Expert advice: why fuel flooding is a problem for drivers

Fuel flooding

If a car starts, then refuses to start again almost immediately afterwards, it could be flooded with fuel

How do you start your car in the morning? Many of us have the same routine. And for some drivers, that could be the cause of a potentially inconvenient breakdown.

I love cars but my job as vehicle and customer data insight manager is all about figures and statistics. It involves analysing numbers and seeing how people – our customers ‑ use their vehicles on a daily basis. The results can be fascinating. Read on to see how the way you start your motor could leave you stranded at the roadside.

The mystery breakdown

As the weather gets colder we see a surge in call outs to the number of vehicles that won’t start. No surprises there. What’s different to a regular winter breakdown is that these are cars that started fine initially. On the button, you might say. Then, mysteriously, they refuse to start again.

It’s all about routine

We all have routines. Perhaps you go outside and move your car so your wife can get hers out of the drive – or vice versa. Perhaps you start the car to warm the interior, scrape the ice off, then go back inside to finish your coffee before leaving for work. Or maybe you have to move the car to avoid getting a ticket from one of those parking attendants who never seems to sleep!

How cars start in cold weather

It’s frequently said that today’s cars have more computing power than the first Apollo space rockets. It’s true: modern motors are phenomenally clever things. When the weather is cold the car knows. It uses that information to increase the petrol-to-air ratio that is injected into the engine’s cylinders. Making that mixture richer helps the engine to start when it’s cold. Usually what happens then is the mixture becomes leaner fairly swiftly so you’re not burning expensive fuel unnecessarily.

The problem

So far so good, the problems only start when you stop the engine. The engine is in the middle of its cold start cycle and has injected a lot of petrol. You then turn it off and there’s nowhere for that fuel to go. When you return to ask the engine to start again it’s already full of unburnt fuel. In old carburettor engine talk, it’s flooded, something we always thought would be impossible for a modern fuel injected engine.

Why fuel flooding is bad for engines

Petrol is a very effective solvent. If you ever want to wash oil off something, petrol is very good at it. Having too much petrol in the cylinders can result in what’s known as bore wash. This is when petrol effectively strips the thin layer of oil from around the cylinders’ piston rings. As you can probably imagine, engines don’t appreciate this, particularly if it happens frequently.

Which cars are affected?

The cars that appear to suffer from this problem tend to have small capacity petrol engines. They’re usually smaller than 1.4-litres and the cars are typically four to six years old. If you own one of these, be careful.

How bad is fuel flooding?

In November 2017, we were called out to 447 breakdowns for this particular problem. That’s an increase of a third. Thankfully, it’s not difficult to put right. And in 98.2 per cent of cases our expert technicians get the cars moving again. But as trends go, it’s significant ‑ second only to alternator problems which is always a massive problem when temperatures drop.

What can you do to combat fuel flooding?

It’s not difficult. If you own a car with a small capacity petrol engine that was registered between 2011 and 2013, try not to drive it a very short distance – metres – on cold mornings.

If you do and your car won’t start again, try the following: jump into the driver’s seat, push the accelerator pedal to the floor and turn the engine over for five to 10 seconds. Then release the throttle and try to start it as normal. The engine should start and you should leave it to run or drive it until it’s warm.

If it still doesn’t start, repeat the above process but only two more times. If it still doesn’t start, call your breakdown provider. It’s worth noting that doing this can damage rotary engines, so if you encounter this problem with a Mazda RX-8 don’t try this fix.

Scott Wilson is vehicle and customer data insight manager for Green Flag


11 comments on “Expert advice: why fuel flooding is a problem for drivers

  1. Gordon Carey 22/12/2017 9:23 AM

    Why only between 2011 and 2013?
    Also, if fuel injected I’m not so sure that the old trick of “Pedal to the metal” will work as the ECU sees an increase of air to the engine and increases the fuel accordingly. Unless that is you know different.

    • Martin Potts 06/02/2018 8:38 PM

      I encountered this problem once about 5 years ago when I moved a car a few yards. Wouldn’t start again and trying just made it worse. I simply unscrewed the spark plugs to let out the excess mixture to air, screwed them back in and hey presto it started. Maybe a bit inconvenient but if all else fails it saves a call out and is ultimately quicker than waiting for your mechanic to arrive.

      • Sylvia 12/02/2018 12:16 PM

        That’s alright Martin if you are a man but me being a woman and living on my own I wouldn’t have a clue that’s what have green flag for .

    • Peter Biggs 10/02/2018 11:47 AM

      There is a sensor on the excelerator that when pressed all the way to floor tells the ECU not to supply fuel to the Injectors thus allowing you clear the flood.

  2. Colin Dawson 13/02/2018 11:40 AM

    And don’t forget running the engine for such a short period can also drain the battery

  3. Heather Knowlson 13/02/2018 12:58 PM

    Is it a good idea to leave the car engine running in cold weather to warm the car up (5 mins plus!) before driving away, I have a 2012 Merc A class, my daughter insists this is ok, I seem to remember that it is best to start the car & drive straight off , after defrosting the windows with spray of course! thanks

    • Peter Biggs 13/02/2018 9:30 PM

      Well you I guess that is down to choice, you could travel approximately 2.5 miles on average in that time so if you did that once day that’s a whopping 910 miles @ £1.25 a ltr = a cost of £147

  4. Rod 13/02/2018 5:56 PM

    Good use of stats to illustrate the problem and very useful info – more please!

  5. Gill Newman 14/02/2018 2:25 PM

    So this could be the issue I am experiencing when driving a short distance to fill up with Petrol. Once I have filled up I always have problems starting.

  6. Sunny Sammut 17/02/2018 11:28 PM

    This is what we used to do when a car engine was flooded back in the 70s as well, it always seemed to work back then as well even though the car engine was not computerised

  7. Sean 20/02/2018 12:44 PM

    Pressing the clutch down whilst starting the car helps too especially on cold mornings

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