Rover MEMS SPi Mini – Diagnostic Workflow
1. Rough or Unstable Idle
Symptoms: Idle hunts, stalls, or surges.
Step-by-step checks:
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Stepper Motor / Idle Air Control
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Check for smooth movement.
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Remove and clean if sticking.
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Observe ECU step count if possible.
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Vacuum Hoses
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Inspect all hoses to MAP sensor, throttle body, and intake for splits or leaks.
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Listen for hissing sounds at idle.
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Throttle Position Sensor (TPS)
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Measure voltage at closed throttle (~0.3–0.5 V) and WOT (~4.5 V).
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Check for smooth linear response.
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Coolant Temperature Sensor (CTS)
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Verify resistance at current temperature matches specification.
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Faulty CTS can cause overly rich or lean idle.
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Lambda Sensor
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Ensure it reaches operating temperature (about 1–2 mins).
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Check voltage swings between 0.1–0.9 V.
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Replace if sluggish or fixed.
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2. Cold Start Problems
Symptoms: Hard starting, excessive smoke, high idle.
Step-by-step checks:
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CTS
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Most common culprit. Confirm resistance is correct for ambient temperature.
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Stepper Motor
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Ensure it opens enough for cold idle (~60–80 steps).
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Clean if necessary.
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Fuel Pressure & Injector
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Confirm pressure at ~1 bar above manifold.
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Check injector for proper spray and no drips.
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Lambda Sensor
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Not critical on cold start (ECU ignores until hot).
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3. Poor Acceleration or Flat Spots
Symptoms: Hesitation, flat spots, stalling under load.
Step-by-step checks:
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Throttle Position Sensor (TPS)
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Ensure smooth voltage ramp.
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Adjust if needed.
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MAP Sensor & Vacuum Hoses
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Check for leaks that cause lean running.
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Injector Pulse & Fuel Pressure
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Ensure correct pulse width under acceleration.
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Air Intake Temperature Sensor
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Confirm sensor reads air temperature correctly; extreme errors affect enrichment.
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4. Cutting Out / Misfiring Under Load
Symptoms: Engine dies at high rpm or load.
Step-by-step checks:
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Fuel Pump
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Confirm stable delivery at pressure under load.
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Fuel Pressure Regulator
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Inspect diaphragm for leaks.
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Crankshaft Position Sensor (CKP)
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Check signal continuity and waveform.
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Intermittent CKP = cutting out at high revs.
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Lambda Sensor
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Ensure responsive feedback.
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5. Emissions / Black Smoke
Symptoms: Excessive smoke, poor fuel economy.
Step-by-step checks:
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CTS & IAT Sensors
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Over-rich running often due to incorrect readings.
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Lambda Sensor
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Faulty sensor prevents closed-loop correction.
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Injector Leaks
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Check for dripping or hanging open.
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Tips for Workshop Use:
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Always check vacuum integrity first, as leaks are the simplest cause of driveability issues.
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Measure sensor voltages/resistances against the table provided earlier.
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Clean stepper motor and throttle body routinely to prevent idle issues.
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Keep a multimeter and diagnostic scope handy for TPS, CKP, and lambda readings.