My 2025 BMW M440i Is Shaking During Start-Up And Showing 0.0 Value Fuel Consumption. Something Wrong or Is This Just What Mild Hybrids Do?

It’s a 2025 model year BMW 440i, with the B58 engine in mild-hybrid configuration. So why does it randomly shake during start-up?

BMW’s six-cylinder engines are legendary and cherished by fans of the Munich brand. But like all modern luxury cars, even BMWs have integrated various stages of hybridity to their powertrains with some features that might appear quirky to new owners, unfamiliar with the experience of how mild-hybrid powertrains operate.

A forum member at Bimmerpost recently experienced this. Henanwang took ownership of a 2025 model year M440i and noticed something unusual about its start-up sequence and his digital dash readout’s fuel consumption values.

Here’s what Henanwang wrote in the B58 6-Cylinder Turbo Engine/Drivetrain/Exhaust Modifications forum over at Bimmerpost:

"Hi guys, I recently got my new 2025 M440i. Something weird happens when the car is at idle. Sometimes the instant fuel consumption is ZERO at idle; otherwise, it is around 1.2l/h. The car will shake once if instant fuel consumption is ZERO after car idles around 10-20s. After car shakes, fuel consumption will go back to normal at around 1.2l/h. I wonder if the mild hybrid engine causes this. It happens randomly, and I have no idea why this happened. I always turn off auto stop-start."

It's a feature

Why is a brand new randomly BMW M440i registering ‘zero’ value fuel consumption at idle and shaking during start-up? Fellow Bimmerpost forum member, Elderado82, replied to Henanwang’s initial post with a reply:

‘Mine does the same. I have a 2025 M340i currently at 1,300 miles. I did notice, though, that when you have the vehicle in Sport or Sport+ mode, it doesn't do it. I also think it’s something with the mild hybrid system. It’s almost as if the system isn't registering that the auto start-stop is turned off.’

BMW’s renowned for the quality of its engines, so what’s up with the ‘shaking’ and zero-value fuel consumption that Henanwang’s experienced with his 440i? It’s all about the tolerances that BMW’s B58 engine’s mild-hybrid systems operate within, and how the 440i manages total system power demands during start-up.

This is what mild-hybrids do

The M440i and M340i run similar B58 3-litre in-line six-cylinder engines and feature mild-hybrid systems, comprising a crankshaft-mounted integrated starter-generator (ISG). These mild-hybrid systems aid performance. How? By adding 11-hp during peak throttle demands – like when you need to overtake at highway speeds.

Enabling the 440i’s mild-hybrid set-up is a 48v electrical architecture. The ISG’s operation is unnoticeable when it adds that 11-hp of additional urge at highway speeds, you’ll just feel more acceleration but no mechanical sensation. But when the 440i is at a standstill during start-up, you will notice a slight shake as the ISG engages or disengages, and the 3-litre gasoline engine fires-up.

If you’ve parked your 440i in direct sun on a sweltering day and need climate control to rapidly cool a cabin, that creates a significant power demand. The kind of thing that can prompt power switching within the mild-hybrid system during start-up.

Why BMW’s in-line six engines matter

BMW has been building six-cylinder engines for decades, but never a V6. There’s a good reason for that. Although longitudinally mounted inline-six engines take up more space, they are mechanically better balanced, than a V6 engine.

Much of BMW’s customer loyalty and the driver appeal of their coupes, like the 4-Series, is built upon the reputation and smooth yet powerful responses of the brand’s in-line six engines, like the B58. And that creates very defined expectations from customers and legacy BMW owners, regarding the powertrain feel of their six-cylinder cars.

Stop-start technology is valuable in reducing emissions and fuel consumption, especially when commuting in dense urban and inner-city traffic. The integrated mild-hybrid system adds to that efficiency but will never be imperceptible in operation. You’ll always notice the step-change between pure gasoline or assisted power as the mild-hybrid engages and disengages.

The mild-hybrid benefits

For BMW owners new to the B58’s mild-hybrid set-up, there’s a touch more mechanical engagement that might be unexpected during start-up, under certain conditions. But it’s nothing untoward.

A mild-hybrid system is designed to engage when needed. And when it occasionally switches from engine to battery load, there might be a momentary shaking sensation, which is to be expected – because the ISG is running as part of the 48V mild-hybrid system.

The benefits of a BMW’s mild-hybrid B58 engine far exceed the random instances of occasional shake and zero fuel consumption reading during start-up as the powertrain alternates between ISG and engine. Who doesn’t want an extra 11-hp, right?

BMW’s 440i has that smooth throttle response that only a BMW 3-litre in-line six can deliver, and with 393-hp and 428 lb-ft, seamless acceleration and overtaking confidence are true to the best BMW six-cylinder traditions. In fact, speaking of the inline-six, Toyota didn't develop an inline-six engine for the GR Supra and used a BMW engine instead. For new 440i owners like Henanwang, the ISG’s engagement and disengagement will soon become a familiar feature of their driving experience – as will the reward of those 11 additional horsepower, it delivers.

The mild-hybrid system in the BMW M440i and M340i offers a unique blend of performance and efficiency. While it may surprise some new owners with its occasional quirks, such as the shaking and zero fuel consumption readings, these are normal behaviors of the system as it transitions between different power sources.

Have you experienced similar behavior in your BMW or another mild-hybrid vehicle? What are your thoughts on the trade-offs between performance, efficiency, and driver experience offered by mild-hybrid technology? Share your insights and experiences in the comments below.

Lance Branquinho has 20 years of reporting experience working with various carmakers and OEMs. His work at Torque News focuses on covering BMW news and opinion about the latest BMW vehicles and their development Follow Lance at LinkedIn and X for the latest BMW-related news.

Image by Henanwang's post on Bimmerpost (linked and referenced above, used under Fair Use copyright for news-reporting purpose.