Limp home mode, also known as a Failsafe mode, is a safety drivetrain feature that prevents damage and, if possible, allows the journey to continue. It is activated by the drivetrain management control modules in case of a component malfunction.
Although most drivetrain management systems are based on the same control units (Bosch, Siemens…), each manufacturer has its own take on these systems and their exact functioning is a well-guarded secret.
However, we don’t need to use CIA techniques to comprehend how the Limp Home functions in BMW; logical thinking would suffice.
How Limp Home Works
BMW uses the limp home feature across drivetrain systems, not only the engine. That means that both engine, transmission, transfer case, and rear differential can use the limp home function in case of fault.
All of these systems can apply different measures to prevent damage and enable a journey. For example, the engine electronics will turn off turbochargers when it’s not capable of controlling them because of a faulty wastegate actuator.
The transmission can lock the 4th gear which will help you drive home in case of an oil pressure problem. The transfer case will disengage the front wheels in case of an actuator fault and help you drive home.
And so on.
Let’s have a look at a few specific faults that will trigger the engine limp home mode.
Both the Digital Motor Electronics (DME) and Digital Diesel Electronics (DDE) engine control systems are based on a virtual energy model. In other words, the engine management system includes a virtual model of the engine, which is used to both control and monitor the system’s functionality.
While the virtual engine model represents a perfect engine operation at the highest efficiency level, in the real world, engines, transmissions, transfer cases, and differentials rarely have the opportunity to do so.
Because of this, the engine management software has the capacity to adapt to the current conditions. This feature is known as “adaptations” and continuously adapts the operating parameters to match the set values.
For example, the DME monitors the engine running smoothness by comparing the engine rotational speed increase at each firing (ignition) cycle. When the post-firing speed increase is the same between all cylinders, the engine is running smoothly and there’s no need for intervention.
Now, if one of the cylinders is not working correctly, the DME will detect it as a post-firing slowdown.
Let’s say that the injector on the 3rd cylinder of our hypothetical N55 straight-six engine is slightly clogged because of carbon deposits, and the 3rd cylinder receives only slightly less fuel during the firing cycle. Nothing serious, but enough to not run perfectly smoothly.
In this case, the DME will alter 3rd injector signal in an attempt to restore running smoothness. So it will adjust the injection time duration until it synchronizes the rotational speed increase between all six cylinders.
This is something you won’t notice while driving a car.
However, if the smoothens cannot be restored, your BMW will go into a Limp Home mode.
Here’s how it works.
Let’s say the ignition coil is worn and cannot supply enough voltage to ignite the fuel mixture at a high load. When you fully depress the gas pedal (WOT), the DME sends all the fuel it is programmed for to give you fast acceleration. The more fuel in the fuel mixture, the harder is to ignite.
So, as our worn coil is struggling to ignite the fuel mixture, the DME detects a complete cylinder misfire. In this case, it won’t even try to correct it. Instead, the DME will shut off 3rd cylinder completely to prevent catastrophic engine damage and save you a few grand in repairs.
Besides shutting off the affected cylinder, it will also shut off turbochargers, as there’s no need to blow the pressurized air into a non-working cylinder.
So, you’re now officially in the Limp Home mode and you can continue driving with reduced power. Don’t worry, you still have about 100HP at your disposal.
Diagnostic threshold.
Each engine management value has a pre-set diagnostic threshold through the map. Crossing the diagnostic threshold will set the diagnostic trouble code (DTC) in the control module. Depending on the fault severity the control module will then take preprogrammed measures.
BMW Limp Mode Symptoms
BMW Limp Mode Reset
The most important thing to remember is that limp mode is a temporary state that will reset with each ignition cycle. The activation of the limp mode is unaffected by the fault memory (DTC) entries.
When you cycle the ignition off and back on, the system clears limp mode. In the case of our N55-powered BMW with a bad ignition coil that means you’ll have all the power available again until the next misfire at a high load.
Limp Home Mode Causes
As we touched on in our example, one of the causes is a cylinder misfire. However, the cause for the activation of limp home mode can be virtually any drivetrain component malfunction.
Here’s the list of the most common limp home causes:
Cylinder Misfire
A cylinder misfire, also known as an engine misfire, can happen for a few reasons. Essentially, a cylinder misfire is when a cylinder stops producing power at a rate that cannot be compensated by adaptations.
The most common causes are bad spark plugs, ignition coils, injectors, leaky intake valves, intake vacuum leaks, and faulty high-pressure pumps.
