Ever have one of those days where your car starts right up, but you try again just a few minutes later and it won’t start? If your vehicle is carbureted, you likely just encountered a flooded engine. If you have a fuel-injected engine, a flooded engine is less likely but still possible. Here’s what happened and how to prevent it.
What is a flooded engine?
A flooded engine won’t start because it’s filled, or “flooded,” with too much gas. In order to fix it, you need to understand how engines use fuel.
An engine needs the proper air-fuel ratio to run properly. The correct air and fuel mixture is 14.7 parts air to one part fuel. After the air enters the combustion chamber, the computer in a modern engine precisely times the pulse of the fuel injector to spray fuel into the turbulent air. The spark plug fires, igniting the mix. At the proper 14.7-to-1 air/fuel ratio, the spark plug easily ignites the fuel vapor. The result generates horsepower and a happy driver.
If anything throws off that air-and-fuel ratio and you try to start the engine with the wrong ratio, the mix won’t ignite. There’s nothing physically wrong with the engine but it won’t start.
This issue was common in carbureted cars until they were replaced by fuel injectors in the early 1990s. A carburetor doesn’t have an air intake sensor and computer to calculate how much fuel is needed based on the current air density. Problems with the jetting or float can cause too much fuel to enter the engine, which is how your engine ends up flooded.
The good news is that unlike a flood-damaged car, a flooded engine is usually a cheap and easy fix.
Signs of a flooded engine
While you won’t see a check engine light, black smoke, dreadful gas mileage or other obvious symptoms, there are several flooded engine symptoms to look for. Depending on the exact cause of the flooded engine, you probably won’t have all the symptoms.
Refuses to start
If you turn the key and the engine cranks but refuses to fire, this is a common sign of a flooded engine. This is doubly true if the engine was running normally just moments earlier. With improper air-to-fuel ratio and potentially gas-soaked spark plugs, continuously cranking the engine is a waste.
Very fast cranking
All that gas flooding the cylinders can act as a solvent, cleaning all the oil off the piston rings so they can’t seal. This lack of compression lets the air-fuel mix dissolve instead of creating the more combustible mix under pressure. Even dry spark plugs can’t ignite the mixture in these conditions.
The carburetor or fuel injectors add fuel to the engine when attempting to start. If the engine doesn’t start and you keep cranking, the fuel flow won’t stop—it builds up in the cylinder and can get into the exhaust system. Your ride quickly starts to smell like a gas station.
Starts, then shuts down
This is usually a spark plug problem. Maybe there is enough compression for one spark plug to fire and get the engine moving, but the others are wet and can’t fire. The engine will start and run just long enough for a celebratory fist pump, then it’s dead again.
How to unflood an engine
Unflooding an engine is usually quick and easy. The process often involves no tools or technical know-how. Here are a few options for how to start a flooded engine.
This one isn’t fun when you’re late for work, but it is effective. Don’t crank the engine for at least 20 minutes. You should find the issue resolved as the fuel vapors disperse.
Pop the hood
This can reduce the wait time. Lift the hood, and everything in the engine bay gets a breath of fresh air. More air makes the engine easier to ignite, but wait a few minutes before trying to start.
Floor it (on older cars)
Putting the throttle pedal to the floor on a car with a throttle cable fully opens the throttle valve, allowing more air into the carburetor and quickly correcting the air-fuel ratio while cranking. Note this won’t work on a modern “drive-by-wire” electronic throttle system.
Pull the spark plugs
Want to do the job right now? Pull out the spark plugs and dry them off with a shop rag or paper towel. Reinsert the spark plugs into the engine, and you should be able to fire up and drive off.
How to avoid a flooded engine
A one-time or occasional flood is easy enough to handle. But what if your engine floods with gas on a regular basis? That means there is a problem, and you should have a look at the following areas.
The carburetor could need an adjustment if the weather suddenly got colder. It could also be time for a cleaning and tune-up.
There are several ways a fuel injector can fire even after the engine is shut down. That fuel can build up and prevent a quick restart. If the engine runs for a moment after you turn off the ignition, you may have a leaky fuel injector. Have a professional check it.
Most cars on the roads today have electronic fuel injection systems, not carburetors. These systems know how to start the engine with the right amount of fuel. By pressing the throttle as you would for a carbureted engine, you throw off the calculations. This may cause a flooding issue.