Rough Idle: An Evidence-Driven Diagnostic Framework
A symptom-first framework for rough idle: misfire counters, fuel trims, air leaks, MAF/MAP, ignition, injectors, compression, engine mounts as symptom-not-cause, timing and sensor plausibility.
Pressure, voltage, torque, pin and waveform values on this page are guidance only. Vehicle-specific verified specification required before acting on any test.
Symptom-first framework
- Clarify complaint — rough at idle only? hot/cold? in gear/neutral? after start?
- Confirm operating condition — temperature, accessories on/off, A/C
- Retrieve DTCs — all modules
- Inspect freeze-frame / live data — misfire counters, trims, at idle
- Identify affected system — ignition, fuel, air, mechanical, mount
- Form differential
- Select decisive test
- Update hypothesis
- Confirm cause
- Repair
- Verify — idle smooth under the condition that failed
Misfire counters
Read per-cylinder misfire counters at idle:
- One cylinder → cylinder-specific (coil, plug, injector, compression on that cylinder)
- Multiple/random → whole-engine (air leak, fuel pressure, MAF, timing)
- None but idle feels rough → not a misfire (mount, vibration, sensor)
Fuel trims
STFT/LTFT at idle and 2500 RPM:
- Lean at idle, normal at 2500 → vacuum leak
- Lean everywhere → MAF, fuel pressure
- Rich → leaking injector, purge stuck open, coolant sensor
See the Fuel Trims guide and P0171/P0172.
Air leaks
Unmetered air after the MAF leans the mixture, rough at idle:
- Smoke/propane test around the intake, PCV, EVAP purge, brake booster
- A leak is a leading rough-idle cause and is found in minutes with a smoke test
MAF / MAP
- MAF plausibility vs calculated load — a contaminated MAF under-reports, rough/lean idle
- MAP at idle — should show stable vacuum; erratic MAP points to a sensor or a real air issue
- See the MAF guide.
Ignition
- Coil / plug / lead — the classic single-cylinder rough idle
- Swap test (coil/plug to another cylinder) — does the misfire follow?
- Coil trigger — confirm the ECM commands the coil (scope/test light)
Injectors
- Restricted injector → that cylinder runs lean, rough
- Leaking injector → that cylinder runs rich, rough
- Injector balance test — flow comparison
Compression
- Compression / leak-down — low on one cylinder → rough idle; on several → timing or wear
- Relative compression — quick screen via cranking current
Engine mounts — symptom, not cause
A worn engine mount transmits normal combustion vibration into the cabin — the idle feels rough but the engine runs smoothly. Before replacing ignition/fuel parts for "rough idle", confirm whether the engine itself is rough (misfire counters, smooth on throttle) or the mount is transmitting normal vibration. Distinguishing amplifier from cause prevents misdiagnosis.
Timing
- Cam phase / VVT mistimed → rough idle, correlation codes (P0016/P0017)
- Timing chain stretch — rough idle + rattle
- See the Crank/Cam Correlation guide.
Sensor plausibility
- Coolant sensor biased → wrong cold idle enrichment
- MAF/MAP biased → wrong air measurement
- O2 stuck → wrong mixture feedback (usually the sensor is right)
Links
- P0300 — random misfire
- P0171/P0172 — lean/rich
- Fuel Trims / MAF — live-data frameworks
- Misfire — the misfire symptom framework
Common mistakes
- Replacing all coils/plugs for a rough idle without reading counters
- Not distinguishing a worn mount from a real rough idle
- Ignoring fuel trims (lean idle is often a leak, not ignition)
- Not smoke-testing the intake
- Condemning the MAF without plausibility testing
Related DTCs
Related MechanIQ guides
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