The phenomenon of “left air” in BMW – what is the cause and how to fix it?

BMW driver’s question

After a long stop, I start the car without problems, but after about three minutes without adding gas, that is, without starting, the revs drop sharply and the car goes out.

Then it fires up already without problems. At idle on a cold engine, the revs are 1100. When it warms up, the revs are in the 600-700 range. I also noticed a second thing.

When I let off the gas and roll with momentum without clutch and brake then at the final stage, when the revs are below 1400, there is a feeling as if something is pushing the car. The temporary fuel consumption indicator rises to about 10 liters per 100 km. The phenomenon disappears when I press the clutch and brake the car to zero or when I continue driving.

I recently replaced the fuel filter as a preventive measure and zero effect. What could it be? Are these two effects two different things? Are they related?

The problem was on a BMW 316i E46, but it may also occur on other models.

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BM Cars service response

The described symptoms suggest a problem with a leak in the air intake system. Very similar engine behavior occurs when so-called ” left air ” enters the engine through leaks in the intake manifold or air lines behind the air flow meter.

The incompatible amount of air with the data that the flow meter transmits causes the engine’s revs to ripple or even go out. RPMs then drop below 400. The described effect of pushing the vehicle at falling engine speeds can also be caused by leaks.

It is necessary to check the tightness of the intake system and eliminate any damage. The described problem is the most common defect in this engine causing the ripple effect.

However, in order to make an accurate diagnosis, a full computer diagnosis of the engine is necessary.

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