An engine sputters when combustion is interrupted: a cylinder misfires, the mixture briefly goes too lean or too rich, or fuel delivery drops out. The most common causes are worn spark plugs or ignition coils, a clogged fuel filter or weak fuel pump, a vacuum leak, a dirty MAF sensor and contaminated fuel. When it happens tells you the most about why.
Sputtering by situation
| When it sputters | Likely causes |
|---|---|
| At idle | Vacuum leak, dirty throttle body or idle control, misfire; see rough idle |
| When accelerating | Weak spark (plugs, coils, wires), fuel delivery, dirty MAF; see car jerks when accelerating |
| At highway speed or uphill | Clogged fuel filter, weak fuel pump, low fuel pressure |
| Cold start only | Coolant temperature sensor, leaking injector, worn plugs |
| After filling up | Water or contaminated fuel |
Fuel, air and exhaust causes people miss
- Bad fuel: water or debris from a contaminated fill-up causes sputtering that starts right after refueling; draining the tank and replacing the filter fixes it.
- Dirty MAF sensor: an oily or dusty sensor under-reads airflow and the engine stumbles; see MAF sensor.
- Restricted exhaust: a collapsed pipe or blocked converter makes the engine sputter and lose power as speed rises.
- Injectors: clogged or leaking injectors upset the mixture on one or more cylinders; see fuel injector symptoms.
Backfiring and popping
A backfire through the intake usually points to a lean mixture or ignition timing problem, while popping from the exhaust often means unburned fuel igniting in the exhaust from a misfire or rich running. Both are signs to diagnose soon, because raw fuel in the exhaust can overheat the catalytic converter. Codes such as P0300, P0171 or P0172 point the way.
How to find the cause
- Read codes, including pending codes, with an OBD2 scanner.
- Check misfire counters to see whether one cylinder or several are affected.
- Look at fuel trims: high positive trims point to a lean condition; see fuel trim explained.
- Inspect plugs, coils and wires; see spark plugs, ignition coils and spark plug wires.
- Check fuel pressure and the fuel filter, and smoke-test for vacuum leaks.
Is a sputtering engine safe to drive?
Mild sputtering can be driven carefully to a shop, but a flashing check engine light, stalling or loss of power in traffic are reasons to stop. If sputtering comes with knocking, smoke or overheating, the engine may have internal damage; see how to tell if an engine is blown and our engines.
More guides: car hesitates when accelerating · car loses power when accelerating.
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