If you were asked to diagnose the problem on this vehicle based on the presence of emulsified oil and water on this oil filler cap, what would your first reaction be? Would you, a) ask for details of stored trouble codes and/or symptoms/drivability concerns, b) hazard a guess and say that the emulsion is clear evidence of a leaking cylinder head gasket or c) would you say that based on appearances, all that is certain is that there is an excessive amount of moisture in the engine?
There is no single “correct” answer to our hypothetical question, but we have all made unfortunate diagnoses in haste. To avoid a possible repeat of this, we will use this article to take a step back and consider defective PCV (Positive Crankcase Ventilation) systems as an often-overlooked cause of not only excessive moisture in modern engines but also as the direct cause of excessive oil consumption, starting with this question-
In this writer’s experience, many technicians only ever concern themselves with PCV systems when they are looking for an engine vacuum leak that might be causing driveability problems and/or abnormal fuel trim values. Beyond that, many technicians tend to consider PCV systems as relatively trouble-free, and therefore, not as systems whose correct operation is critically important to the correct and proper operation of the emission control systems of all modern vehicles.
As a practical matter, there is very little that can be done to improve the thermal efficiency of internal combustion engines, and particularly the efficiency of petrol engines, so from the perspective of engine designers, the only viable way of increasing engine efficiencies is to maximize combustion processes. We need not delve into all the new technologies that are aimed at accomplishing this beyond saying the most advanced combustion processes come at the cost of having to overcome severe engineering challenges in terms of cylinder sealing and oil control on cylinder walls. Let us look at one particular engineering challenge, this being-
We are all familiar with excessive oil consumption on a wide range of modern vehicles and we don’t have to rehash the basics here, but what is not generally known is that many, if not most issues with excessive oil consumption do not result from poor engine design per se. Most oil consumption issues stem directly from poorly balanced trade-offs between extremely high compression ratios and combustion pressures to reduce emissions on the one hand, and the need to increase piston strength while saving on piston weight(s) at the same time, on the other.
These requirements are incompatible in practice, so what happens is that the pistons in many engines suffer gross deformations under heavy loads, which makes it impossible for the piston rings to maintain positive cylinder sealing under all operating conditions. The next thing that happens is that piston ring blow-by increases, and unless a crankcase ventilation system can extract these gases effectively and efficiently, the pressure in the crankcase can increase to the point where oil seals start to leak, or worse, be forced out of their housings.
The above is one aspect of poor cylinder sealing; another is that unless a crankcase ventilation system is supplied with sufficient fresh air to replace evacuated gases, the crankcase vacuum can build-up to the point where air and moisture are sucked into the engine past oil seals. This typically happens when the PCV systems' air inlet is restricted or blocked, but in cases where a leak in the PCV system exists before the inlet, the PCV system can stop working altogether because the vacuum it requires to work cannot be established and/or maintained.
Moreover, if a PCV system cannot extract the mixture of blow-by gases and ambient air (which contains moisture) from an engine, the moisture will condense on internal engine surfaces every time the engine cools down. When the moisture content exceeds a certain threshold, it will form liquid water, which will then collect in the bottom of the sump. When the engine starts up, this water will mix with the oil, and some of it will become entrained in the oil, and some will be released from the oil to exist in vapour form when the oil temperature exceeds the boiling point of water.
Eventually, though, there will be enough water in the oil for some water to form an emulsion with the oil, which is the milky white substance we see on oil filler caps. Barring other sources of moisture such as from leaking cylinder head gaskets or cylinder liner seals, this emulsion typically forms when one or more of the following happens-
The above is the short version of how and why oil/water emulsions form in many engines, but defective PCV systems also have other, more insidious consequences, so let us look at-
Limited space precludes a comprehensive discussion on how PCV systems cause excessive oil consumption on all known oil burners, but we can do the next best thing, which is to focus on the so-called “Valve of Doom”, which is the regulating valve in the PCV systems on many BMW engines. Consider the example waveform below-
Image source: https://www.searchautoparts.com/sites/www.searchautoparts.com/files/images/ma0919-d10%20copy.png
In this example, the red trace at the top represents ignition events taken from an ignition coil on a turbocharged N55 engine, while the blue trace represents the negative pressure in the crankcase. The red trace should be obvious- it shows when the engine was started and when it was shut off in 10-second intervals. However, note the blue trace that starts immediately upon start-up: although the maximum vacuum is achieved almost immediately and is maintained throughout the time the engine was running, the crankcase vacuum took almost 75 seconds to decay to ambient atmospheric pressure after the engine was switched off.
