Misfire at Idle But Fine When Driving: Causes
Key Takeaway
Misfire only at idle? Drives fine otherwise? Here are the specific causes.
Why Misfires Happen Only at Idle
At idle, the engine runs at very low RPM and load. Components that work fine under load can fail at idle because the margins are thinner.
Idle-Only Misfire Causes
| Cause | Why It's Idle-Only | Fix Cost |
|---|---|---|
| Vacuum leak | Less air at idle makes leak proportionally larger | $100 – $400 |
| Dirty throttle body | Carbon restricts idle airflow | $15 (DIY clean) |
| Weak spark plug | Enough spark at high RPM, not at low | $20 – $40 |
| IAC valve stuck | Controls idle air; doesn't affect driving | $100 – $250 |
| PCV valve stuck open | Excess crankcase vapor at idle | $20 – $50 |
| Low idle speed | ECM idle setting too low | Reset/relearn |
Top Fix
Start with spark plugs and a throttle body cleaning — these together fix ~60% of idle-only misfires.
ProHow Steer Helps
Steer detects idle-specific misfires and distinguishes them from load-based misfires for faster diagnosis.
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