Bad fuel and misfuelling: what to do and what not to do

Fuel is the only consumable you buy on trust. Its quality cannot be checked at the pump, and the consequences of a bad tankful can cost more than a year of price differences saves.

The symptoms of bad fuel

They do not appear immediately but tens of miles after filling, once the new tank reaches the engine.

Loss of power, especially uphill and when overtaking.

Rough running, shunting, misfires — and a Check Engine light, often flashing.

Hard starting, or stalling at idle.

Detonation — the characteristic metallic rattle under load on a petrol engine.

A sharp rise in consumption with no change in driving style.

The most important clue: the link to a specific filling station. If symptoms began after a specific tankful, keep the receipt — it is the only document you will have in any dispute.

Water in the fuel

It arrives as condensate in the station’s tank or in your own.

On petrol: misfires, hard starting, and in winter freezing in the fuel line.

On diesel it is more serious. Diesel fuel lubricates the pump and injectors. Water does not lubricate — it causes scoring, and those are the most expensive components in the engine; covered in the common rail injector article.

A water-in-fuel sensor is fitted to many diesels, and its warning must not be ignored: it warns about exactly this.

Prevention is simple: do not run permanently on an empty tank. Condensate forms on the walls of empty space.

Misfuelling

The most expensive mistake at a filling station, and the response is the same in both directions.

The main rule: do not start the engine.

While the fuel is in the tank, this is a drain and a flush. Once it has been started, it is distributed through the whole system and the bill multiplies.

Petrol in a diesel is the worse case. Petrol strips the lubricating film from the high-pressure pump, and scoring begins within minutes of running. That is the most expensive scenario available.

Diesel in a petrol is milder. The engine smokes, runs rough, and stalls. The consequences are usually limited to flushing the system, spark plugs, and sometimes the catalyst.

What to do: do not switch on the ignition, do not crank, call recovery, drain the tank.

What diagnostics shows

Misfire codes — P0300 and per-cylinder ones; with bad fuel they are usually random rather than confined to one cylinder; covered in the misfire article.

Fuel trims. A sharp shift after refuelling is strong evidence that the fuel composition changed; mechanics in the fuel trim article.

Knock codes — the module detects detonation and retards ignition timing, which takes power away. That explains a loss of performance with no obvious symptom.

Rail pressure on a diesel — commanded versus actual; a shortfall points at the pump or blocked injectors.

Freeze frame — showing the conditions when the fault set, which can be compared against when you filled up.

What to do if you suspect bad fuel

1. Keep the receipt.
2. Do not “top up good over bad” — that dilutes without solving.
3. Read the codes and do not clear them before the investigation.
4. Replace the fuel filter — it takes the first hit.
5. On a diesel, check for water in the filter and drain the sediment.
6. If the symptoms are serious, drain the tank — cheaper than injectors.

What not to do

Do not pour in “cleaning” additives hoping to solve an acute problem. They work as prevention, not as repair.

Do not drive with a flashing Check Engine — those are active misfires destroying the catalyst right now.

Do not start the car after misfuelling — not even “just to check”.

In short

• Symptoms appear tens of miles after filling; the receipt is your only document.
• Water in diesel is more dangerous than in petrol: there the fuel is also the lubricant.
• Misfuelling: do not start it — that is the difference between a flush and a repair.
• Petrol in a diesel is worse than diesel in a petrol.
• A sharp fuel trim shift after refuelling is strong objective evidence.

Download the app — codes, fuel trims, and freeze frames are read with an ELM327 adapter over Bluetooth SPP on Android.