So, you just had a "drivetrain malfunction" pop up in your N63 powered vehicle (BMW 550, 650, 750, X5, X6, etc). It can be a scary thing, but don't fret. With a proper diagnostic approach you can figure out exactly what's going on.
1. First Considerations
You've probably already pulled off the road or arrived at your destination now, but here are some considerations for when you get a drivetrain malfunction while driving.
Every drivetrain malfunction is different. Some are just warnings and will not impact your driving after the engine "recovers" from the fault condition (which can be instantaneous). Some will engage safety features (like "limp mode") and reduce the output of the engine. So the first thing to do should always be evaluate how the engine is running right after the drivetrain malfunction.
If the engine appears to be running rough or does not have full power, pull over!
Normally if this is the case, the "drivetrain malfunction" warning will stay in the information area of your cluster. You want to pull over because the power output of your engine can be unpredictable. Once safely stopped, it is worth shutting off the engine and seeing if a restart will return the engine to full power. Some drivetrain malfunctions like misfires will enable a safety feature to cut fuel injection in a certain cylinder until the engine is restarted. Restarting the engine and taking it easy on the rest of the drive can get you home to do further analysis.
If the engine stutters or cuts power for a second then returns to normal, you can continue to drive
Normally when this one happens, the "drivetrain malfunction" warning will disappear from the information area of the cluster. The engine management was able to recover from the error condition and you can continue to drive. It is usually best to take it easy the rest of the trip until you can get the codes pulled to see what happened.
PULL THE CODES! PULL THE CODES! BEFORE YOU ASK FOR HELP PULL THE CODES!
Whether you fire up ISTA or just head to AutoZone to get the codes checked, you need that information before starting diagnosing your issue.
You don't drive to the doctor and say what you're feeling and get handed a prescription.
Getting basic code information pulled from a basic scanner is perfectly fine. With resources like my BMW Fault Code Index it is easy to see what the codes mean.
2. Common Faults
In this section I will outline many of the common fault codes that occur on the N63 platform.
A misfire means that the engine was not able to combust the air and fuel mixture in a cylinder. The engine management system is monitoring the acceleration of the crankshaft and can detect abnormalities in the acceleration to determine if a single cylinder is not combusting. To protect the catalytic converter from being exposed to raw fuel, the engine management system will disable fuel injection on this cylinder until the engine is restarted. This causes the engine to run rough with reduced output. Codes: 140101, 140201, 140301, 140401, 140501, 140601, 140701, etc (many others) Potential Causes: A misfire is caused when the fuel and air in a cylinder are not ignited. The DME monitors the rotation of the crankshaft to detect abnormalities in its speed to determine which cylinders are potentially not firing. To combust, a cylinder must have: fuel, spark, and compression. Here is what I would recommend checking in order to best diagnose this issue with many potential causes:
If the misfire is occurring on 3+ cylinders, skip to #3.
The most common cause of misfires on this platform is a weak coilpack that cannot generate enough spark energy to ignite the dense fuel/air mixture. The quickest way to test this is to move the affected coilpack(s) to other cylinders. Keep track of where you moved them to, then clear the codes. If and when the misfire returns, you can see whether the misfire follow the coilpack. If it did, you know it needs to be replaced. I recommend Eldor coilpacks.
If the misfire did not follow the coilpack, here are the other possibilities:
Leaking injector: If your injectors are below index 12 you can assume they are leaking. Index 12 injectors can fail but it is less common. It's hard to confirm this directly, so usually you use a process of elimination to determine this as the cause.
Spark plugs: Spark plugs only commonly fail at high mileage (100k+). If budget is important you can try the same migration trick as with the coilpacks, but they are generally cheap enough if they are old it makes sense just to replace them as a normal maintenance item.
Compression: If the engine has suffered some physical damage it is possible the cylinder cannot compress the fuel and air mixture enough to compress. You can use a compression tester with the fuel pump fuse disconnected in order to check the compression of each cylinder and look for any that are much lower than the others.
