Hello!

I've been scouring the forums for old threads, but haven't come across this problem. the car is a 2005 Cooper s with 120k on it, mods are 15% pulley, intake, exhaust.

Under hard acceleration on the highway (noticeable in 4,5,6 gear) the car starts to stutter. If you drive the car without flooring it, there's no problem. If you continue to press the gas when the stuttering starts, the car will go into limp mode and runs like crap producing the P0300 random misfire code. The check engine light comes on, but no other lights.

Once you turn the car off and on, it drives normal again and the light will go out after a couple days.

Any thoughts? Was thinking about trying a new bypass valve this weekend.

 

What makes you think that its the bypass valve? Only curious, not asking in a judgmental way.

If its having missfire issues, my assumption would be lack of fuel, or possibly ignition issues. What makes me think this way is during hard pulls, mostly WOT, its probably not getting enough fuel, or running really lean due to the increased air flow(reduction pulley), resulting in detonation, ie miss fire, and putting it in limp mode to prevent damage.

Do you have a way to monitor air fuel ratios? That would be an easy way to determine if its getting enough fuel and WOT.

 

Make sure the wires are pushed in all the way on the coil. While you are at it, check to see if the coil electrodes are rusty.

And, what light is on, the Service Engine light on the tach or the EML?

 

I am having the same issue, only happens when the engine is warm. I also have a 15% pulley on my car and Alta intake.

I don't think it's the BPV. You can check the function of the BPV by sight. Look at MODMINIs youtube video to see how it should work (also a good description of what its like when it goes bad). I stuck my phone in the engine bay where it could film the bypass valve while i reved the engine. You can see the "plunger" opening and closing the diaphragm as you rev. That will confirm operation of your BPV.

I replaced my throttle body, coil, wires, plugs, belt, crank pulley and still have the same issue. I am now thinking T-MAP sensor or vacuum leak. I am getting different codes though, mass airflow codes.

Good luck in your search, and please keep posting everything you do! I'll let you know if I figure anything out on my end too.