This sensor registers the actual vacuum inside the intake manifold, as opposed to the absolute manifold pressure, as on almost all other vehicle makes. Nonetheless, the best place to start diagnosing idling issues is at the differential pressure sensor on the intake manifold. Bear in mind though that when you are diagnosing rough or uneven idling issues, the degree of throttle opening is almost imperceptible, so resist the temptation to condemn the throttle body as the cause of the rough idling problem.
This vacuum is determined by the percentage of throttle opening, which as we have stated elsewhere, is maintained at between 3% and 4% during idling. Therefore, the DME is programmed to maintain the intake manifold vacuum at around 50 millibar, which is equal to about 1.5 inches of Mercury when the engine is idling. For instance, the low rate of airflow during idling causes a low intake manifold vacuum, and although a BMW engine can run at idle when the pressure in the intake manifold is equal to atmospheric pressure, the crankcase ventilation and EVAP systems require a vacuum to function. We have all used intake manifold vacuum readings as diagnostic aids, but things are a little different on Valvetronic-equipped BME engines. The point is that under some operating conditions, the amount of valve lift does indeed control the amount of air entering the cylinders, which has major implications for- Manifold vacuum Whether or not this constitutes true engine control is debatable, but that is somewhat beside the point. Although the range of valve lift adjustment during idling is very small, the fact that some adjustment is possible during idling clearly illustrates the concept of engine control through valve lift, as opposed to controlling the engine through making changes to the throttle opening. While this distinction can be confusing, the best thing to do is to remember that when a BMW engine runs at idle, the throttle opening is held to an opening of between 3% and 4% by the DME, while the amount of commanded intake valve lift controls the volume of air that enters the cylinders.ĭuring idling, the DME can adjust the intake valve lift (without increasing the throttle opening) to maintain and improve both idling speed and quality. Some BMW literature also refers to this as an “engine control mode” that is distinct and separate from a control mode in which the throttle opening controls the engine load. Regardless of the age of the Valvetronic system on any given BMW model, all iterations of the system are said by BMW to produce an effect known as “throttle-free” load control.
*See the article “Understanding BMW’s Valvetronic System” for more details. The redesigned N52 inline six-cylinder engine, which was introduced during 2006, received the second-generation Valvetronic system, while the third, and current iteration of Valvetronic systems is in use on almost all BMW engines across the range of BMW vehicles. The first iteration of the Valvetronic system* was introduced in 2002 on the N62 (V8), and N73 (V12) engines.
Let us start with- A brief overview of Valvetronic technology While Valvetronic systems are reasonably robust and reliable, they are not immune to faults and effects that affect their operation, and in this article, we will discuss some of these issues, as well as provide some diagnostic and servicing tips. In a previous article titled "Understanding BMW's Valvetronic System", we discussed the workings of a mechanical system BMW had developed to alter the lift of the intake valves on equipped engines in some detail.