For any modern diesel engine to run efficiently it has to have the following:
Correct fuel and fuel pressure.
Correct boost/inlet air pressure.
Correct cylinder compression.
A free flowing and gas tight exhaust system.
This guide is for the most common engine power and smoking problems associated with BMW turbo diesels.
The lifeblood of any modern turbo diesel is the VTG (variable vane) turbo, without this your engine is basically a low powered and useless animal.
So 99% of all power problems are usually turbo related in some way or another.
There are various faults that can cause turbo related power problems and these need to broken down into categories
The basic turbo operation principle
Boost pressure leaks
This is where the turbo is functioning correctly but the boost pressure is being lost before it can enter the engine due to cracks, splits or poorly fitted hose work on the induction side, it can be a notorious power problem creator and the engine can emit more black smoke than usual with this problem, due to being too rich in fuel and not enough air.
Turbo not creating the boost to start with
This one has more than one variable to it:
Firstly the exhaust system that feeds the turbo has to be gas tight with no leaks, any leaks will cause loss of exhaust volume delivery to the turbo and create power and smoking issues
Blocked or gummed up air filters will cause the turbo to be starved of deliverable air and create power and smoking problems.
Variable vanes seized on the turbo due to soot and carbon build up will cause boost problems, as the pitch angle of the vanes are stuck, so it will either be poor on power pick up or power will drop off further up the rev range depending on how it's stuck.
If the variable vanes are not actuating due to loss of the actuator control, then on vacuum actuated models it is nearly always due to a failure on the vacuum system itself (very common) or a faulty mechanism.
On electronic actuators it is usually a failure of the module on the turbo that is the cause.
Variable vane sequence
Boost control can also be affected by the ECU receiving duff or faulty sensor info from other sensors on the engine, thus the turbo is not being instructed correctly by the ECU, this is where diagnostic software will help tell you of suspect sensors or problems.
There are more basic things like the actual health of the turbo but this is too vast a topic to cover it all and you realistically should know if your turbo is in good basic health or not but your VTG variable vane system is key to your turbo’s performance..
FUEL and ignition related problems
Again diagnostics is the key to a modern diesel.
Firstly fuel pressure is all important on a common rail engine the fuel pressure has to be correct or you can get poor power, erratic running and starting and smoking issues..
Fuel filters and pumps etc must be checked for any suspected fuel delivery issues
Fuel injection system faulty or worn injectors can give power problems usually combined with lumpy running and black smoke, due to the high pressure any serious leaks would be instantly noticeable on common rail engines on the high pressure side..
Again sensor issues can affect high pressure fuel pump control so sensors should be checked with diags software for any issues…
Faulty pressure regulators or relief valves can cause loss of fuel pressure and power issues
Compression and ignition issues
Poor cylinder compression can cause power and running issues, along with poor ignition and combustion and smoking, again diagnostic software like INPA can help with the diagnosis of cylinder compression or injector and ignition issues to individual cylinders on the engine, but ultimately a compression test maybe needed if you have cylinder compression suspicions.
MAP (manifold absolute pressure or boost pressure sensor) and MAF (mass air flow) sensors
These perform similar functions and rarely do engines have both but BMW diesels do have both, symptoms of either of these can be broad but usually include any or all of the following: poor power delivery, excessive smoking,rough running and poor fuel consumption, both these sensors are common failures on diesel engines, the MAF sensor measures ambient incoming airflow, the MAP sensor measures actual air pressure in the inlet manifold.
Typical MAP sensor location in the inlet manifold area
EGR valve (exhaust gas recirculation)
The EGR is a very common problem or failure on diesels, the valve reduces the amount of NOx by recirculating a small amount of exhaust gas into the intake. It is a controlled opening between the intake and the exhaust. By mixing this inert gas into the air fuel charge the cylinder temperature is reduced. Reducing the temperature reduces the amount of nitrous of oxides produced in the chamber. This prevents the engines cylinders from reaching damaging temperatures..
There are different types of EGR systems used on different engines. The vacuum operated EGR valve used in the illustration above is a typical design. The valve is opened by vacuum supplied through a duty cycle solenoid and closed by a spring. The solenoid controls the valve to open in proportion to throttle opening.
The valve remains in the closed position when the engine is started, at idle, and at wide open throttle. A faulty EGR valve or related component may leave the valve in either the stuck open or stuck closed position. The symptoms of a stuck open EGR valve are a rough idle with a lean air fuel mixtures. The symptoms of a stuck close EGR valve are an engine that has pre-ignition ping and knock. This is the result of high cylinder temperatures caused by the lack of the cooling exhaust gas introduced into the chamber by the EGR.
Blocked or faulty cat
A blocked or faulty cat can cause massive reductions in power and serious fuelling/smoking issues on the engine, effectively choking the engine and the turbos ability to generate air for the engine to start with.
As mentioned earlier your engine sensors can have a detrimental effect on your diesel engine, the ECU relies on these sensors to calculate engine fuelling and air mixture.
Fuel pressure sensor, fuel temp sensor, coolant temp sensor etc etc also can have bearings on engine control, again diagnostics software is the key to diagnosing sensor issues because the ECU is smart enough to expect a certain operating range of the sensor, if it steps out of this range for too long due to being faulty the ECU will usually flag a fault code pertaining to it, but not always.
Vacuum leaks or failure
Turbo diesels do not create a natural vacuum system being forced induction, so they have to have vacuum pumps to create the vacuum to power the various equipment on the engine, Turbo's. EGR’s, swirlflaps,pressure regulators etc etc can all rely on vacuum control so it is essential that your vacuum system is in good serviceable condition.
This will not usually cause power issues but will eventually cause premature turbo bearing failure and excessive engine smoking (blue oil smoke) on the engine due to excessive internal crankcase pressures. The M47 engine is common for turbo failure or smoking issue from the old type loo role breather becoming blocked and should be renewed with a Vortex filter.
Vortex breather (bottom) old loo role type (top) that becomes blocked
Failure of operation of the swirl flaps can display some symptoms but they will be negligible and not create serious power or smoking or engine related issues of any note, unless they fail and are ingested by the engine of course...
More info on swirl flaps: http://www.bmwforums.info/general-gu...irl-flaps.html
As I have already mentioned modern diesels are complex engines so diagnostics software is essential to get to the bottom of any niggling problems on them, you can obtain a BMW INPA or DIS diagnostics setup for diagnosing your car for as little as £7 if you have a laptop handy...
ECU Diagnostics and Tuning
If your modern diesel does not get long open road runs with good periods of high RPM and linear throttle useage (motorway use) you should run a tank of fuel treatment through the car on a regular basis to keep the engine,EGR and turbo free of soot and carbon deposits from low speed and RPM throttle usage this will keep your engine cleaner and hopefully free from some of the issues mentioned in this guide.