Other causes: This list was non-exhaustive, but it covers all of the main causes. There are many other possibilities with wiring or DME issues that can cause misfires but they are too many for me to cover in this guide.
If you are having misfires in 3 or more cylinders:
If the misfire happened at a high RPM, I have noticed that the misfire detection algorithm can sometimes incorrectly detect a multiple misfire condition. I would recommend clearing the codes once and see if you can reproduce to confirm that the misfire really is occurring on multiple cylinders.
If the misfire is occurring on cylinders of the same bank (1-4, 5-8), you most likely have an issue with the high pressure fueling system of that bank. If there are no fuel pressure codes stored, the high fuel pressure sensor of that bank is suspect. If low rail pressure codes are store, the HPFP of that bank has likely failed.
If the misfire is occurring all over the engine, you should examine the low pressure fuel system. If no fuel pressure codes are store, the low pressure fuel sensor has most likely failed. If both rail pressure codes are store or a low pressure fuel system code is stored, then likely the low pressure fuel pump has failed.
Example: "air mass too high"
This code or codes like it when the DME detects MAF values out of range for what it expects. These faults usually are recovered from quickly and you are able to use the full power output of the engine after the initial cut out with no engine restart needed. Codes: 102302 Potential Causes: Your MAF sensor likely needs to be replaced on that bank. The early MAF sensors used on the N63 were notorious for failing, enough so that BMW included them in the US' Customer Care Package soft recall. These can become an issue especially once tuned since they are working past the normal operating conditions. Also, if you are running an aftermarket intake and have the MAF placed after a bend it can cause this code at higher power levels. Replacing the MAF may have no change in that situation.
Charging Pressure Codes
Example: "Charging pressure control, plausibility Pressure too low"
These codes are normally purely informational. They will not impact the engine running and you can continue to accelerate and drive with no issues. Codes: 120308 Potential Causes: The most common faults generated by the charge control system are pressure too high and pressure too low. This means that the boost pressure in the charge path (after turbos, before throttle body) was out of range for what was expected by the DME (either too high or too low).
Pressure too low: If using a piggyback such as a JB4, this fault can occur because the piggyback is not able to achieve the boost increase it wants to. The best course of action is to grab a log and post it for others to review before continuing diagnoses.
Besides that, here are the most common possibilities:
Issue in the wastegate vacuum control system: The wastegates are operated via a vacuum pump. If there is any line disconnection or leak in the vacuum control system, the wastegates will not close and boost will not be made. Ensure all vacuum lines are connected to the wastegates and that nothing is ripped or torn. It is less common, but the diaphragm inside the wastegate itself can also be leaking. You can confirm this by applying vacuum to the wastegate and watching for loss of vacuum. Another common source of issue is if one disconnects the vacuum line from the exhaust muffler valve and does not cap it off.
Boost leak: The N63 charge path is very short and stout, so boost leaks are not common. The most common sources for vacuum leaks are:
Diverter valves: If tuned, the stock diverter valves can fail and let out boost pressure
Intercooler hose o-rings: Between the turbo and the intercooler charge hose flange there is an oring that can possibly leak
Wastegate adjustment: If the turbos wastegate arms are not properly adjusted, they can not allow the wastegate to close fully and prevent boost from being built. Using a vacuum pump, you can adjust each wastegate arm so that when -300hPa is applied to the wastegate the flap is fully closed in the turbo.
Pressure too high: If using a piggyback such as a JB4, this fault can occur if the piggyback is overshooting its boost increase target. The best thing to do there is post logs for others to review before continuing diagnosis. Other possibilities are:
Wastegate adjustment issue: If the wastegate arm is too tight, the wastegate can close earlier than expected causing boost levels that are too high. Using a vacuum pump, you can adjust each wastegate arm so that when -300hPa is applied to the wastegate the flap is fully closed in the turbo.