Put petrol in diesel car? What to do in the next ten minutes
Filled a diesel with petrol? The next ten minutes decide whether this costs a drain or a fuel system. Do not start the engine, and read this.
Put petrol in diesel car, now what? It happens to careful people every week, usually in a hire car, a partner's car, or the first week with a new car after years of the other fuel. The difference between a modest drain and a bill in the thousands comes down to what you do in the next ten minutes, and the single rule is this: do not start the engine. Do not even turn the key to the ignition position if you can avoid it.
Below is what petrol does inside a diesel, the three scenarios we see, what a drain and flush involves and costs, where insurance stands, the rule about draining on a forecourt, and how to stop it happening again.
What petrol does inside a diesel engine
Diesel is an oily fuel, and the high pressure pump and injectors in a modern diesel rely on it for lubrication. Petrol is a solvent. The moment petrol reaches the pump, it strips that lubrication away and the pump's finely machined parts start running metal on metal. The wear produces tiny metal particles that travel on into the injectors.
That is why misfuelling is a bigger problem than it was twenty years ago. An old mechanical diesel would run badly on a petrol mix and survive. A common rail diesel with a pump running at enormous pressure can be damaged in minutes of running. The tank and the pipes do not care. It is the pump and injectors that pay.
Put petrol in diesel car: the three scenarios
Everything about the cost and the fix depends on how far the petrol got. Be honest with whoever comes out, because it changes what they need to do.
Petrol in a diesel car and it didn't start
This is the best case. The petrol is sitting in the tank and has not been pumped forward. The fix is a tank drain, a flush with clean diesel, and a refill. No parts, no pump damage. One caution: on many cars, turning the ignition on without starting runs the lift pump for a few seconds, which pushes fuel towards the engine. So leave the key out and push the car rather than start it.
Started, idled, stalled
You started it, it ran for a minute, maybe it coughed and died. Petrol is now in the lines, the filter, the pump and the injectors. The job becomes a tank drain plus a full flush of the fuel system and usually a new fuel filter. In most cases the pump survives a minute of idling, but we cannot promise that from the roadside, and a garage should check it runs cleanly afterwards.
Driven until it stopped
If the car was driven out of the forecourt and down the road before it lost power, smoked, rattled or stalled, assume the pump has been working without lubrication the whole time. A drain and flush is still the first step, but the car needs to go to a garage for a proper look at the high pressure pump and injectors, and there is a real chance some of them need replacing. The amount matters too. A splash into a near full tank of diesel is survivable. A full tank of petrol driven for ten kilometres usually is not.
The forecourt rule: nobody drains on the forecourt
If you realise at the pump, tell the staff and do not start the car. Nobody drains fuel on a forecourt, not the station and not us, because pumping fuel out beside live pumps and other customers is a fire risk. The car has to be moved off the forecourt first, either pushed to a safe corner of the site if the staff allow it, or lifted onto the flatbed and taken somewhere suitable.
That is the normal sequence for a wrong fuel recovery in Kilkenny: we arrive, winch the car onto the truck without starting it, move it to a safe spot or to the garage in Three Castles, and do the drain there. Ring 087 288 2280 from the forecourt and tell us the car, the fuel and whether it was started.
What a drain and flush involves
The contaminated fuel is pumped out of the tank, either through the filler neck or through the tank's drain point depending on the car. Modern tanks have anti-siphon valves, baffles and saddle shapes, so this is slower than it sounds and sometimes means dropping the tank. Once empty, the tank gets a rinse with clean diesel, which is pumped out again. If the engine was run, the lines are flushed through, the filter is replaced and the system is bled.
The old fuel is waste and has to be disposed of properly. Then the car gets enough clean diesel to run and is started and run until it idles smoothly with no smoke or misfire. For a car that was not started this is an hour or so. For one that was driven, the diagnosis afterwards is what takes the time.
What it costs, in general terms
A drain and flush on a car that was not started is a few hundred euro depending on where the car is and how awkward the tank is, plus the cost of the fuel to refill it. If the engine was run, the fuel filter and the extra flushing add to that but it is still in the hundreds.
The expensive outcome is a damaged high pressure pump and injectors on a car that was driven, where parts and labour run into the thousands and can exceed what an older car is worth. That is the whole reason for the do not start rule. We agree the price on the phone before the truck moves, and there is no call-out fee.
Diesel in a petrol car
This is harder to do, because the diesel nozzle is wider than most petrol filler necks, but it happens with older cars and with jerry cans. Diesel in a petrol engine does not cause the same wear, because a petrol fuel system does not rely on the fuel for lubrication. What happens instead is that the engine will not ignite the diesel properly. It runs rough, smokes heavily, misfires and may not start at all, and if run for long it can foul the spark plugs and clog the catalytic converter.
The advice is the same: do not start it, get it drained. A splash of diesel in a nearly full petrol tank can sometimes be diluted by topping up, but with a catalytic converter it is not worth the gamble.
Will insurance cover misfuelling?
Some comprehensive policies in Ireland cover damage from misfuelling and some exclude it specifically, so read the wording or ring the insurer rather than assume. Even where it is covered, claiming for a drain that costs a few hundred euro can cost a no claims bonus worth more, so most people pay for the drain themselves and only involve the insurer if the pump and injectors are damaged.
Breakdown cover is separate. If your policy includes it, ring the insurer's assistance line first, because the cover normally only pays for recovery they arrange, not for help you organise yourself. If you have none, or the wait leaves you stuck on a forecourt for hours, ring us. Hire cars are different again: most rental agreements make the driver responsible for misfuelling in full, so ring the rental company first.
How to stop it happening again
Nearly every misfuelling we attend has the same story behind it: a car that was new to the driver, a fill done while talking or on the phone, or an early morning on autopilot. A few habits remove most of the risk.
- Read the inside of the fuel flap every time. Most cars have the fuel type printed there.
- Say the fuel out loud before you lift the nozzle. It sounds daft and it works.
- Do not rely on pump handle colours. They are not consistent between stations.
- For the first few weeks in a new or borrowed car, put a note on the fuel flap or dashboard.
- Fit a misfuelling prevention insert to the filler neck of a diesel if you share the car or run a fleet.
Questions people ask about this
I put petrol in my diesel car and didn't start it. Is it damaged?
Almost certainly not. The petrol is sitting in the tank. Leave the key out, do not turn the ignition on, and have the tank drained and flushed before it is started. No parts should be needed.
How much petrol in a diesel is too much?
Any amount is a risk in a modern common rail diesel, but a small splash into a nearly full tank is far less serious than a full fill. If in doubt, drain it rather than hope dilution will do.
Can the fuel be drained at the petrol station?
No. Stations do not allow draining on the forecourt because of the fire risk. The car is pushed or lifted off the forecourt first and drained in a safe place, usually at our garage in Three Castles.
Is diesel in a petrol car as bad?
It is less damaging because a petrol engine does not depend on the fuel for lubrication, but the car will run badly or not at all and can damage the catalytic converter if driven. Drain it before starting.