What happens when you put petrol in a diesel engine
What happens when you put petrol in a diesel engine?

WRONG FUEL!!! ( more likely petrol in a diesel engine )

Putting the wrong type of fuel (petrol in a diesel engine) in your car can be a costly mistake. As soon as you’ve put the wrong fuel in and turned the key then, the engine is gone and you’re stuck at the garage.

Putting petrol in a diesel car, or diesel in a petrol vehicle is a surprisingly easy mistake to make. From an over-tired parent simply reaching for a pump, to being distracted by another work email, or simply forgetting to check the pump, it’s no surprise to see so many drivers join this ever-growing fuel mix-up statistic.

95% of misfuelling happens when petrol in a diesel engine or car, this is because diesel has a wider nozzle, which is harder to fit into a petrol car.

Let’s start our detailed discussion on what you should not do if you put petrol in a diesel engine by accident.

What to do when you put petrol in a diesel car?

Important: If you’ve used the wrong fuel ( petrol in a diesel engine ), do not switch on the engine or put the key in the ignition.

Putting petrol in a diesel engine can cause more damage than putting diesel in a petrol. If this does happen, don’t fret –  there is significant damage limitation if you realize you’ve put the wrong fuel in your car before you start the engine. All you have to do is :

First of all, and this is most important: Do not start your engine. Don’t even put the key in the ignition.

If you’ve realized your mistake before starting the engine, here’s what you do:

  • Tell the petrol station what’s happened.
  • Put the car in neutral.
  • Have someone help push your car to a safe place.
  • Call your insurer as soon as you can.
  • If you have breakdown cover, give them a call – they should be able to drain, flush and refuel your car.

If you aren’t so lucky and you drive off with the wrong fuel, you’ll soon realize what’s happened when the car grinds to a halt.

What is going to happen in Garage

Then it is just a case of draining your tank and refilling it with the correct fuel. If you spot your mistake quickly, you should be okay as it’s commonly agreed that you can mix up to 5% diesel in a petrol tank without breaking your car or the bank.

The real damage happens when you start the car and drive away. As soon as you turn the key in the ignition and the dashboard lights up, your fuel pump has come to life. The engine is supposed to be primed with diesel before it starts, but when you accidentally put petrol in it, it will be sucking the offending liquid into the fuel lines. So, not only will your tank then need to be drained, your fuel pipes will need to be flushed as well – which isn’t cheap!

What’s The Worst Case Scenario When You Put Petrol in a Diesel Car?

Knowing the worst-case scenario (remember the worst-case scenario is always petrol in a diesel engine) has its positives – especially when it comes to putting the wrong fuel into your tank. After all, if you know the potential dangers of what could happen it might stop you from doing it. As we mentioned earlier, Sticking your key in the ignition is the worst thing you can do. If your engine does actually start, then you’re in for some real trouble.

This is because a modern diesel engine utilizes a plethora of technology to save mpg figures and minimize emissions. The fuel pumps are greased with diesel as it passes through.

When petrol enters these diesel pipes, rather than lubricating the tubes, it’s solvent properties create internal friction. This is because the metal surfaces grind together, which causes them to disintegrate, and create swarf. This means microscopic particles of metal could be created, causing further damage to your car’s engine.

After the petrol has gone through the pumps, the engine’s high-pressure injectors are the next thing to go. This is because they force fuel through the engine’s cylinders via incredibly fine holes with an engineered spray pattern. If this swarf gets into the injection system then it will block some or all of the holes. This really is the worst-case scenario as a common rail injector system could cost thousands of rupees to replace.

Petrol in a diesel engine


Diesel in a petrol engine

What happens when you put diesel in a petrol engine?

Diesel pump nozzles tend to be larger than most petrol fuel necks, so misfuelling in a petrol car doesn’t happen all that often.

Luckily, putting diesel in a petrol engine isn’t as bad as the other way around. Because diesel needs to be compressed before it’ll ignite, chances are you won’t even be able to start the engine.

