Fault Tree: Rough Idle / Misfire

Quick triage & tools

Scan first: read codes (all modules if you can) + live data that matches the symptom.
Power basics: battery voltage (resting + while cranking) and quick fuse checks before deep dives.
Have ready: basic scan tool, multimeter, and a way to confirm the symptom (road test / idle test).

Tip: if a step says “check wiring”, use Wiring diagrams basics → and Voltage drop testing →

Start with the pattern

Only at idle → vacuum leak, EGR stuck open, PCV/boost leak, intake leak, injector drip.
Only under load → ignition breakdown (coil/plug), fuel pressure drop, boost leak, knock control.
Cold only → coolant temp plausibility, air leaks, enrichment issues, weak spark.
Single cylinder → plug/coil/injector/compression. Random → air/fuel metering, pressure, timing, exhaust restriction.

Related: Misfire Diagnosis deep-dive →

Decision flow

Scan codes + look at Mode $06 / misfire counters (if available)
Note which cylinder(s) and whether it’s idle or load related.
Check fuel trims at idle and at ~2500rpm steady
- High positive trims at idle that improve off-idle → vacuum leak / PCV leak / intake leak.
- High positive trims that stay high at rpm → low fuel pressure/volume, MAF under-reporting, exhaust leak before O2.
- Negative trims → leaking injector, high fuel pressure, EVAP purge stuck open, MAF over-reporting.
Use: Fuel trim patterns (advanced) →
Quick air-leak test
- Inspect intake boots, PCV hoses, brake booster hose, turbo pipework (if boosted).
- If you can: smoke test (best) or use a vacuum gauge for pattern changes.
Use: Smoke testing done right → • Vacuum gauge testing →
Ignition vs fuel: do the easy swap test (only if single-cylinder)
Swap coil/plug with another cylinder. If the misfire follows, you’ve found it.
Fuel pressure check (especially if under-load misfire)
A pump can pass at idle and fail at load. Look for pressure drop when revving/under load (if safe).

Use: Fuel pressure testing →
5V reference / sensor grounds sanity check (if multiple sensors look wrong)
A shorted sensor can pull the 5V rail down and create “phantom” issues.

Use: 5V reference & sensor grounds →
Mechanical check if it won’t resolve
- Compression / leak-down (burnt valve, ring issue).
- Timing correlation codes (P0016/P0017) or VVT issues can cause rough running.
Use: P0016/P0017 timing correlation → • VVT performance (P0011–P0014) →

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Print / save checklist

Tick these off as you work. If you need to hand this to a mechanic, print it as a short job card.

Freeze-frame captured / conditions noted
Battery voltage checked (resting + cranking)
Basic visual checks (hoses, connectors, grounds, fuses)
One test at a time (don’t change multiple variables)
Confirm fix by reproducing the original condition

Print / Save as PDF Back to symptom selector

What to do next

Use the links below to deepen the test you’re about to perform, cross-check related codes, or jump to a faster symptom-led flow.

Find another symptom flow Jump to the symptom selector to locate the closest decision tree. Workshop Guides Deep-dive how-to tests: voltage drop, wiring diagrams, smoke testing, fuel pressure and more. Diagnostic Codes Look up DTC meanings, common causes, and related checks. AI Tools Use AI assistance to summarise symptoms, plan tests, or sanity-check a diagnosis.