A 2016 BMW M235i with a BM3 Stage 2 91 octane tune and approximately 60,000 miles experienced stumbling on cold starts. The engine RPM would drop to around 500 RPM before recovering. This issue occurred only on the first start of the day. Subsequent starts were normal. The car had new spark plugs, and data logs sent to BM3 showed no issues.
The battery tested fine, ruling out a power issue. Spark plugs were recently replaced, addressing the spark aspect. Fuel remains a potential problem area. The car stalled once in early June but restarted immediately. After refueling and driving, the problem seemed to improve but returned after filling up at the same Shell station with 93 V-Power.
Similar issues have been reported online, particularly in M235i forums, with some accompanied by videos showcasing the stumbling. Some owners described the problem as sporadic, resolving itself without intervention. Others, under warranty, had fuel pumps replaced, but this didn’t always fix the issue.
While bad gas is a possibility, especially given the recurrence after fueling at a specific station, other potential causes need investigation. The correlation between refueling and the stumble suggests a possible link to fuel quality or a fuel system component, like the Mass Air Flow (MAF) sensor. When tuning a car, the MAF sensor plays a crucial role in measuring the amount of air entering the engine. A dirty or faulty MAF sensor can provide incorrect readings, leading to an improper air-fuel mixture and causing stumbling or stalling. Cleaning or replacing the MAF sensor is a relatively simple procedure that can often resolve this issue.
Furthermore, considering the car is tuned, verifying the tune itself is crucial. While BM3 indicated the logs were fine, a closer examination of fuel trims and MAF sensor readings within the logs might reveal subtle inconsistencies. Consulting with a BMW tuning specialist experienced with BM3 could be beneficial. They might offer insights into specific parameters within the tune that could contribute to the cold start stumble.
Other potential culprits to investigate include vacuum leaks, fuel injectors, and the crankshaft position sensor. A thorough inspection of these components is recommended. A smoke test can help pinpoint vacuum leaks, while testing fuel injector pulse width and the crankshaft position sensor’s signal can identify potential problems with those components. Additionally, checking for any pending or stored Diagnostic Trouble Codes (DTCs) using a BMW-specific scan tool might provide further clues.
Addressing this issue requires a systematic approach, starting with the most common causes related to fuel delivery and air metering, specifically focusing on components affected by tuning, such as the MAF sensor. If the problem persists after these checks, seeking professional diagnostics from a qualified BMW technician is advisable.
