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What Happens if You Put Mixed Gas in a Lawn Mower?

Mowing your lawn is one of those chores you still need to do even when you live in the city. 

However, for some of my neighbors who have a large lawn and don’t want to spend a lot of time doing it, they normally consider renting a small tractor (if they can find one). They’re not cheap, but they can be a huge time-saver if you have a really large lawn or multiple properties.

The big question is, what happens if you rented but instead of putting gas, you unknowingly added diesel in a lawn mower? Is it a big deal? 

Putting diesel in a gasoline (petrol) engine is a common mistake, and if you’re not careful, you could end up with a broken engine.

As someone who has seen his share of messing things up, I can tell you that engine knock produced by confusion between diesel and gas in a lawn mower is a fairly common mistake.

Diesel and gasoline are two kinds of fuel, and they contain slightly different chemical components. If a lawn mower engine is designed to use one kind of fuel and you use the other, it can’t process the fuel into energy, and this can cause serious damage to the engine.

Let’s explore more of that in the next section.

Diesel in gas engine?

These days, I see people taking for granted the fact that diesel and gasoline are completely different fuels. 

However, this wasn’t the case in the early days of the automobile and I totally understand that only a few people know this.

The first commercial diesel engine was designed in 1897, which means that for almost half a century, gas engines could run on either fuel. 

When World War II broke out, the US government made a bold move by banning the sale of all diesel vehicles. 

The ban was officially lifted in the 1980s, but by then the gas/diesel divide was firmly established and there was little need for consumers to learn how to cross the divide.

Diesel engines are often used in agricultural machinery and heavy trucks—vehicles that are designed to withstand the rigors of continuous operation. 

But when diesel is accidentally put in a petrol engine, the fuel may cause severe damage to the engine. 

Diesel engines use a different compression mechanism than petrol engines. 

When diesel fuel is put in a gasoline engine, the high compression rate of the diesel engine can cause the fuel to ignite within the engine, potentially damaging the pistons and causing the engine to seize. 

Diesel fuel is much thicker than gasoline and contains more lubricating oil. If you put diesel into a gas engine you can potentially destroy the fuel pump and you will probably clog up the injectors with the unburnt fuel, causing it to run roughly or stop altogether.

Diesel contains more energy than petrol, but it is harder to ignite. When you put diesel in a gasoline engine by mistake, the diesel may not be completely burned. 

In essence, putting diesel fuel into a gasoline engine can result in performance issues, damage to your lawn mower’s fuel system, and even costly repairs down the line. 

Effect of gasoline on diesel lawn mower engine

For a start, gas is much lighter than diesel, having a specific gravity of 0.71 to 0.77, while diesel has a specific gravity of 0.82 to 0.88.

Since, gasoline is thinner than diesel, you’ll probably notice your engine is harder to start after using gas.

But if it does, the spark plug in the engine will probably get fouled, and it will need to be replaced. The exhaust filter will also need to be cleaned, and the injectors cleaned or replaced.

How can you prevent misfueling from happening?

When you are out buying fuel for your lawnmower, it can be easy to accidentally buy the wrong gas.

Here are a few tips to deal with that.

  • Purchasing a lawn mower that has a clear fuel tank is a great start. Gas and diesel fuel tanks are clearly marked, usually with a picture or a number. 
  • In a lot of countries, diesel and also fuel nozzles have a tendency to be a bit different in size. To prevent misfueling, you can buy a misfuel avoidance tool / device that will be mounted on fuel tank entry and will certainly not permit you to put the diesel or a gasoline nozzle right into a mower with a gas or diesel engine.
  • Another tip to prevent misfueling is to consider a fuel type that is easy to recognize while purchasing either a diesel or gas lawn mower.
  • Take extra precautions when refueling a lawn mower. The only time you should add fuel to a lawn mower is when the mower is off.