How to Recognize and Treat a Flooded Engine

If you cold-start your engine and shut it off soon after, unburned fuel can wet the spark plugs and make the next engine start-up difficult.
Written by Andrew Biro
Reviewed by Melanie Reiff
Contrary to popular belief, the term “flooded engine” does not refer to water in the engine, but rather a large quantity of unburned fuel dampening your engine’s spark plugs. When this happens, it can be difficult to re-start the vehicle the next time you switch on the ignition.
If your engine is flooded with too much fuel and not enough air, your engine’s spark plugs may no longer be able to ignite the mixture, causing you to lose power. Fortunately, fixing a flooded engine is a pretty simple process, but it can be an irksome inconvenience—one that’s easily avoided with the proper know-how.
Knowing what causes a flooded engine, how to fix it, and how to prevent it from happening again are three things every car owner should learn—which is why
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What is a flooded engine?

Simply put, the term “flooded engine” refers to the phenomenon by which your engine receives too much fuel and too little air, causing the fuel mixture to exceed its upper explosion limit. When this happens, your spark plugs are unable to ignite the fuel and instead ground out, resulting in a loss of engine power—something you may not notice until the next time you try to start your vehicle.
Generally speaking, older vehicles are much more susceptible to engine flooding than more modern vehicles, as newer engines have computer-controlled fuel injection systems that carefully monitor the amount of fuel being received.
That said, even modern vehicles can experience a flooded engine in the right circumstances—particularly in winter when the engine is fed more fuel to run smoothly—so it’s a good idea for all responsible car owners to familiarize themselves with the telltale signs of a flooded engine.

Signs of a flooded engine

If your engine has flooded, you will likely notice the following:
  • A strong gasoline smell: If your engine floods and receives more fuel than it can burn, you may notice a distinct gasoline smell when trying to start your vehicle
  • Starting trouble: A flooded engine can often make it difficult to start or restart your vehicle, as the spark plugs are unable to ignite the fuel mixture and provide power
  • Odd engine noise: If your engine sounds different or produces a fast ‘whirring’ noise when you go to start the vehicle, it may be due to a flooded engine
Key Takeaway A flooded engine is the result of too much fuel being injected into the engine, throwing off the air-to-fuel ratio and making it so the spark plugs are unable to ignite the fuel mixture, resulting in a loss of power.

How to fix a flooded engine

To fix a flooded engine, you need to clear the excess fuel and restore the air-to-fuel ratio—something that can be achieved by following the steps below:
  1. Crank the engine while pushing the gas pedal to the floor
  2. Continue to crank the engine for at least 10 seconds until you hear the engine trying to start
  3. Once started, let the engine rev for a few seconds and then hold it at a higher-than-normal idle speed for a couple more seconds
  4. If you do this and the engine fails to start after 10 seconds, you may be dealing with some other underlying issue, in which case you should have a mechanic inspect your vehicle to identify the problem

How to avoid flooding the engine

Oftentimes, an engine will flood in cold weather after briefly starting the engine and shutting it off soon after, before the fuel has had time to sufficiently warm. To avoid this, always let your engine sufficiently warm up after a cold start—this will only take a minute or two and you’ll know it’s warm enough when the engine note changes and the idle speed drops slightly.

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An engine may flood in cool weather after the vehicle has undergone a cold start and then shut off again soon after. This is largely due to two factors playing off each other: 1) when temperatures are low, your engine needs more fuel to run smoothly and 2) shutting off the engine before the fuel has had time to sufficiently warm can allow unburned gasoline to accumulate in the engine cylinder.
Ultimately, this throws off the air-to-fuel ratio and makes it so the spark plugs can no longer ignite the fuel mixture properly, resulting in a loss of engine power.
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