Bad fuel: telling from the live data
The car got worse after a fill-up. Here is how to tell bad fuel from coincidence.
«Filled up at an unfamiliar station and it got worse» is heard often and confirmed less often than you would think. The live data tells one from the other, and it takes a single trip.
What points at the fuel
Misfires across all cylinders at once. Bad fuel does not pick a cylinder — it reaches all of them. Counters ticking in one cylinder point to a plug or a coil, not to the filling station.
Fuel trims that moved after the fill-up. Diluted fuel needs different delivery and the unit shows it. The key word is «after»: a trim that was plus twelve before the fill-up has nothing to do with the petrol.
Ignition timing pulled back. The unit hears knock and retards the timing. On low-octane fuel that shows up immediately.
What does NOT point at the fuel
- A single sensor fault. Fuel does not break wiring.
- Consumption that crept up over a month. That is wear, not a fill-up.
- Hunting idle when cold. Look at the thermostat and air leaks.
Checking it in one trip
- Note the trims and misfire counters before the fill-up, if you can.
- After it, drive your usual route and look at the same numbers.
- If misfires appear across all cylinders and the trims have shifted, the fuel is suspect.
What to do
Do not top up on top of it. Adding good petrol to a tank of bad dilutes the problem; it does not remove it.
Run the tank down and refill at a station you trust. In most cases that is enough: the car recovers within a fill-up or two.
Do not «treat» it with additives at random. An additive will not fix water in the tank and will not add ten points of octane.
Honestly about the limits
A scanner does not measure octane and does not see water in the tank. It shows how the car RESPONDS to the fuel — and that is enough to tell «bad petrol» from a breakdown that merely coincided with it.
