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A flooded engine is an internal combustion engine that has been fed an excessively rich air-fuel mixture that cannot be ignited. This is caused by the mixture exceeding the upper explosive limit for the particular fuel. An engine in this condition will fail to start until the excessively rich mixture has been cleared. It is also possible for an engine to stall from a running state due to this condition.

Engine flooding was a common problem with carbureted cars, but newer fuel-injected engines are immune to the problem when operating within normal tolerances. Flooding usually occurs during starting, especially under cold conditions or by pumping the accelerator before or during starting. It can also occur during hot-starting; high temperatures may cause fuel in the carburetor float chamber to evaporate into the inlet manifold, causing the air/fuel mixture to exceed the upper explosive limit. This is most often experienced in hot weather.

A severe form of engine flooding occurs when excessive liquid fuel enters the combustion chamber. This reduces the dead volume of the combustion chamber and thus places a heavy load on the starter motor, such that it fails to turn the engine. Damage (due to excessive compression and even dilution of the lubricating oil with petrol) can also occur.

Some Mazda-made Wankel engines are notable for occasionally flooding themselves when turned off before reaching normal operating temperatures. When so flooded they are very hard to start and may require major shop work.^{[citation needed]}

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