In the case of a severe cylinder misfire, the limp mode algorithm will deactivate the affected cylinder completely to prevent a fuel wash (also known as a cylinder wash).
Low Fuel Pressure
The low fuel pressure is mostly caused by a worn-out high-pressure pump, which has been giving BMW owners headaches since it was first introduced in 2006.
The first sign of a failing HPFP on gasoline-powered BMWs is a longer crank before the engine starts than usual and a low fuel pressure fault code. After the activation of limp mode, the DME will also permanently turn on the engine cooling fan and increase the output of the electric water pump.
Diesel unfortunately rarely shows mild symptoms. Instead, most often they’ll just cut the power completely during the driving. The reason is that Diesel uses much higher fuel pressure to operate. When the HPFP fails, the fuel pressure drops are drastic.
Turbocharger Control
In the event of an excessively high or excessively low boost pressure, the engine electronics will turn on the limp mode. All BMW direct injection turbocharged engines heavily rely on a specific boost at a given load. When the DME is not able to control the boost, the only logical measure is to turn off turbocharging completely.
In most cases, the engine will work just fine as a naturally aspirated engine and you can drive until the first opportunity to fix the problem.
The most common causes are wastegate or vane control actuators, boost leaking on a pressure side, intake vacuum leaks, and faulty sensors like air flow meters, manifold air pressure sensor, or intake air temperature sensors.
The damaged turbocharger itself will of course trigger the same measures.
Valvetronic Faults
BMWs have been using the Valvetronic system to control the engine power output instead of throttle actuators for more than 20 years.
BMW “throtleless” engines control the combustion, and thus the power output, by managing the intake valve openings rather than activating the flat butterfly valve of the throttle body.
The result is better engine response and increased efficiency. When you think about how the system works, it may look scary at first, especially if you think the Valvetronic motor could stuck at WOT.
For this reason, BMW has built a few levels of redundancy. When the Valvetronic motor fails completely, the standard throttle can take over the task. If the mechanical components jam, the DME can still control the engine’s output via fuel injection control.
However, over the years the system proved itself more reliable than standard electronically actuated throttle bodies.
Most Valvetronic faults are caused by poor wiring connections, a few software-related issues, or extremely rarely – worn-out gears of eccentric shafts. In case of a system fault, the engine management automatically activates limp mode and applies appropriate management measures.
Because the system can have a major effect on safety, it has built-in redundancy so it will work even if the Valvetronic motor completely fails.
Vanos Faults
VANOS is a variable valve timing system used by BMW for gasoline engines since 1992.
A faulty VANOS won’t always push your car into limp mode, but it very well could. A check engine light is a common occurrence as well, especially on later BMW models.
The most common Vanos faults include broken gear bolts, and engine management software errors, faulty electro-hydraulic Vanos solenoids,
Throughout the years, many times I found the Vanos solenoid connectors mixed up during repair causing the system to shut off and consequentially limp mode.
Sensor Faults
Engine output is controlled by actuators (injectors, ignition coils, Vanos solenoids, etc.) after data from sensors are processed in the control unit (DME) by the engine management software. The DME will employ “backup” values to allow the engine to run when the sensor data is missing or the values are improbable.
For example, the DME may still manage ignition and injection even in the event of a malfunctioning crankshaft speed sensor by using information from the camshaft sensor, albeit with some restrictions. In this situation, the engine enters limp mode (on most models, not all).
If the airflow meter, oxygen sensor, or boost pressure sensor malfunctions, the engine will still run since the DME will use average substitute data.
Depending on a number of factors, the DME will only activate limp mode if it is absolutely necessary. Because the engine can run just fine without O2 sensors or an airflow meter, for example, the typical check engine light will be sufficient to notify you of an emission-related problem.
In Conclusion
One of the main drawbacks in automotive redundancy models is that all automotive systems are non-linear in nature and cannot thus be compared to a linear system. The virtual engine model map is of great help, but it also has its own limits.
Therefore DME uses logical actions based on available data and totally turns off the affected cylinder in the event of a single-cylinder misfire because it lacks the data necessary to properly diagnose the issue.
Additionally, your BMW won’t let you know when it is in limp mode. The single red flag is a reduced power output or restricted drivetrain functionality. The Limp-Home mode is not always involved with “Drivetrian,” “Transmission,” or “Engine Malfunction” check control warning messages.