This waveform is normal for this particular engine because it shows a) that the engine is properly sealed, b) that there are no leaks in the PCV system, and c), that the regulating valve is functioning as it should. Before we get to specifics though, let us consider an excerpt from BMW TSB SI B11 03 13, and particularly the part that relates to excessive oil consumption-
"The crankcase ventilation system uses different crankcase ventilation valves, depending on the engine type. Although the valves all look different, they function similarly, using a spring and diaphragm assembly to control the crankcase pressure. A properly functioning pressure control valve is designed to maintain a slight vacuum (under-pressure) in the crankcase, which assures reliable crankcase venting during all engine operating conditions. One of the results of a malfunctioning crankcase ventilation system can be increased engine oil consumption.” Source: https://blog.bavauto.com/16943/bmw-high-engine-oil-consumption-tsb-si-b11-03-13/
On BMW engines there is a wide range of prescribed crankcase values, and while some variation is allowed in some cases, the allowed variation is typically very small, which makes it critically important that the regulating valve in the PCV system on any BMW engine functions correctly. Below is an example of the internals of such a Valve of Doom, aka PCV pressure regulation valve-
Image source: https://www.e90post.com/forums/attachment.php?s=f0cd622a4beb2abc4a41b6a93577889a&attachmentid=1321752&stc=1&d=1448087781
While this particular example is from an N55 engine, all BMW PCV pressure-regulating valves conform to this general pattern but note that there are differences in both the appearance and methods of mounting between valves that suit different BMW applications. Moreover, even though two valves might appear to be identical from the outside, two seemingly identical valves might be calibrated differently to suit different applications, so take extreme care not to install and/or replace an incorrect or unsuitable valve on any BMW engine.
So what can go wrong with these valves, considering that their function is to maintain a slight negative pressure in a running BMW engine, as opposed to separating oil mist from (relatively) clean air? To understand the distinction, it is necessary to understand that a) the separation of air and oil largely takes place in a series of baffles and other mechanisms that are built into the tappet (valve) cover, and b) that for these mechanisms to work as intended, the pressure-regulating valve has to be in a perfect working condition. Having said that, let us look at this failed valve in some (generic) detail-
In its simplest form, the valve is divided into two chambers by a flexible rubber diaphragm, with one half connected to the crankcase, and the other connected to the intake system. Incorporated into the diaphragm is a metal sealing surface that mates with a port in the bottom half: as intake vacuum creates a low-pressure above the diaphragm, spring tension is overcome, the metal sealing surface is lifted off the port, and the crankcase is evacuated through the now open port.
However, consider the image above that shows a clear leak path in the sealing surface as indicated by the red arrow. This very common failure mode is most commonly caused by the build-up of carbon inside the valve that prevents it from closing properly, but regardless of the cause, the effect is that the maximum allowable negative pressure in the engine can be exceeded under some operating conditions. The secondary effect of such a leak is that the mechanisms that separate oil and air in the tappet cover stop working, which typically results in large quantities of oil being sucked into the intake system, thereby contributing to oil consumption in the worst possible way.
Some other symptoms of this condition could include one or more of the following, but note that the severity of some symptoms may vary, depending on the size of the leak and the engine variant-
One other common failure mode on older engines involves perforations or cracks and tears in the rubber diaphragm, which if big/serious enough, could prevent the valve from opening at all, thus raising the positive crankcase pressure to above allowable thresholds. Typical symptoms of such a condition could include one or more of the following-
Note that since the list of symptoms above is not exhaustive, all diagnostic processes that involve PCV systems on BMW engines must be performed strictly as per OEM service information to avoid inadvertent mistakes and misdiagnoses, which could cause extensive engine damage
Note though that issues such as cracked or degraded vacuum hoses, defective MAF sensors, damaged/worn turbochargers, damaged crankshaft/camshaft oil seals, leaking tappet cover gaskets, poorly sealing oil filler caps, and restricted exhaust systems can mimic many of the symptoms of defective PCV pressure regulating valves. Therefore, all diagnostic process should start with a thorough visual inspection of all PCV and forced induction components to ensure you do not miss obvious faults, which brings us to-
Since diagnosing PCV issues on BMW engines can be a tricky and time-consuming affair, it is perhaps easier to list a few do’s and don’ts when dealing with these systems. Let us start with a few do’s-
Do use proper test equipment
While it is sometimes possible to obtain reasonably accurate test results with manometers and even with jury-rigged vacuum gauges and modified oil filler caps, these methods should only be used as screening tests to verify that a problem vehicle does have a PCV related problem. The problem with such equipment is that the maximum allowable deviations from specified engine vacuums are very small, and less than optimal test equipment may not always be accurate enough to reveal these deviations, which could lead to a misdiagnosis.
