All First Time Tuners to this platform should read this sticky. This car has a history of misfires and they are all well documented and typically easy to fix. If you are new to tuning your vehicle and you have over 25,000 miles on your vehicle spark plugs the first thing you should do before installing your tune is get fresh spark plugs. This problem is so common for newly tuned vehicles that even Cobb has made a video.
Cobb's Video on Misfires
Here is a list of things you can try in no particular order and there may be more. I would suggest starting with the least expensive ideas first.
-Spark Plug Gap
- New Spark plugs
- Swapping Coilpacks
-High Pressure/Low Pressure Fuel Pump
-Carbon Build up in intake valves
Check Your Spark Plug Gap:
The cheapest quick fix is to ensure your spark plug gap is within spec. BMW 135i and BMW 335i Stock Spark Plug gap for 2007-2010 is .028".
Replace Spark Plugs:
If this fix does work then you still may want to consider changing your spark plugs to new, especially if they have over 25,000 miles on them.
If the misfire is on one or two cylinders:
Try swapping coil packs to see if the problem moves to another cylinder: i.e. if cylinder 1 is misfiring, swap coil 1 to 3 and see if the misfire now happens on cylinder 3. If it moves the coil pack needs replacement.
If the misfires does not move then you need to check your spark plug gap and or replace them before moving on to the next steps.
If changing spark plugs and moving/replacing coil packs does not work it could be the following:
Your spark plug(s) would show an indication of fouling and smell of fuel. This would be an indication your injectors may be leaking at the seals. A leaky injector in most cases also causes a rough start and or idle.
Weak/ Failing Low Pressure Fuel Pump:
Not as common but a weak LPFP can cause misfires. You can measure this by trying to get logs from your tune of the LPFP voltage/pressure. You can also check your fuel trims to see if they are exceeding 31-34%. (However, your tune may just need adjustment). We advise to contact your tuner of choice in regads to fuel trims and if your low pressure pump may be on the fritz.
Weak/Failing High Pressure Fuel Pump:
Not as common but a weak HPFP can also cause misfires.
Excessive Carbon on Intake Valves:
This will happen to all N54 Engines. It is a result of not having fuel sprayed over the intake valves like traditional Port Fuel Injection. Since the N54 is direct injection over time the intake valves gunk up with carbon deposits. We've seen historically that after 35,000 miles all N54 vehicles could use a Intake Valve Cleaning. The benefits for most are smoother power delivery, increased power, smooth idle and in some cases a fix for misfires.
Usually attributed with Vanos codes, the Vanos Solenoids can cause misfires. A quick fix is to clean them. See DIY.