How Fuel Injection Works on the Classic Mini with Rover MEMS ECU

When the classic Mini transitioned from carburettors to electronic fuel injection in the early 1990s, Rover introduced the Single Point Injection (SPi) system, controlled by the Modular Engine Management System (MEMS) ECU. This system used a single central injector mounted in the throttle body, replacing the carburettor while retaining a familiar manifold layout.

Unlike multi-point systems, which inject fuel directly into each intake runner, the SPi sprays fuel into a single throttle body, where it mixes with air before entering the engine. The MEMS ECU continuously monitors engine conditions via a network of sensors and adjusts fuelling, ignition, and idle speed accordingly.

Key Components

1. Throttle Body and Injector

• Houses the single fuel injector, which is pulse-width controlled by the ECU.

• Injector duty cycle varies from very short pulses at idle (about 1–3 ms) to much longer pulses under load (up to 15 ms).

• Provides atomised fuel into the air stream entering the engine.

2. Stepper Motor (Idle Air Control Valve, IACV)

• A four-phase stepper motor  for fine throttle plate control.

• Allows the ECU to maintain target idle speed, compensate for load (e.g. alternator or fan kicking in), and control cold start idle.

• At cold start: stepper opens wider (more air, ~60–80 steps) → higher idle for warm-up.

• At warm idle: stepper retracts (around 20–30 steps) → stable 850–950 rpm.

3. Throttle Position Sensor (TPS)

• Potentiometer attached to throttle spindle.

• Voltage range: ~0.3–0.5 V closed throttle, up to ~4.5 V wide open.

• Tells ECU driver demand; also used for acceleration enrichment and deceleration fuel cut.

• If faulty: ECU may assume “limp home” mode with fixed throttle map.

4. Manifold Absolute Pressure (MAP) Sensor

• Located within the ECU, connected by a vacuum hose to the throttle body.

• Measures intake manifold pressure (typically 20–40 kPa at idle, 100 kPa at WOT).

• Primary load sensor – determines base fuelling and ignition advance.

• Faults include split vacuum hose (causes lean running, high idle, poor driveability).

5. Coolant Temperature Sensor (CTS)

• Negative temperature coefficient (NTC) sensor screwed into the thermostat housing.

• Resistance ~5 kΩ at 20°C, ~300 Ω at 90°C.

• Provides enrichment on cold start, leans mixture as engine warms.

• ECU Operating Temperature is 80°C.

• Faults: if reads cold all the time, ECU over-fuels → poor economy, rich idle. If open circuit, ECU may default to 60°C→ hard cold starting.

6. Air Intake Temperature Sensor (IAT)

• Located in the throttle body.

• Resistance ~2.5 kΩ at 20°C, ~300 Ω at 50°C.

• Corrects fuelling for air density (colder = denser = more fuel needed).

• Less critical than CTS but contributes to fine-tuning mixture.

7. Lambda (Oxygen) Sensor

• Heated zirconia sensor in exhaust downpipe.

• Voltage range: ~0.1 V (lean) to ~0.9 V (rich).

• Closed-loop operation once warmed (~600°C, usually after 1–2 mins running).

• Maintains 14.7:1 AFR at light load.

• Faults: slow response or dead sensor → poor fuel economy, hunting idle.

8. Crankshaft Position Sensor (CPS)

• Reads timing teeth from flywheel.

• Provides engine speed and position reference for both ignition and injection timing.

• Without CPS signal, engine will not run.

9. Fuel Pump and Pressure Regulator

• Electric pump delivers fuel at ~1.0 bar (15 psi) above manifold pressure.

• Regulator in throttle body maintains pressure differential.

• Faults: weak pump or stuck regulator → lean mixture, misfire, cutting out.

Operating Modes

Cold Start

• CTS shows low temperature → ECU lengthens injector pulse (rich mixture).

• Stepper motor opens extra bypass air → high idle (~1200–1500 rpm).

• Ignition timing may be advanced slightly.

• Lambda ignored until sensor is hot.

Warm-Up

• As CTS resistance falls, ECU progressively leans fuelling.

• Stepper slowly closes to bring idle speed down to target (~950 rpm).

• Once lambda sensor is hot, ECU switches to closed-loop mixture control.

Normal Running (Warm, Light Load)

• ECU targets stoichiometric mixture using MAP, TPS, IAT, CTS, and lambda feedback.

• Injector operates with short pulses, adjusting for load.

• Stepper adjusts only for idle stability.

Acceleration

• Rapid change in TPS triggers acceleration enrichment (extra fuel pulse).

• Prevents hesitation.

Deceleration

• Closed throttle + engine above idle speed → fuel cut-off.

• Stepper motor may open slightly to prevent stalling when throttle reopens.

Common Faults

• Vacuum hose to MAP sensor split or leaking → very poor running, over-fuelling.

• Stepper motor → erratic idle, stalling, hunting.

• CTS failure → cold starting issues, excessive fuelling, black smoke.

• Lambda sensor or Lambda sensor heater → poor MPG, rough idle, high emissions, running in open loop.

• Throttle pot → flat spots, hesitation, poor drivability.

Summary

The Rover MEMS ECU in the single-point injected Mini is a clever compromise between carburettor simplicity and modern fuel injection control. By carefully monitoring sensors like the MAP, CTS, IAT, TPS, and lambda, it adjusts fuelling and ignition for all driving conditions. The stepper motor plays a central role in managing idle stability and cold start behaviour. When healthy, the system gives the Mini reliable starting, good emissions, and improved driveability compared to a carburettor – but as these cars age, sensor faults and vacuum leaks are now the most common causes of trouble.