[Efthreeoh]

With the age of the E9X chassis now a good 6 years past its last production date and most cars still on the road are with their second or third-tier owners (who probably are missing the original BMW documentation), I decided a write up of how the oil monitoring system works in the N52/54/55 engines was in order. A lot of owners have no idea how the oil sensor works, and misunderstand how the e-dipstick operates and what it tells them; the BMW owners manual never really gave a good explanation in the first place. A lot of owners do not understand the e-dipstick bar graph and think it represents the entire 7-quart quantity of oil in the engine. It only represents the top quart of oil.

Oil Sensor Purpose:
The N-series engines in the E9X all use VANOS, which is a camshaft timing system to vary the timing of valve operation dependent upon engine load (power) requirements. VANOS is operated by relatively constant oil pressure in conjunction with varying oil volume flow though computer controlled solenoid-operated oil control valves. To insure proper operation of VANOS, the engine computer needs to determine the level of the engine oil, which is why the engine has an oil level sensor. The primary purpose of the oil level sensor is for the VANOS system, rather than to just notify the driver of the engine oil level. The e-dipstick is an adjunct function of the oil level sensor even though it offers the driver convenience of reviewing engine oil level from the driver’s seat. The oil level sensor operation is not well understood by most owners, and appears to give faulty readings, which is why owners suspect its reliability.

Oil Sensor Design:
The sensor design is quite simple. It uses a dual stack of concentric-ring capacitors to determine the oil quality (contamination level) and oil level. Dual stacked means one capacitor is on top of the other capacitor. Capacitors are an electrical device normally used to store electrical energy for short periods to provide electric devices with short bursts of electrical energy when needed. Capacitors use a dielectric material to store the electrical energy. For the BMW sensor, engine oil is the dielectric material; the BMW oil sensor uses the electrical-design theory of the capacitor as a measurement device rather than an electrical energy storage device. The oil sensor, called “OZS” by BMW, uses the lower capacitor to measure the oil quality and the upper capacitor is used to determine the oil level. The sensor measures the dielectric value of the oil against a dielectric constant (value) to determine the contamination level of the oil. The upper capacitor uses the value (amount) of the capacitance in the upper capacitor based on the amount of oil present to determine the engine oil level. Concentric-ring means the design of the capacitor stores the oil in between conductors of the capacitor to determine the dielectric value of the oil (imagine a small pipe inside a larger pipe with oil in between the two pipes). There are small openings at the top and bottom of the OZS housing that allow engine oil to slowly pass through the capacitors. There is very little to mechanically or electrically to break in a capacitor, which makes the BMW oil sensor device highly-reliable. For the OZS to correctly measure the oil, the temperature of the oil must also be known, so there is an oil temperature sensor built into the bottom of the OZS.

E-Dipstick Operation:
Mechanical dipsticks are direct-read devices, meaning the level of the engine oil in the crankcase is directly determined by presence of oil on the dipstick. The e-dipstick is not a direct-read device; it takes data and data analysis to determine the oil level, which also means it takes time to determine the oil level; BMW calls this the "dynamic oil level" measurement. To get an accurate oil level reading the oil must be at operating temperature (one of the reasons the OZS has a temperature sensor built into it), and the engine must have been in operation for a minimal time period and gone through a series of operational parameters to ensure oil has moved throughout the lubrication system while the engine is running. The engine operating temperature must reach and be above a minimum of 140 deg. F. Engine speed must have reached greater than 1,000 RPM, and the engine must have reached acceleration levels transverse (side to side) and longitudinal (forward) of greater than 12 – 15 feet per-second squared. It takes about 5 minutes to reach these parameters in normal driving and the e-dipstick will not provide a measurement until such time, which is why if the engine is cold, a small clock symbol appears in the e-dipstick window and no reading is provided. Once the time and other parameters are met, the e-dipstick updates to a new level reading. The engine computer then continuously monitors the oil level and updates the e-dipstick display every 20 minutes, which means you can only get a new level reading after every 20 minutes of continuous driving.

Static oil level measurement. The OZS also has a function to measure the oil level when the engine is cold (like after an oil change) to confirm you refilled the engine. Static oil level measurement at engine OFF is only a reference measurement as the oil condition sensor (OZS) is flooded when the engine is turned off and can only detect the minimum oil level [the oil level is measured correctly only when the engine is running]. After switching on the ignition, the static oil level measurement provides the driver with the opportunity of checking whether there is sufficient engine oil for safely and reliably starting the engine. You follow the same procedure as checking the dynamic oil level, the action will return the "OK" notification in the gauge cluster.


The Confusion:
This is where the problem occurs with oil filling of the E90 N-series engines. Most owners don’t understand the e-dipstick doesn’t immediately update the oil level measurement (as a mechanical dipstick does). The e-dipstick reports only the top-level quart of oil, meaning it only shows how much of the 7th quart of oil is in the engine. The bar graph, which is analogous to the upper and lower marks on a mechanical dipstick, is just 1-quart long, so if the bar graph shows halfway between MIN and MAX the engine is missing just 1/2-quart of oil (i.e. not 3.5 quarts). If after an oil change and complete refill of the engine crankcase (7 quarts), if the engine is still above 140 deg. F (or soon does reach 140), but the engine has not gone through the full 5-minute time period of reaching the operational parameters, if the oil level is read, the e-dipstick will report the last known oil level. So if you changed the oil when the engine was, for example ¾-quart low, the e-dipstick will still read ¾-quart low even if you added the full 7-quarts of new oil. Conversely, if you top off the engine when it is not a full 1-quart low, say again at ¾-quart low, and then immediately read the oil level, it will still show ¾-quart low. This gets confusing to owners, because they don’t trust the oil sensor and don’t understand how it works, so they will add more oil and overfill the engine; I had a good friend, who is a very experienced DIY’er, do this exact scenario and blow the front crankseal of his son’s N52. This is also why, if you read the owner’s manual, it tells you to only add 1-quart of oil to the engine only after the CBS notification (“+1QT”) appears in the information display; if you add the quart and then immediately check the oil level, it will still show 1-quart low.

So as an owner of an E9X with the N52/54/55 engine, when changing the oil, or topping off, you must allow the e-dipstick at least 20 minutes to provide an update to the previous engine oil level. Also, you do not need to keep the engine oil level at MAX. Allow the engine to consume the 7th quart of oil and only add replacement oil at 1-quart increments to top off the oil level as notified by the CBS. Add the 1-quart of top-off oil within 125 miles of receiving the "+1QT" notification.