Diagnostic guideIntermediateEvidence-led
A warm-engine rough idle is a condition to measure, not a reason to order an idle valve, injector, or ignition coil.
Why this matters
Heat changes fuel strategy, purge operation, electrical resistance, and mechanical sealing. A useful diagnosis records exactly when the idle becomes unstable and which cylinders or fuel trims react.
Evidence to capture first
- Cold and fully warm idle speed, short-term trim, and long-term trim
- Cylinder misfire counts and whether one cylinder leads the event
- Purge command, electrical load, and manifold pressure when the symptom begins
A disciplined path
- Confirm the symptom with all accessories off, then repeat with normal electrical and A/C loads.
- Check for stored, pending, and permanent codes before clearing anything.
- Compare fuel trim at idle with trim held near 2,500 rpm; the direction of change helps separate load calculation from an idle-only air leak.
- Test the suspected circuit or leak directly before disconnecting parts that could temporarily change the symptom.
Decision point
Replace a part only when its input, command, output, or sealing test fails. A trim number by itself does not identify which component caused it.
Source desk
Use current model-specific service information for specifications, connector locations, disabling procedures, and tightening values. The following public resources support the broader safety or system context used in this guide.