Flooded engine

What's a flooded engine and how do you fix it?

Learn what to look out for if your car's engine is flooded

Car engine flooding is a common cause of breakdowns, especially on cool or colder days when the engine needs more fuel to run smoothly until it's warmed up.

If you can't start your car, this could be the problem. Read on to find out what causes a flooded engine and how to fix it.

Woman looking at overheated engine with steam coming out

What does flooding the engine mean?

A petrol engine floods with fuel when it’s switched off too soon after being started from cold. After switching off, the unburned fuel remains in the engine. That wets the spark plugs, which prevents ignition and makes the car difficult to re-start.

The petrol can also cause ‘bore wash’ when it removes the oil around the cylinder’s piston rings. It lowers the compression, which is why the engine sounds different and turns over more quickly.

Why would an engine be flooded?

One of the most common causes of a flooded engine is when a vehicle is started in colder weather and switched off again before it’s properly warmed up most callouts for flooded engines happen in winter. The low temperature makes it more likely that the fuel won’t ignite properly, and the problem is exacerbated by the fuel pump continuing to add more fuel.

Cars tend to need more fuel when it’s cold because some of the vaporised petrol condenses on the cold surfaces inside the engine, and cannot be burned. To compensate, extra fuel is added until the engine's cold surfaces have warmed up. In older cars with carburettors, this is done via a "choke" which achieves the same thing by reducing the air drawn into the carburettor.

The downside is that without enough air, engines won't run at higher revs and are difficult to drive until the choke is switched off. Modern fuel-injected engines do this more efficiently by controlling the fuel-to-air ratio constantly during warm up, allowing the car to be driven immediately. Despite this, switching off the engine while it is still cold and then trying to restart it within a relatively short period can lead to flooding.


What causes a flooded car engine?

A flooded engine often happens when an engine is switched off when it's still cold. This can happen in certain situations like when a car is moved out of a garage onto the drive or moved a short distance to let someone else out. It’s most common in the autumn and winter months.

Some engines are more susceptible than others. It’s most likely with small, four-cylinder engines but can happen to any petrol engine in the right conditions.

We provide 24/7 roadside assistance.

Buy breakdown cover

How can I tell if my engine's flooded?

You can tell if your engine's flooded when you spot these signs:

  • Very fast cranking (the engine sounds different when you turn the key – usually a 'whirring' sound)

Every car engine has rotating and moving parts, meaning it’s sometimes normal to hear sounds when starting the engine. However, when you have a flooded engine it’s more likely that you’ll hear a louder, more concerning noise as the engine is unable to fire up. The is caused by lowered compression, which in turn causes the engine to turn over faster without firing, leading to the distinctive ‘whirring’ sound.

  • A strong smell of petrol, especially around the exhaust

In a normal functioning vehicle, the fuel system supplies fuel to the engine at the right time, exactly when it needs it. If you have a flooded engine, the fuel is being supplied to the engine at a rate higher than it can be burned. This fuel will then sit in your engine when you’re trying to start your car, and you’ll be able to smell it.

  • The car doesn’t start, or starts briefly and cuts out again

A likely scenario is that you start your car briefly, just to move it to a new parking space. Then, when you go to start your car again, it won’t work. This is because the engine has been supplied more fuel than it could burn in the short time that the engine was running when you first moved the car.


What are the symptoms of a flooded engine?

Flooded engine symptoms are fairly easy to identify. The first likely symptom that you’ll encounter is a strong smell of petrol. But you may also hear abnormal whirring sounds when trying to start up your engine.

Your car may also start up but then cut out again right afterwards.


How does a flooded engine sound?

When you try to start up a flooded engine, it will usually make a loud cranking or whirring sound. This sound can be quite high-pitched and it usually will go on for a while as the engine is unable to start up properly.


How to fix a flooded engine

Before you start trying to clear a flooded engine, it's a good idea to close any doors and windows to your house. That's because the exhaust is likely to spew out smoke for a few seconds when the engine starts. If the car is in a garage, make sure the garage door is open and there's plenty of ventilation.


How do you clear a flooded engine?

  1. Crank the engine with the accelerator pedal pushed to the floor.
  2. Continue cranking the engine (for up to 10 seconds) until you hear the engine trying to start.
  3. Once the engine’s started, allow it to rev up for a second or two, then hold it at a higher-than-normal idle speed for a few more seconds
  4. If the engine doesn’t start within 10 seconds there may be other issues and you should switch off and call us for assistance.
  5. Once the engine’s warmed up, you can switch off safely or use the car normally.


Can flooded engines be repaired?

By following the steps above, there’s a chance you could unflood an engine yourself. However, if you get to step 4 and the engine still doesn’t start, you may have a more complicated repair on your hands. It could be that your spark plugs and filters need replacing – so make sure to get your car checked by a professional mechanic. If you were wondering ‘will a flooded engine fix itself?’, the answer is no. Steps need to be taken to clear the excess fuel from the engine or repair any damage that may have been caused. This won’t happen if you leave your car alone.


Is it hard to fix a flooded engine?

If you’re unable to clear a flooded engine yourself, the repairs needed can be quite complex. For example, if you have wet spark plugs, these may have become damaged due to the spark plug tips being soaked in excess fuel. This will result in a misfire as the spark plugs ground out instead of firing. It’s recommended that you take your car to a garage so that professional mechanics can handle the problem.


How to start a flooded engine

If you need to know how to start a car with a flooded engine, you can refer to our 5-step process above. This process can be attempted by anyone, but just make sure not to push your car too far. If it won’t start up after a couple of attempts, it’s best to call out your breakdown cover provider.

If you’re wondering what actually happens when you start a flooded engine, you’ll usually hear a loud high-pitched whirring sound. It’ll sound like the engine is trying to start up but can’t quite get there. This is caused by an excess of fuel, which means more air is required to clear it and “dry” the spark plugs.

Your flooded engine may also start up briefly and then cut out. This is because the engine’s management system can still see that additional fuel is required, but isn't able to determine how much air to add to keep the engine running. This only happens once flooding has occurred, and requires the driver to control the air via the throttle (accelerator) pedal until the engine is warm enough to take over control. This usually doesn’t take long.


Have you broken down and need help?

Whether you think you’ve got a flooded engine or some other kind of problem with your car and you can’t get going, we’re here to help. Even if you’re not already signed up for breakdown cover with us, we can come out and help you today. Just give us a call on 03330 046 046.


How can I avoid flooding the engine?

It’s best to always allow your petrol engine to warm up once it’s been started from cold. This only takes a few minutes – it’s warm enough once the engine note changes and the idle speed drops slightly. You’ll also see the temperature gauge move off the minimum.


Can you flood a diesel engine?

In short, no. Diesel engines work differently to petrol engines. They're not usually at risk of being flooded with fuel. For petrol cars, it’s also possible for both manual and automatic engines to become flooded.


What if the engine is flooded with water?

If your engine is flooded with water rather than fuel, that’s a different problem. It could cause a lot of damage depending on how much water there is. To avoid flooding your engine with water, it’s best to drive carefully through fords and floods.

Published: 9 May 2019 | Updated: 13 November 2023 | Author: The AA

Buy breakdown cover

We get more people back on the road than anyone else