Bad fuel: how to recognise it over OBD and tell it apart from a real fault

A familiar scenario: you fill up at an unfamiliar station, drive six miles, and the car is a different animal. Pulls worse, stumbles under load, sometimes lights the lamp.

The question you need answered quickly: is this the fuel, or did something break just now? The answer determines whether you drain the tank or book a workshop. The data settles it in ten minutes.

The strongest sign is timing

The most powerful argument is not in the values themselves but in when they changed.

Open the freeze frame of the stored code and look at the mileage and conditions. If the fault was recorded within a few tens of miles of the fill-up — and nothing preceded it — the fuel theory moves to first place. What else that snapshot reveals is covered in its own guide.

Mechanical faults do not behave this way. An injector does not die precisely as you pull away from the pump.

Petrol: octane below specification

The most common case is fuel with a lower octane rating than advertised.

What the ECU does. The knock sensor hears detonation, and the ECU starts retarding ignition timing. The engine survives, but loses power and drinks more.

What the data shows. Continuously active knock correction under load. On decent fuel the correction sits near zero and moves occasionally; on bad fuel it subtracts degrees without pause.

That is the most reliable indicator of fuel rather than failure. How the system works is in the knock sensor guide.

Codes: P0325 appears when the sensor itself is faulty, not when the fuel is poor. With bad fuel there may be no code at all — only the correction behaviour.

Petrol: water or contaminants

The worse case. Water does not burn, and the cylinder that receives it misfires.

What you see: P0300 or individual cylinder codes, misfire counters climbing, unstable idle. Frequently the picture wanders: present, then absent, depending on how the water has mixed in the tank.

How it differs from a failed coil: a coil produces a stable misfire in the same cylinder every time. Water produces misfires that migrate between cylinders. The way to separate them is in misfire troubleshooting.

Petrol: enrichment or enleanment

Fuel of a different density or with additives shifts the mixture ratio, and the module compensates through fuel trims.

What you see: long-term trim moving negative (rich) or positive (lean) on both banks simultaneously and by roughly the same amount.

That symmetry matters. A vacuum leak almost always affects one bank more. An equal shift on both is something that arrived through the fuel. Details in the fuel trim guide.

Diesel: a different picture

On a diesel, bad fuel shows differently.

• Low cetane number — hard starting, harsh combustion, whitish smoke when cold.
• Water — the dangerous one. The injection system is lubricated by the fuel itself; water washes the film away and destroys the pumping elements. Here the stakes are the cost of a high-pressure pump.
• Excess sulphur — attacks the particulate filter and catalyst.

What the data shows: unstable fuel rail pressure, a gap between commanded and actual values, codes P0087 and P0089. The fuel system is covered in common rail injector diagnostics.

Separately: modern diesels have a water-in-fuel sensor in the filter. If it has triggered, the question is closed — it is the fuel, and you should not drive on.

Telling it apart from a genuine fault

Sign	Fuel	Fault
When it started	Immediately after a fill-up	Gradually, or unrelated
Both banks	Shift is equal	Usually one worse
Misfires	Migrate between cylinders	Tied to one cylinder
After changing fuel	Clears within 1–2 tanks	Unchanged
Knock correction	Continuously active	Occasional

The last row is the simplest test available: put decent fuel in and run the tank through. Symptoms gone, question closed. Symptoms remain, go looking for a fault.

What to do right now

1. Keep the receipt. Without it no complaint is possible.
2. Do not clear the codes. The freeze frame with its mileage stamp is your main evidence.
3. If water in a diesel is suspected, do not drive. The gap between draining a tank and replacing the injection system is too large.
4. On petrol, dilute. Add decent higher-octane fuel and drive gently, off load.
5. Watch the trims across the next two tanks.

Prevention

• Do not fill to the brim at an unfamiliar station — half a tank is enough to get home and check.
• Do not fill up while a tanker is unloading: fresh delivery stirs sediment off the bottom of the storage tank.
• In winter, keep an eye on the fuel filter — condensation in the tank collects exactly there.
• Look at fuel trims periodically as a general health indicator, not only when something already feels wrong.

In short

• Symptoms tied to the moment of fuelling are the strongest argument.
• Petrol: continuous knock correction, symmetric trim shift, migrating misfires.
• Diesel: unstable rail pressure, and a water sensor is a verdict on its own.
• The test is easy: put good fuel in and run a tank through.
• Keep the receipt, do not clear the codes.

Download the app — knock correction, fuel trims, and misfire counters are read with an ELM327 adapter over Bluetooth SPP on Android.