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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 not 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 ones are immune to the problem when operating within normal tolerances. Flooding usually occurs during starting, especially under cold conditions or because the accelerator has been pumped. 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. High temperature fuel may also result in a vapor lock, which is unrelated to flooding but has a similar symptom.
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 fuel) can also occur. This condition is known as the engine "flooding out."
Liquids inside an internal combustion engine are extremely detrimental because of the incompressibility of liquids. Although not the most common cause, a severely flooded engine could result in a hydrolock. A hydrolock occurs which is when a liquid fills a combustion chamber to the point that it is impossible to turn the crankshaft without a catastrophic failure of the engine or one of its vital components.
Some Mazda-made Wankel engines are notable for occasionally flooding themselves when turned off before reaching normal operating temperatures. Worn fuel injectors can also cause flooding in rotary engines. When flooded, rotary engines become very difficult to start. To unflood a rotary engine, fuel flow must be cut while the engine is turned over by the starter in order to clear out excess fuel.
The conventional remedy on a carbureted engine is to steadily hold the throttle wide open (full throttle) while cranking (starting) the engine. This permits the maximum flow of air through the engine, flushing the excess fuel out the exhaust. On a fuel-injected engine, ignoring the throttle (no fuel) while starting permits electronic logic systems to choose the correct fuel mixture, often based on exhaust gases. In either case, 'pumping' the throttle will only make it worse as that pumps even more excess fuel into the engine. However, some fuel injection computers interpret 'pumping' the throttle to indicate a flooded engine, and accordingly alter the fuel-air mix.
In worst cases, the excess fuel can foul spark plugs, sometimes necessitating their replacement before the engine will start. This is most likely to occur on a carbureted engine in cold weather, after the engine has been running, then shut off for about a half-hour. This tends to fool the (cooling) choke to set as though the (warm) engine is cold, creating a too-rich fuel mixture, thus flooding the engine.
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