FAQ
TL;DR: A 10 % diesel-in-petrol mix typically drops octane by ~4 RON [Shell, 2021]; “the mixture up to 10% DOES NOT bother” [Elektroda, Ermax, post #15406160] Most engines will run but expect hard starts and plug fouling.
Why it matters: Even small octane loss can trigger destructive knock in modern high-compression petrol engines.
Quick Facts
- Safe dilution threshold: ≤10 % diesel in petrol [Elektroda, Ermax, post #15406160]
- Octane drop: ≈0.4 RON per 1 % diesel [Shell, 2021]
- Cold-start crank time may rise by 30 %–50 % [Elektroda, wowka, post #15408308]
- Tank drain service cost: €100–€200, roadside average [AA, 2023]
- Turbo GDI knock damage can occur within 5 km under load [Bosch, 2020]
Will 6 L of diesel in 60 L of petrol harm my gasoline engine?
Most port-injected engines survive a 10 % mix; power drops and starts are harder but no lasting harm if refilled soon [Elektroda, Ermax, post #15406160]
Why does diesel lower octane and cause knock?
Diesel molecules resist spark ignition yet dilute petrol’s antiknock additives, cutting octane ~4 RON at 10 % concentration [Shell, 2021]; lower octane increases knock risk in 95-RON-calibrated engines [Elektroda, Xantix, post #15409167]
What symptoms should I expect while driving on the mix?
Expect lazy throttle, rough idle, white or grey smoke, and longer cold starts [Elektroda, bula17, post #15406525] Spark-plug fouling can suddenly stall the engine after several kilometres [Elektroda, wowka, post #15408308]
Is it safer to drain the tank or burn the fuel off?
Drain if diesel exceeds 10 % or if you drive a knock-sensitive turbo GDI. Otherwise, gentle driving and topping with fresh petrol usually works [Elektroda, jas67, post #15406424]
How can I dilute the contaminated fuel quickly?
- Add at least 30 L of correct petrol.
- Drive 10 km at light throttle to mix.
- Refill to full again at half-tank.
This halves the diesel share each refill [Elektroda, Anonymous, post #15406046]
Can modern FSI/GDI or turbo engines handle the mix?
They run higher compression; even 5 % diesel can trigger knock under boost and damage pistons [Bosch, 2020]. Avoid load and dilute below 3 % for safety [Elektroda, ociz, post #15408488]
Will the spark plugs really foul?
Yes. “Candles will throw syfem” [Elektroda, andrzej20001, post #15406035] Unburned diesel leaves carbon and oil that bridge electrodes, killing all plugs at once [Elektroda, wowka, post #15408308]
Can the fuel pump or injectors suffer long-term damage?
Gasoline pumps cope; but if diesel settles, waxes can clog filters and varnish injectors after weeks [Elektroda, andrzej20001, post #15408438]
Why doesn’t the diesel nozzle fit most petrol fillers?
Modern petrol fillers are 21 mm; diesel nozzles are 25 mm by ISO 17268. Older cars lacked this safeguard, so cross-fueling was possible [Elektroda, janek1815, post #15409715]
What if I accidentally fill a diesel car with petrol instead?
Petrol strips diesel’s lubricity; a 20 % mix can scar injection pumps within minutes, demanding €1 000+ repairs [AA, 2023]. Drain immediately [Elektroda, michal_rybka, post #20314112]
Is separation a problem if the car sits?
Yes. Petrol and diesel stratify within hours; the next cold start can pull near-pure diesel, stalling the engine [Elektroda, jas67, post #15406424]
Edge case: What happens in freezing weather?
Diesel can gel below −7 °C, blocking petrol filters and starving the engine, even at 5 % concentration Winter Fuel Guide.