330ci Weird rough idle + early rev limiter issue

[dpickett769]

Hi,

so ive just bought a Msport 318ci that has been engine swapped to an M54b30 and its developed a weird issue just hours after buying it.

To start with it has a remapped ECU supposedly with a raised rev limiter too 7K and an ABS delete.

Now when cold it seems to misfire and run rough on idle yet allow to me to reach the factory rev limiter but not the raised one, but when warm runs and idles much smoother yet the rev limiter decides it wants to be at 5500rpm. Throughout all of this it does seem a little down on power whether it hits the limiter early or not. Now i know that doing an ABS delete can cause an early rev limiter but surely if its mapped out then it shouldn't appear and from driving the car the ABS and DSC are for sure not working. Both rear wheel sensors are connected but the fronts have been removed because it has modified front hubs for extra lock. ABS module is still in place as are all the fuses and relays.

Now having plugged it into some fancy snap on code reading machine, it appeared to show faults for misfires for two cylinders and wheel sensors not working despite being plugged in and a fault from the second O2 sensor. Aswell as ABS TC etc but i was expecting that from them being mapped out.

Cleared the codes and the only ones that returned where the O2 sensor and wheel sensors but the car is still very obviously misfiring when cold, i have replaced spark plugs and checked the coil packs one by one checking to make sure they all make the spark plugs spark and they all do.

I cant see that its the ABS causing the early rev limiter because whether or not it rev limits early seems to be temperature related as does the misfire, only thing i can think of is possibly coil packs breaking down at higher temps but then if that was the case i would get a misfire when hot as well as cold but i dont? when cold car seems to have a bit more grunt than when warm but still feels a bit down on power but when warm power band flat throttle response is awful and has that early rev limiter and have no clue why.

can anyone help?

[Muzz258]

I think you have two problems

1) The rev limiter has not been remapped properly in all places in the DME
2) You have vacuum leaks causing the rough running

When the engine is cold, a vacuum leak will cause rough running as the DME is running in open loop mode. It has slightly rich mixture and the vacuum leak leans it out.

When the engine is warm, the DME runs in closed loop mode and the O2 sensors provide the feedback loop. This will enrichen the mixture for any extra unmetered air getting in due to the vacuum leak. As the vacuum leak gets worse, the mixture gets richer until you get an "out of range" O2 code where it can't change the mixture anymore.

Usual vacuum leak suspects are:
1) Cracks in the rubber boots between the MAF and the throttle body/idle control valve
2) Breaks in the CCV hoses. valve cover to CCV and CCV to dip stick tube
3) Broken blank off caps on the vacuum ports on the back end of the manifold.

[dpickett769]

Okay so ive had a check around and unplugged the maf and it runs fine with no maf plugged in, no lumpy idle or rev limiter issues and as soon as the maf is plugged back in it goes shit again so appears that is the problem. Going to double check for any vac leaks also just to be sure. Thanks!

[MarcoZandrini]

Unplugging the MAF forces the DME to run an overly rich fuel/air mixture. This masks vacuum leaks. You need to have the intake smoke tested to find all of the vacuum leaks.