The most accurate test equipment remains oscilloscopes and pressure transducers because you can "follow" the engine vacuum in (almost) real-time, which removes all the guesswork from PCV problems and diagnostics.
Do investigate and/or repair all oil leaks
The second part of this heading reads “...before testing a PCV system, and particularly on a BMW engine.” It is not always easy to say if an oil leak through an oil seal is the result of excessive crankcase pressure or the result of an oil seal that had suffered mechanical or other damage.
However, damaged oil seals will almost certainly cause PCV issues such as excessive engine vacuum or even excessive oil burning as explained elsewhere, so the problem becomes one of which came first; excessive engine pressure that caused a seal to fail, or a damaged seal that is now causing an excessive engine vacuum. The customer may not be able to tell you if the oil leak started concurrently with the driveability problems, so replacing oil seals and fixing other oil leaks is often the quickest way to confirm or eliminate a defective PCV pressure-regulating valve as the cause of oil leaks and/or driveability problems.
Don’t assume anything
All BMW engines are extremely sensitive to both excessive and insufficient engine vacuum values, which means that neither “some” vacuum, nor a “little” positive pressure is acceptable. Service information for each BMW engine always specifies exact engine vacuum values, and while we realise that relevant service information is not always easy to obtain, proper diagnosis of PCV issues is often very difficult, if not always impossible to accomplish without it.
Don’t neglect to scan for fault codes
PCV system failures will a) typically set multiple fault codes, and b), typically set multiple fault codes at the same time, or within a few km of each other, such as in this example of a set of fault codes on an N55 engine. In this example, the fault codes below set as the result of the PCV pressure-regulating valve sticking in the open position-
|
Trouble Code |
Description |
Mileage |
|
0029CC |
DME: Combustion misfires- several cylinders |
210362 |
|
0029CD |
DME: Combustion misfire- cylinder #1 |
210362 |
|
0029CE |
DME: Combustion misfire- cylinder #2 |
210362 |
|
0029CF |
DME: Combustion misfire- cylinder #3 |
210362 |
|
0029D1 |
DME: Combustion misfire- cylinder #5 |
210362 |
|
002C9C |
DME: Oxygen sensor heater before cat converter- Activation |
210362 |
|
002C9D |
DME: Oxygen sensor heater #2 before cat converter- Activation |
210362 |
|
002C9E |
DME: Oxygen sensor heater after cat converter- Activation |
210362 |
|
002C9F |
DME: Oxygen sensor heater #2 after cat converter- Activation |
210362 |
Note that in this example (which is typical of this kind of problem) from this writer's records, the primary clue that the PCV valve was in the open position was the random misfires, since these valves do not have position sensors to tell the ECU what the valve is doing. The position of the valve was confirmed by checking the engine vacuum with a pressure transducer and an oscilloscope, and replacement of the valve resolved all the fault codes, including those relating to the oxygen sensors, which leaves us with-
One other common fault code that relates to PCV systems on BMW engines is DME 0028A0- “Intake pipe absolute pressure- plausibility: Pressure too high.”
Many pre-conditions have to be met for this code to set, but the short version is that it relates to the relationship between the throttle position and the volume of air being measured by the MAF sensor under some strictly defined conditions. Thus, given that one of the enabling conditions is the fact that the fault must be present for more than 10.5 consecutive seconds, the most likely source of the excess air is through an open PCV system pressure-regulating valve since air/vacuum leaks elsewhere in the intake system will set other codes almost immediately upon start-up.
Although PCV systems can be tricky to diagnose in general, and on BMW engines in particular because you often need OEM service information to perform some diagnostic procedures, these systems are not so complicated that they cannot be diagnosed and repaired at all without comprehensive OEM service information. If you understand the need for effective crankcase evacuation on modern engines, have a sound knowledge of engine and/or fuel management systems, and have access to a pressure transducer and an oscilloscope, you should be able to draw valid conclusions and inferences based on the symptoms you observe. And, of course, the realisation and knowledge that PCV systems are not as reliable and trouble-free as you may have believed.
