Sometimes a gas station has water, debris, or wrong-octane fuel sitting in the underground tank, and you draw the short straw. Here is how to confirm bad gas vs other engine problems, and what to do.
Most common bad-gas problem. Symptoms start within 50 miles of fueling: misfires, stalling, hard start. Drain the tank or use HEET for mild cases.
Get a full diagnosis →Gas degrades after 30-90 days. Old fuel = poor combustion, varnish, hard starting. Common after winter storage. Dilute with fresh fuel and additives, or drain.
Get a full diagnosis →Putting 87 in a car that requires 91+ = knock, pinging, retarded timing. ECU pulls timing to protect, but you lose power and economy. Fill up with the correct octane next time.
Get a full diagnosis →Underground tank sediment after a tanker delivery can get into your fuel. Plugs the filter, eventually the injectors. Replace the filter first.
Get a full diagnosis →Misfueling. Do not start the engine if you suspect this, towing and a tank drain are required. Starting circulates the wrong fuel through the entire system.
Get a full diagnosis →Some stations have E15 mislabeled or pumps with stale lines mixing in E85. Check fuel labels, get a sample tested if you suspect.
Get a full diagnosis →| What You Notice | What It Usually Means |
|---|---|
| Engine stumbles right after fill-up | Bad gas, suspect water or stale |
| Pinging / knocking on acceleration | Low octane in a premium-required car |
| Misfires at speed, fine at idle | Water contamination |
| Filter clogs quickly after fill-up | Debris in fuel |
| Black smoke at tailpipe | Wrong fuel or rich-running from bad gas |
| Long crank, hard to start | Stale fuel, low volatility |
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If your scanner is showing one of these, that's your starting point. Tap any code for full causes and repair costs.
Symptoms start within 50-100 miles of fill-up. If your car ran fine before the gas station and badly after, you have your answer.
Sometimes, with documentation. Save the receipt, ask for a sample of the fuel, get a shop diagnosis in writing. Most major chains will reimburse for proven contamination.
Usually no, if you catch it within a few hundred miles. Long-term contaminated fuel ruins fuel pumps and catalytic converters.
Only if symptoms are mild. Misfires kill catalytic converters fast, $500-$2000 to replace. If the engine is running rough, drain or dilute.
Mostly. Use Top Tier stations (Costco, Shell, Chevron, Mobil, etc.), avoid filling up when a tanker is at the station (sediment is stirred up), and avoid low-volume stations.
Do not start the engine. Tow to a shop, drop the tank, drain, refuel. Starting circulates diesel through the fuel pump and injectors, multiplying damage 5-10x.
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