What happens when you put diesel in a gas engine

Let’s start with a basic question – can you use diesel oil in a gas engine. Well, you can put diesel in a gas car, but your car won’t be happy about it! It will let you know be dying as you pull away from the gas station.

While putting diesel in a gas car is bad, it’s nowhere near as bad as what happens if you put gas in a diesel engine (which may do serious damage to the engine).

Watch out for the green nozzles!

Here in the US, the green pump nozzles are for diesel fuel. Unfortunately, it’s possible to insert them into a gasoline car filler neck. Since a gasoline car cannot run with diesel fuel, the engine will stall as soon as the diesel makes its way from the tank to the injectors.

This happens fairly often. Usually, it’s someone borrowing a car or a son or daughter new to driving. They’re trying to do the right thing and fill the tank before bringing the car back and oops! You can’t really get mad about it can you?

So, we’ve covered whether it’s ok to put diesel in a gas car (it’s not), and what happens if you put diesel in a gas car (it stops working), but what do you do if it’s too late? The diesel has been added to the tank. What now?

How do you know if there’s diesel in the gas tank?

Before we talk about removing diesel from the gas tank, let’s talk about how to find out someone has added diesel to a gas car. Sometimes the driver knows. It was a brain fart. They were thinking about something and grabbed the wrong nozzle.

However, MOST of the time the car is towed in, and we’re told the car just stalled. The engine cranks, but it won’t start.

Most of the time people who don’t know, don’t know. They don’t put it together that the car died right after visiting the gas station. Why would that have anything to do with it?

You can tell someone added diesel to the gas in a few ways. First, the engine seems to have everything it needs to run. Fuel pressure, injector pulse, spark, compression and timing.

However, if you sniff at the tail pipe it’ll smell a bit like diesel. And if you pull a spark plug out it will be wet and oily and smell strongly of diesel.

How to fix diesel in a gas engine

What should you do if you accidentally fill your gas car with diesel? Well, the good news is that putting diesel in a car won’t hurt the engine. To fix this, all of the diesel fuel must be removed from the tank, and you’ll need some fresh gasoline in the tank.

The more the gas you add the better because there will be some left over diesel, so we want to dilute it as much as possible. Sometimes the spark plugs must be cleaned or replaced before you’ll be able to start the engine.

Is this a DIY project?

Can you fix this yourself? Maybe. It depends on how handy you are.

However, the hardest part of the project is disposing of the gasoline / diesel mix that you remove from the car. Figure this out before you do anything!

Don’t empty the tank and think you’ll be able to figure it out later. It may not be as easy as you think. You might think your local government would make it easy for a person to properly dispose of waste as long as they were willing to pay for the cost.

If you live near us, you’d be wrong. Here’s a case in point. Maybe it’s better where you live. I don’t know.

How to remove diesel from a fuel tank

Please remember that gas is easily ignited!

In the old days, there was usually a drain plug on the bottom of the fuel tank. This feature started disappearing on all of the cars we repair in the late 80s and early 90s.

So how do your remove the diesel if there’s no drain plug? Your car comes with a built in fuel pump. You should use that to pump the mixed diesel and gas from the tank.

We typically disconnect a fuel line in the engine bay and run a hose to a DOT approved container provided by the company that handles our liquid waste products. Then we use the factory scan tool to command the fuel pump to run until there’s nothing left to pump.

What if you don’t have a bi-directional scan tool handy? Well, you can still bypass the fuel pump relay, although this may not be that easy because many cars aren’t using as many simple push-in relays these days. The other option would be to power the pump directly at the tank, but don’t blow yourself up making sparks next to a fuel source!

You may need to replace the spark plugs

Diesel fuel will foul the spark plugs. You may be able to get the car started with some persistence, but don’t burn out your starter beating a dead horse. Remove the spark plugs and either clean them or replace them.

If you have a car that has difficult to replace spark plugs (as many do these days), I’d recommend you replace the plugs instead of cleaning. Nothings worse than spending 3 hours doing a project only to find you’ll need to do it again.

Will adding diesel damage your engine or car?

What happens to a gasoline car when you add diesel? Not much. Other than not starting while the diesel is still in the tank, it won’t hurt anything; there will be no long-term damage.

Yeah. I know. That guy on the internet said the whole car would be ruined. He’s wrong.

Once you get the car started, it will be fine once the clouds of smoke from the tail pipe settle down. It won’t hurt the fuel filter, fuel system, or fuel injectors. Everything will be OK.

If you’re local and own one of the Japanese cars we repair, we’d be happy to remove the diesel from your tank. If you, like most of the people on the internet, live somewhere else, hopefully you found this post useful.