A BMW 4 Series from 2010 was experiencing misfires, specifically after reaching operating temperature (around 250F). Initial troubleshooting involved replacing spark plugs, suspecting potential damage from previous misfires, but this didn’t resolve the issue. However, clearing the error codes temporarily restored smooth performance until the engine warmed up again.
Further investigation revealed a whining noise emanating from the passenger side of the valve cover. A mechanic suspected vacuum leaks in the aging and potentially cracked air/vacuum lines. Removing one line temporarily eliminated the noise and altered the engine idle, suggesting a vacuum leak was present.
Additionally, the oil cap was tightly sealed shut, indicating a strong vacuum. Forcefully removing the cap caused the engine to stutter and stall, further supporting the vacuum leak theory. A diagnostic scanner monitoring misfire counts showed cylinder 4 registering occasional misfires before the vacuum line test. During the test, misfires occurred across multiple cylinders.
The next steps involve replacing the vacuum lines and, if necessary, the oil cap due to a suspected faulty seal. The PCV valve is considered a less likely culprit as the whining noise originates from the smaller lines connected to the valve cover, not the PCV valve itself. The PCV valve typically produces a whistling sound when failing, which wasn’t observed in this case.
Addressing the vacuum leaks is prioritized as they can disrupt engine performance and cause misfires. A properly sealed system ensures accurate air/fuel mixture and optimal engine operation. If the issue persists after replacing the lines and oil cap, further diagnosis will be necessary to pinpoint the root cause of the misfires in this BMW 4 Series 2010.