What happens when petrol is put in a diesel car

Read more about petroleum at “https://en.wikipedia.org/wiki/Petroleum” #petrol in a diesel engine

Petrol nozzles can easily fit into most modern diesel filter necks. This makes it far easier to make the mistake of putting petrol into a diesel tank, than vice versa. More importantly, putting petrol into a diesel tank causes more damage.

That’s because diesel acts as a lubricant, helping the fuel pump to do its job. Petrol, on the other hand, causes the reverse to happen: it actually increases friction between parts in the diesel engine. And that’s never good news for an engine. So the more petrol that gets pumped through your diesel fuel system, the more damage it will do.

Common rail (or HDi) diesel engines are particularly susceptible to damage from misfuelling. If the damage is extensive, you could be looking at new fuel pumps, injectors, pipes, filters, fuel tanks – or even a whole new replacement engine. Not good.

petrol in a diesel engine

Diesel Fuel

When we talk about diesel fuel here, we’re talking about #2 diesel fuel – on-road or off-road, it doesn’t matter. 

When you’re trying to predict what kind of issues might arise from accidentally adding one to the other, you have to take into account the biggest differences between the two fuels.

Diesel fuel is heavier than gasoline (because it’s made up of large molecules). It atomizes differently due to a different density and viscosity.  And its flash point and autoignition temperatures are significantly higher. And given these, the converse can also be applied.  Gasoline is lighter and flashes at a lower temperature than diesel.

These differences in physical properties are what cause problems in engines and fuel systems when you put in fuel that isn’t supposed to be there.

Putting Gasoline In Diesel Fuel

Let’s say you accidentally drop a small amount of gasoline into your diesel fuel (petrol in a diesel engine).  The first thing it’s going to do is depress the flashpoint of the diesel, which can be dangerous given that pockets of higher concentrations of gasoline can develop in a tank. So the flashpoint wouldn’t be consistent throughout the entire tank.

Given the large difference in flashpoint temperature between gas and diesel, it doesn’t take very much gasoline to depress the flash temperature significantly. As little as 1% gasoline contamination will lower the diesel flash point by 18 degrees C. This means the diesel fuel will prematurely ignite in the diesel engine, which can lead to engine damage. here is a piece of advice don’t put petrol in a diesel engine 🙂

Gasoline contamination can also damage the fuel pump and mess up diesel injectors.  This happens because of a drop in lubrication. Simply speaking, gasoline is a solvent while diesel is an oil.  Diesel has enough lubricity to lubricate the fuel pumps and the injectors. Swapping in some gasoline takes away this lubrication, leading to damage.

Beyond these, you’ll get incomplete combustion, initially characterized by large amounts of black smoke. Beyond being an aesthetic problem, the vehicle’s computer will try to compensate for this combustion lack by adjusting the fuel-air mixture. This is going to cut your power and performance considerably. And if you continue to use the fuel, you can cause real damage to the vehicle’s computer sensors by either overheating them or covering them in soot such that they can’t detect anything.

Putting Diesel Into Gasoline

Now, let’s look at the reverse – you’re mixing a higher flash, heavier fuel into a base fuel (gasoline) that’s lighter, more volatile and burns at a much lower flash temperature.  Some might think that this “diesel-in-gasoline” scenario isn’t as serious as the reverse.  But that’s not really the case. 

One big concern with contaminating your gasoline with diesel fuel is the reduction in octane. Thinking about how gasoline burns in an engine, octane rating is the measurement of gasoline’s ability to ignite at the right time – not too early.  Gasoline with a lower octane rating will ignite too quickly once it is injected into the chamber. The gasoline ignites and explodes, but the piston is still on its way up and the resulting pressure wave collision gives you (at best) a knocking sound and (at worst) damage to the piston and rod.  In a sense, octane slows down combustion, it delays it.

