank times. I replaced my fuel filter, fuel pump and the fuel valve next to the supercharger. I still haven't replaced the fuel pressure regulator.

If you want to do the fuel pressure gauge, I can tell you how to.

 

Interesting. I don't have a boost gauge and I still want to check for vac leaks.but I may install a boost gauge soon or hopefully my one scanner comes in and I can check with that. The crans used to be longer and used to stumble on start until I ran some tachron through the car and switchd to better gas.

Next I'm checking for vac leaks, then boost, and somewhere in there I'll check fuel pressure. Have any pictures of your gauge?

I wouldn't say I have longer starts but that are a hair longer than what I was used to on my X5.

 

The stumble in the 2700-3300 rpm range is built in the ecu as there is a fuel dip there. When you accelerate through that rpm range you hit the dip and see the stumble. We see people buy bypass valves and many other parts trying to fix it but none of that will ever take care of it. The best thing if you have no lights and found all mechanicals in good condition is have a basic ECU tune done and that will smooth it out.
https://www.waymotorworks.com/wmw-ec...-cooper-s.html

 

Thanks for that information! I do find it odd that this stumble, misfire is how the car is supposed to operate judging by the lack of questions surrounding it in the forums. I suppose I'll keep driving it until something else surfaces.

Does anyone know if there is a way to check what firmware is running on my ECU and if the dealer can update it to the latest?

 

I thought I'd bump this thread w/a suggestion. I never noticed this dead spot w/my original exhaust, I replaced it recently (along w/other mods) w/an ebay special w/60 mm throughout and the dead zone was very noticeable. I ditched for a much better fit ebay special w/reduced runners before the mufflers and the dead spot is not as noticeable, almost like original. It's possible that the newer quieter exhaust is allowing me to go deeper into the pedal as this seemed to be a way to get around it when it was very noticeable. I've played w/different accelerator pedal launches however and do believe there's an improvement. It may be likely that there's improved scavenging w/the smaller diameter exhaust and a bump in torque in that region as a result:

https://motordyneengineering.com/sca...20applications.