What to Do If You Accidentally Put Diesel Fuel in an Unleaded Engine

Accidentally putting diesel fuel into a petrol (unleaded) engine is a common mistake that can cause serious damage if not handled quickly and correctly. If you find yourself in this situation, knowing the right steps to take can save you from costly repairs and long delays. This guide explains what happens when diesel fuel enters an unleaded engine, how to spot the problem early, and what actions to take immediately.

Petrol pump nozzle in car fuel tank

Diesel Fuel in Unleaded Engine

Petrol engines and diesel engines work differently and require different types of fuel. Petrol engines rely on spark ignition, while diesel engines use compression ignition. Diesel fuel is thicker and less volatile than petrol, which means it does not evaporate or combust properly in a petrol engine.

When diesel fuel enters an unleaded engine, it can cause:

• Clogged fuel injectors and filters due to diesel’s thicker consistency

• Poor combustion or engine misfiring because diesel does not ignite properly in petrol engines

• Damage to the fuel pump and engine components if the vehicle is driven with the wrong fuel

• Increased smoke and exhaust emissions as the engine struggles to burn the diesel

  
  

The severity of the damage depends on how much diesel was added and whether the engine was started or driven.

How to Recognize Diesel in Your Petrol Tank

If you accidentally fill your petrol car with diesel, you might notice some warning signs before serious damage occurs:

• Engine won’t start or struggles to start: Diesel fuel won’t ignite properly in a petrol engine.

• Rough idling or stalling: The engine may run unevenly or stall soon after starting.

• Loss of power: The car may feel sluggish or unresponsive.

• Excessive smoke from the exhaust: Diesel combustion produces thick smoke in a petrol engine.

• Warning lights on the dashboard: The check engine light or other indicators may come on.

  
  

If you notice any of these symptoms shortly after refueling, stop driving immediately.

Immediate Steps to Take If You Put Diesel in an Unleaded Engine

Do Not Start the Engine

If you realize the mistake before starting the car, do not turn the ignition on. Starting the engine will circulate diesel fuel through the system, increasing the risk of damage.

Call for Professional Help

Contact a specialist service like Misfuelled Car Fix at 0800 999 1182. They have the expertise and equipment to safely drain and clean your fuel system.

Drain the Fuel Tank

The fuel tank needs to be completely drained to remove all diesel fuel. This process requires professional tools and knowledge to avoid spills and contamination.

Flush the Fuel System

After draining, the fuel lines, injectors, and filters should be flushed to remove any remaining diesel residue.

Replace Fuel Filters

Diesel can clog petrol fuel filters, so replacing them is essential to prevent future problems.

Refill with Correct Fuel

Once the system is clean, refill the tank with the correct unleaded petrol.

What Happens If You Drive With Diesel in a Petrol Engine

Driving even a short distance with diesel in a petrol engine can cause:

• Fuel pump damage: Diesel’s viscosity can strain the pump designed for petrol.

• Injector clogging: Diesel leaves deposits that block injectors.

• Engine misfires and stalling: The engine struggles to combust diesel.

• Potential engine failure: Prolonged use can cause severe damage requiring expensive repairs.

  
  

If you have driven the car after misfuelling, it is even more important to get professional help immediately.

How Professionals Fix Diesel in Unleaded Engines

Specialist mechanics follow these steps to fix misfuelled cars:

1. Assess the situation: Check how much diesel was added and if the engine was started.

2. Drain the fuel tank: Remove all contaminated fuel safely.

3. Flush the fuel system: Clean fuel lines, injectors, and pumps.

4. Replace fuel filters: Install new filters to avoid clogging.

5. Test the engine: Run diagnostics to ensure no lasting damage.

6. Refill with unleaded petrol: Use the correct fuel to restart the engine.

   
   

Using a service like Misfuelled Car Fix (phone: 0800 999 1182) ensures the job is done properly and reduces the risk of long-term damage.

Preventing Diesel Misfuelling

Avoiding this mistake is the best way to protect your car. Here are some tips:

• Double-check the pump before filling: Look at the nozzle and fuel type.

• Use different fuel caps: Some drivers use petrol caps that don’t fit diesel nozzles.

• Pay attention at the pump: Avoid distractions while refuelling.

• Educate other drivers: If others use your car, make sure they know the correct fuel type.

• Use warning stickers: Place a visible label near the fuel cap.

  
  

What to Expect After Repair

After professional cleaning and repair, your car should run normally again. You might notice:

• Improved engine performance

• No unusual smells or smoke

• Normal fuel consumption

  
  

If problems persist, return to the mechanic for further checks.