Gasoline needs to have an octane rating of 87-91 to fit today’s car engines. Diesel fuel has an octane rating of 25-40.  Mixing 2% diesel fuel into gasoline will lower the overall octane rating by 1 point. Getting 10% diesel contamination lowers octane by 5 points, which is enough to create problems in most engines. The octane depression rises linearly with increasing percentages of diesel fuel in the gasoline.

And that’s just the first potential problem. 

  • Because diesel fuel is heavier than gasoline, it can sink to the bottom of your gas tank, resulting in the injection of both gas and diesel into the intake manifold or the cylinder. Depending on the mix, you can get partially-burned diesel fuel which leaves bigtime deposits on pistons, valves and spark plugs. You get a car or truck that runs terrible, and if you keep driving it, you can cause serious damage
  • If enough diesel fuel gets in the cylinder, you can hydro-lock the cylinders, resulting in a blown head gasket, cracked cylinder head or other serious problems that can lead your vehicle down the road to a quick and final death.  This diesel fuel in the cylinder can also seep past the piston rings into the oil crankcase, diluting the lubricating oil. This can damage all internal engine lubricated parts resulting in major engine failure from rapid wear.
  • If unburned diesel fuel makes its way into the exhaust system, it will ignite in the catalytic convertor. The fire will plug the holes in the catalyst, destroying it and leaving you with a repair job well into the four-figures.

How to avoid putting the wrong fuel in your car

The obvious answer here is to be attentive while filling up, but that’s sometimes easier said than done.

An alternative way to prevent this from happening is by fitting a diesel fuel cap to your fuel neck. This cap stops petrol nozzles from fitting into fuel tanks they’re not meant for.

These caps are designed to fit into specific makes and models, so be careful you choose the right one. Depending on your model, they tend to cost between £15 and £30.

This problem is not so silly that the government of the UK is doing this.

The Department for Transport (UK) has announced that all fuel pumps are to be given new labels to make it clear which fuel you should be using in your car.

There’s no change in the fuel itself – just the labels.

Ironically, this may cause even more confusion in the short term as motorists used to seeing ‘unleaded’ and ‘diesel’ are now greeted with ‘E5’ and ‘B7’.

And with that, it may happen that you choose the wrong pump and accidentally put diesel in your petrol engine. But worry not – there’s a way out of this.

New petrol and diesel pump labels explained (UK)

The new labels are designed to tell you exactly what you’re putting into your car.

First is the shape on the label. A circle means petrol, and a square means diesel.

The letter on the label tells you what renewable fuel source has been added to help with the environment.

E means ethanol, and B means biodiesel.

Finally, the number tells you what percentage of that renewable fuel is mixed in.

E5 means your petrol is made up of 5% ethanol, and B7 means your diesel is made of 7% biodiesel.

As well as fuel pumps, the government plans to have these labels put on the fuel caps of cars from April 2020.

Put petrol in a diesel engine? or Put petrol in a diesel Car? 1 thing you must avoid.
Put petrol in a diesel engine? 1 thing you must avoid

The Bottom Line – Don’t Drive It if you ever put petrol in a diesel engine

Because it’s impossible to know exactly how much of the wrong kind of fuel is in your tank and fuel system, the bottom line advice is that if you have good reason to believe you (or someone else) put the wrong kind of fuel in your gasoline or diesel engine, you need to have it towed to a mechanic’s garage where they can remedy the problem. 

Once at the garage, they will remove all of the fuel from the filter and flush the system to remove the problem fuel. 

Some might respond with well, my ________ (fill in with friend, coworker, relative, general practitioner) accidentally got some in his tank, and he drove it and it was just fine.

In those situations, there’s no way to know how your situation compares to theirs (and human nature is such that we always want to minimize our description of potential problems if it stems from a mistake that we’re responsible for).  If you drive the vehicle after you think the wrong fuel has been dispensed, you’ve been warned. We recommend that you don’t take that chance in any case.

Read more automobile-related topics on our blog at “https://www.themechanicals.in/category/blog/


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