What do I do if I put the wrong fuel in my car?
The single decision that separates a drain-and-go from a fuel system rebuild, why petrol in a diesel is the expensive way round, and what to do if you have already driven off.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 4 min
- Safety
- warning
Short answer
Do not start the engine. If the key has not been turned, the wrong fuel is sitting in the tank and a drain and flush usually fixes it completely. Tell the forecourt staff, do not try to push the car yourself, and call a fuel-drain service or your breakdown provider. If you have already driven, stop as soon as it is safe and switch off — every additional minute of running spreads the damage.
This happens to a large number of drivers every year, and it is far more often petrol into a diesel than the other way round — partly because a petrol nozzle fits easily into a diesel filler, while a diesel nozzle is deliberately made too wide for most petrol fillers. The direction matters because the consequences are different. Petrol in a diesel is the damaging combination: diesel fuel lubricates the high-pressure pump and injectors, and petrol acts as a solvent that strips that lubrication, so running the engine can wreck the fuel system. Diesel in a petrol car mostly causes misfiring, smoke and a refusal to run, which is unpleasant but usually less destructive.
Safety
Step by step
- Stop filling the moment you realise.The less that goes in, the more likely the tank can be diluted or drained without complication. A small amount in a nearly full tank of the correct fuel is a much better position than the reverse.
- Do not turn the ignition on at all.This is the whole ballgame. On most modern cars the fuel pump primes as soon as the ignition is switched on, which draws the wrong fuel out of the tank and into the lines, filter, pump and injectors. Leaving the key alone keeps the problem in one place.
- Tell the forecourt staff straight away.They deal with this regularly, they know where it is safe to leave the car, and many sites have a designated bay. They can also help push the car safely away from the pumps with the right number of people, which is not something to attempt alone in traffic.
- Call a fuel-drain specialist or your breakdown cover.Mobile fuel drain services attend forecourts routinely and can often drain, flush and refill on the spot in under an hour. Check your breakdown policy and your insurance — misfuelling is covered by some policies and by some new-car assistance packages.
- If you have already driven, stop immediately and switch off.Do not try to reach home or a garage. The engine will usually be running badly, losing power and smoking. Every minute of running circulates more of the wrong fuel through parts that are expensive to replace.
- Be honest with whoever works on it.Say exactly how much went in, whether the ignition was switched on, and how far the car was driven. That is the information that determines whether it is a drain, a drain plus filter, or a fuel system strip.
- Prevent the repeat.It most often happens with an unfamiliar car — a hire car, a courtesy car, or the first fill after changing car. A label on the filler flap or a note on the key is a trivial precaution and works.
Common mistakes
- Starting the engine "just to move it off the forecourt" — It is the exact action that converts an inexpensive drain into a fuel system repair. Ask the staff for help moving it instead.
- Topping up with the correct fuel to dilute it — It might reduce the concentration, but it also means far more contaminated fuel to dispose of and does not remove the wrong fuel from the system. Get advice first.
If it doesn't work
Petrol in a diesel, engine never started
Cause: Wrong fuel confined to the tank — Fix: Drain, flush and refill. Usually a straightforward job with no lasting damage, and the cheapest possible version of this problem.
Petrol in a diesel, driven several miles
Cause: Petrol circulated through pump and injectors — Fix: Stop, do not restart, and have it recovered. It may need the fuel system flushed and, in bad cases, the high-pressure pump and injectors replaced.
Diesel in a petrol car
Cause: Diesel does not vaporise or ignite properly in a petrol engine — Fix: Drain and flush. It usually causes heavy white smoke, misfiring and a refusal to run rather than mechanical damage, and often clears completely after a proper flush and new plugs if needed.
A small splash of the wrong fuel into a nearly full tank
Cause: Heavily diluted — Fix: Get advice before assuming it is fine. A very small proportion is sometimes tolerable, but the ratio matters and it is not a judgement to make by feel with a modern common-rail diesel.
AdBlue accidentally added to the diesel tank
Cause: Adjacent fillers — Fix: Same rule and even more strictly: do not start it. AdBlue in the fuel system is very damaging and needs professional draining.
Questions people ask
How much does a fuel drain cost?
It varies by provider and location, but it is very substantially less than any fuel system repair, which is the comparison that matters. Some insurance policies, breakdown memberships and new-car assistance packages cover it — check before assuming you are paying.
Why is it always petrol in a diesel?
Because a petrol nozzle is narrower and slides into a diesel filler neck easily, while a diesel nozzle is made wider precisely so it will not fit most petrol cars. Many diesel cars now also have a misfuelling device in the filler neck for the same reason.
Will my insurance cover the damage?
Misfuelling damage is often excluded from standard comprehensive cover because it is not accidental damage in the usual sense, though some policies and add-ons include it. Read the policy rather than assuming either way, and note that the drain and the damage may be treated differently.