Symptoms
- Loud popping or banging sound from the engine or exhaust while cranking
- Engine does not smoothly catch and run
- May see a flash or smoke from the exhaust or intake
- Could be a new symptom or have happened once before
Likely causes
- CommonIgnition timing significantly off (belt/chain slipped a tooth)Fuel ignites at the wrong point in the engine cycle, causing backfiring through the intake or exhaust.
- PossibleBad or crossed spark plug wires (on wire-equipped engines)Firing the wrong cylinder's spark at the wrong time causes backfiring.
- PossibleStuck or burnt exhaust or intake valveA valve that doesn't seal lets combustion escape into the intake or exhaust tract.
- PossibleVacuum leak causing a dangerously lean mixtureAn overly lean mixture can ignite unpredictably and cause backfiring.
- Less commonFailing camshaft or crankshaft position sensor confusing ignition timingBad sensor data can cause the computer to fire at the wrong time.
What to do
- Stop cranking the engine immediately after a backfire rather than trying again right away.
- Do not continue repeated attempts, since backfiring can indicate a timing problem that risks further engine damage if ignored.
- If you recently had spark plug wires, a distributor, or timing components serviced, suspect a mis-installation first.
- Check for obvious timing marks being misaligned if you or someone else recently worked on the timing belt or chain (best done by someone experienced).
- Have a mechanic check for a slipped timing belt/chain, especially if the car recently had a lot of miles on the original belt.
- Avoid opening the hood immediately after a backfire in case of a small fire; check for smoke or flame from a safe distance first.
- Have compression, timing, and spark tested together by a shop rather than replacing parts by guesswork.
- Budget for the possibility of a timing-related repair, which tends to be more involved than a simple part swap.
Tools and parts
Timing belt or chain kit (if needed)Search: “timing belt kit for [year make model]”
Spark plug wires (if applicable)Search: “spark plug wires for [year make model]”
When to call a pro
Backfiring during cranking should be diagnosed by a mechanic checking timing, compression, and ignition components before further attempts to start the car.
Safety
Use caution
A backfiring engine can occasionally produce a brief flame at the intake or exhaust; step back if you see smoke or fire and let it dissipate before approaching, and avoid repeated cranking attempts.