What Is a Flooded Engine?
Engine flooding happens when excess petrol enters the cylinders, making the air-fuel mixture too rich to ignite. It's more common on older carburetted vehicles and cold petrol engines, but can happen on modern fuel-injected cars too — especially if you've been cranking repeatedly without the engine starting.
The petrol saturates the spark plugs, preventing the spark from igniting the mixture. The more you crank, the worse it gets.
How to Tell If Your Engine Is Flooded
- Strong smell of petrol around the car or from the exhaust
- The engine cranks strongly but refuses to fire
- It may have nearly started then died immediately
- You've been cranking and re-trying the ignition multiple times
- Cold morning start attempt on a car that sat for a while
How to Clear a Flooded Engine
Method 1 — The "floor it" method (modern fuel-injected cars):
Many modern ECUs have a "clear flood" mode. Press the accelerator pedal fully to the floor and hold it there while cranking for 10–15 seconds. With the throttle wide open, the ECU cuts fuel delivery significantly, allowing fresh air to clear the excess petrol. Release the pedal and try a normal start. Repeat once or twice if needed.
Method 2 — Wait it out:
If you have time, simply stop cranking and leave the car for 15–30 minutes with the bonnet open if possible. Petrol evaporates fairly quickly, especially in warm weather. After waiting, try a normal start without pumping the accelerator.
Method 3 — Remove and dry the spark plugs:
For persistent flooding, removing the spark plugs, wiping them dry, and reinstalling them clears the fuel from the cylinders. Crank the engine briefly with plugs out to blow out excess fuel, then reinstall and try starting. This works well but requires basic mechanical confidence.
What Not to Do
- Don't keep cranking — it floods the engine further and drains the battery
- Don't pump the accelerator on a modern fuel-injected car (this is an old habit from carburettor days and makes flooding worse)
- Don't spray starter fluid (ether) into the intake — dangerous on fuel-injected engines and can cause engine damage
When It Might Be Something Else
If the engine floods regularly or you're sure it isn't flooded but it still won't start with a petrol smell, there may be an underlying fault:
- Leaking fuel injector — allows fuel to dribble into cylinders when the engine is off
- Cold start injector fault — delivers too much fuel on a cold start
- Faulty coolant temperature sensor — causes the ECU to run an overly rich mixture thinking the engine is cold
A mechanic can check fuel trims and injector leakage with a scan tool.
Costs
Clearing a flooded engine yourself costs nothing. If a mechanic does it, labour for a simple flood clear is typically $60–$100 NZD. Diagnosing and fixing an underlying cause (leaking injector, sensor fault) is $150–$400 NZD depending on parts needed.