Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Computerized Engine Controls: Notes
The Motronic emission control system, or Digital Engine Electronics (DME), precisely controls fuel injection and ignition under variable operating conditions. The system also operates in conjunction with other computer-controlled systems.
These systems can influence the entire engine control system, depending on requirements. The Motronic Control Unit (MCU) assumes control of functions listed briefly here:
- Fuel Injection Control
- Ignition Control
- Cold Start Control
- Engine Speed Control
- Fuel Evaporative (Purge) Control
- Relay Controls
- Self-Diagnostics
The Motronic emission control system is a microprocessor system controlling air/fuel mixture and exhaust emissions by adjusting fuel injection and ignition timing. The MCU receives and processes sensor input and sends output signals to adjust injector pulse widths and ignition timing.
Various engine sensors supply the MCU with operating information. These include: intake airflow, intake air temperature, throttle position, coolant temperature, engine speed, vehicle speed, crankshaft position and exhaust gas oxygen content.
The MCU can run the engine in an emergency or limp mode, allowing vehicle operation despite component failure. The MCU also has adaptive capabilities to compensate for component wear and other negative factors, such as minor air (vacuum) leaks. Once a fault is recognized, it is stored in MCU memory as a defect code. The system automatically substitutes a fixed replacement value for the incorrect value caused by defective component or circuit.
| Application | Location |
|---|---|
| 325 Series & 735i | Under Right Side of Dash |
| 525i & 535i | Behind Right Front Strut Tower |
Use the Manual With the Right Hardware
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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