How does misfuelling affect your car?
Petrol and diesel engines are more different than you might think, and so what actually happens when you put the wrong fuel in will depend on what type of engine your car has.
Putting petrol in a car with a diesel engine is more likely to cause damage than the other way round. Metal parts of the engine rely on the diesel fuel to lubricate them so they work properly. Petrol, however, acts as a solvent, prevents diesel's lubricating function, and causes the metal components to rub against each other.
This can cause debris to clog up the fuel injector. The petrol itself can also cause damage to the fuel lines, and if it gets into the fuel injectors, they will likely need replacing. This tends to come at vast expense.
Putting diesel in a petrol car isn't quite as bad. In most cases it won't cause significant lasting damage, but is likely to cause your car to misfire if you try and start it as the diesel coats key components like the spark plugs. Your car won't start, and might start kicking out nasty smoke, but should be fine once the fuel tank has been drained.
What to do if you put the wrong fuel in your car
It's perhaps best to start with what <em>not to do: do not start your car.
If you realise as the pump that you've put the wrong fuel in, here's what you should do:
If you have already started your car, pull over as soon as you can safely and switch the engine off, the proceed from step 2 above.
Be warned that if it's a diesel car you've misfuelled, and you have started to drive off, the chances are high that the damage will be significant and costly to repair.
Preventing misfuelling
Aside from the obvious (always concentrate when you fill up your car), there are a couple of tips you can follow to make sure you don't put the wrong type of fuel in:
Is damage from misfuelling covered by my car insurance?
It depends. Some insurers cover issues caused by misfuelling under accidental damage clauses, some don't. Check your policy documentation to be sure - if misfuelling isn't covered, it will normally say so explicitly. If you're still unsure, contact your insurance company - if you get a cheaper car insurance policy, it might not include a clause on this.
Most of the time, misfuelling isn't covered as standard. However, you may find that you can take out specific misfuelling cover as an optional extra. Even if you can this cover, it will normally only mean you get a pay out to cover the drainage and flushing of your fuel tank, but not any damage that occurs as a result of you driving off.