Rover MEMS SPi Mini – Diagnostic Workflow

1. Rough or Unstable Idle

Symptoms: Idle hunts, stalls, or surges.
Step-by-step checks:

  1. Stepper Motor / Idle Air Control

    • Check for smooth movement.

    • Remove and clean if sticking.

    • Observe ECU step count if possible.

  2. Vacuum Hoses

    • Inspect all hoses to MAP sensor, throttle body, and intake for splits or leaks.

    • Listen for hissing sounds at idle.

  3. Throttle Position Sensor (TPS)

    • Measure voltage at closed throttle (~0.3–0.5 V) and WOT (~4.5 V).

    • Check for smooth linear response.

  4. Coolant Temperature Sensor (CTS)

    • Verify resistance at current temperature matches specification.

    • Faulty CTS can cause overly rich or lean idle.

  5. Lambda Sensor

    • Ensure it reaches operating temperature (about 1–2 mins).

    • Check voltage swings between 0.1–0.9 V.

    • Replace if sluggish or fixed.

2. Cold Start Problems

Symptoms: Hard starting, excessive smoke, high idle.
Step-by-step checks:

  1. CTS

    • Most common culprit. Confirm resistance is correct for ambient temperature.

  2. Stepper Motor

    • Ensure it opens enough for cold idle (~60–80 steps).

    • Clean if necessary.

  3. Fuel Pressure & Injector

    • Confirm pressure at ~1 bar above manifold.

    • Check injector for proper spray and no drips.

  4. Lambda Sensor

    • Not critical on cold start (ECU ignores until hot).

3. Poor Acceleration or Flat Spots

Symptoms: Hesitation, flat spots, stalling under load.
Step-by-step checks:

  1. Throttle Position Sensor (TPS)

    • Ensure smooth voltage ramp.

    • Adjust if needed.

  2. MAP Sensor & Vacuum Hoses

    • Check for leaks that cause lean running.

  3. Injector Pulse & Fuel Pressure

    • Ensure correct pulse width under acceleration.

  4. Air Intake Temperature Sensor

    • Confirm sensor reads air temperature correctly; extreme errors affect enrichment.

4. Cutting Out / Misfiring Under Load

Symptoms: Engine dies at high rpm or load.
Step-by-step checks:

  1. Fuel Pump

    • Confirm stable delivery at pressure under load.

  2. Fuel Pressure Regulator

    • Inspect diaphragm for leaks.

  3. Crankshaft Position Sensor (CKP)

    • Check signal continuity and waveform.

    • Intermittent CKP = cutting out at high revs.

  4. Lambda Sensor

    • Ensure responsive feedback.

5. Emissions / Black Smoke

Symptoms: Excessive smoke, poor fuel economy.
Step-by-step checks:

  1. CTS & IAT Sensors

    • Over-rich running often due to incorrect readings.

  2. Lambda Sensor

    • Faulty sensor prevents closed-loop correction.

  3. Injector Leaks

    • Check for dripping or hanging open.

Tips for Workshop Use:

  • Always check vacuum integrity first, as leaks are the simplest cause of driveability issues.

  • Measure sensor voltages/resistances against the table provided earlier.

  • Clean stepper motor and throttle body routinely to prevent idle issues.

  • Keep a multimeter and diagnostic scope handy for TPS, CKP, and lambda readings.