What Happens If You Put Diesel in a Petrol Car? Here’s What You Should Do

You can accidentally fill a petrol car with diesel, although drivers make this mistake less often than putting petrol in a diesel car. Diesel dispenser nozzles are generally larger, so they may not fit easily into a petrol car's narrower filler neck. However, the physical difference does not guarantee that you will avoid misfuelling.

The important part is what you do after noticing the mistake. If you have not started the engine, the wrong fuel remains largely contained in the tank. That gives you the best opportunity to have a professional remove it before it reaches the engine's fuel system.

What Happens When Diesel Goes Into a Petrol Car?

Petrol and diesel engines ignite fuel in different ways. A petrol engine uses a spark plug to ignite the fuel-air mixture, while a diesel engine injects fuel into highly compressed hot air, causing the fuel to ignite. Diesel also differs from petrol in its physical and combustion properties. Diesel fuel has greater lubricity and works with compression-ignition engines, while petrol suits spark-ignition engines.

As a result, a petrol engine cannot burn diesel properly through its normal combustion process. Instead of running normally, the engine can begin to misfire, lose power, produce heavy smoke, stall or refuse to restart.

What Should You Do If You Put Diesel in a Petrol Car?

First, do not start the engine. If you realise the mistake while you are still at the fuel station, leave the car switched off and tell the station staff what happened.

Do not try to solve the problem by simply filling the rest of the tank with petrol and driving away. The concentration of diesel represents only one part of the problem. Once the contaminated fuel circulates through the fuel system, it can affect more components.

Instead, arrange for a professional to drain the contaminated fuel before you drive the car. A recovery service or qualified workshop can determine whether the tank and fuel system need draining, cleaning or further inspection.

Follow this checklist

• Engine off: Do not start the engine or deliberately turn the ignition on.
• Still at the pump: Tell the station staff and move the car only if you can do so without starting it.
• Already started: Switch the engine off as soon as it is safe to do so.
• Already driving: Pull over somewhere safe and switch the engine off.
• Call for help: Arrange professional fuel draining instead of attempting to drive to a workshop.
• Tell the mechanic exactly what happened: Explain that you added diesel to a petrol car and say whether you started the engine.

The earlier you stop the contaminated fuel from circulating, the simpler the recovery can be.

What If You Already Started the Petrol Car?

Do not panic, but stop driving as soon as it is safe. A petrol engine running on diesel may start misfiring, lose power, produce excessive smoke or stall. Diesel can also contaminate components such as the fuel filter and spark plugs, particularly after the wrong fuel circulates through the system.

The next step remains the same: have a professional remove the contaminated fuel. Depending on what happened, a mechanic may need to inspect the fuel system and ignition components before returning the vehicle to normal use. The fact that the car still starts does not mean you can safely continue driving with the contaminated fuel.

What If You Drove Several Kilometres?

Driving several kilometres after misfuelling creates a more serious situation than noticing the mistake before starting the engine, but it does not automatically mean you have ruined the engine. Several factors determine how much attention the car needs, including how much diesel entered the tank, how much petrol was already present and how long the engine ran with the contaminated mixture.

Symptoms such as persistent misfiring, heavy smoke, loss of power or stalling indicate that you should stop driving and arrange a professional inspection. Do not keep driving simply because the car still moves. Continuing to circulate contaminated fuel can turn a problem that a simple drain might have resolved into a more involved repair.

Diesel in Petrol vs Petrol in Diesel: Which Is Worse?

Putting diesel in a petrol car generally causes less damage than putting petrol in a diesel car, but you should not treat either mistake casually.

The difference partly comes from how the two fuel systems work. Diesel fuel provides important lubricating properties within diesel fuel systems, while petrol has much lower lubricity. As a result, diesel fuel systems can become particularly vulnerable when petrol replaces or heavily contaminates the diesel.

The key point is that “less serious” does not mean “safe to drive.”

Can You Fix Diesel in a Petrol Car by Adding More Petrol?

Do not rely on topping up with petrol as a DIY solution. The amount of diesel in the tank, the vehicle's fuel-system design and whether you have already started the engine all affect what you should do next. No universal percentage applies safely to every petrol car.

Professional draining removes the uncertainty. After removing the contaminated fuel, a mechanic can check the vehicle before you return it to normal driving.

How Can You Avoid Misfuelling Again?

The simplest way to avoid misfuelling is to pause before picking up the nozzle, especially if you regularly drive more than one car or have recently switched between petrol and diesel vehicles. Check the fuel type marked on the vehicle before filling.

Do not rely solely on the colour of the pump handle because fuel stations can use different colour conventions. Reading the label takes only a few seconds and removes the guesswork. If someone else normally fills your car, clearly tell them whether it takes petrol or diesel. You can also place a small reminder inside the fuel flap if different people occasionally drive the car.

What Should You Do If You Put Diesel in a Petrol Car?

If you realise you have filled a petrol car with diesel, do not start the engine. Arrange for a professional to drain the contaminated fuel and check the car before you drive it normally. If you have already started or driven the car, stop as soon as it is safe and get professional help. Diesel in a petrol car can cause serious running problems, but catching the mistake early can prevent a much bigger headache.

At the fuel pump, a five-second double-check costs less than a very expensive “petrol ya diesel?” conversation later.