Under the bonnet…

When thinking about this blog, I wanted to start where most people naturally star and that’s under the bonnet, in the engine bay. So let’s look inside a typical engine bay (under the bonnet) and see what there is, then we can look at what each part (component) does.

Engine Bay

Brake Fluid Reservoir

Your brake fluid reservoir is simply a place to store brake fluid when its not being used. The brake fluid reservoir connects to a master cylinder underneath. We’ll take about the braking systems later in the blog. On the side of most brake fluid reservoirs there is a small line and the words ‘Min’ and ‘Max’. As reservoirs are mainly see-through, you can see where the brake fluid is against the side of the reservoir and your brake fluid needs to sit between the ‘Min’ and ‘Max’. Brake fluids are hygroscopic, which simply means that once they they absorb moisture from the air around them. We’ll explain why that’s essential to know later, but in short, when you’re topping up brake fluid use a new sealed bottle of brake fluid. If you go to EuroCarParts.com and type your registration number in, it will tell you what sort of brake fluid you need. You can also check the owners manual for your vehicle. Most will be Dot3, Dot4, Dot5 etc.


Oil Cap Fill Up

This should be a simple screw cap on the top of the engine, where new oil enters the engine. To find your vehicles oil type, either look in the owners manual or you can visit a site such as EuroCarParts.com, enter your vehicle registration number and then look up engine oils. See Oil Dipstick below to find out how much oil is in your car and when you need to top it up.


Engine Cover

An engine cover is simply a piece of plastic that sits over the top of your engine. It’s purpose mainly is to make the engine bay look tidier and more visually appealing, in some vehicles there maybe plastic connectors that help to route certain wiring looms (a collection of wires together) through the engine bay and it helps to protect the engine from certain spills.



The car battery is there to hold sufficient charge to get a car started and also to store energy generated by the alternator for use on the vehicles electrical systems. We’ll take about starting systems, vehicle electronics and using a multi-meter later in the blog, but here are two figures for you to remember :

  • When the engine is not running the battery should have a charge of around 12.6 volts or more, anything less than this and you’re battery either needs charging or it’ll soon need replacing.
  • When the engine is running, the battery should have a charge of between 13.7 volts to 14.7 volts. If it’s between these figures you know your alternator is producing enough power to keep your vehicles electrical systems running whilst charging the battery at the same time. But we’ll take more about batteries and charging systems later on.


Fuse box

The fuse box is an essential on a vehicle and it contains a variety of fuses and relays. We’ll take about them later in the blog, but they’re similar to the fuses you would find in a plug in your house and they protect your electrical items from burning out as they restrict how much power is sent through the circuit.


Screen wash bottle

This is the lid to the screen wash bottle and contains the fluid that will eventually make its way through various pipes to be splashed on your windscreen. You can top this up with either tap water, or if you want to stop the water from freezing you can add screen wash to the water or simply fill the bottle with concentrate screen wash.


Coolant / Expansion Tank

The coolant / expansion tank is an essential part of the engine cooling system and the coolant / expansion tank is a little like the brake fluid reservoir tank where coolant fluid is kept when not being used. The fluid flows through the engine block, cooling the engine down by removing heat. After it has been through the engine block, the coolant fluid needs to be cooled itself and so passed through the car’s radiator before being returned to the coolant / expansion tank. The coolant / expansion tank has a ‘Min’ and a ‘Max’ level on the side of the tank and your coolant should always be between these two levels. Do not open the coolant lid when the engine is still warm or has been recently run. They say a picture speaks a thousand words, so here is why :


Oil Dipstick

Engine oil is the vehicles equivalent of blood. The oil covers the metal on metal parts reducing friction, essentially reducing metal on metal contact and protecting the engine. It generally does this when the oil is warm. In order for the oil to be able to protect the engine, there needs to be enough of it to circulate throughout the engine and this is where your dipstick comes in. Your dip stick measures how much oil is in the engine and in order to make sure you get an accurate measurement, your vehicle needs to be level and it also needs to be cold. To see what engine oil does and why its so important, have a look at this video from Castrol.


Mass Air Flow Sensor (MAF)

Typically engines found in cars are called internal combustion engines. If we break this down, internal means occurs on the inside, combustion means to consume by fire and engine means a machine with moving parts that convert power in to motion. In other words an internal combustion engine is the process of burning fuel (petrol/diesel) inside the engine to turn the power of the combustion in to movement. In order to burn the fuel, it needs to be mixed with air and the mixture needs to be perfect. The mass air flow sensor measures how much air is flowing in to the engine and the quality of it. These measurements of quantity and quality are sent to the engines computer which then tells the vehicle how much fuel is needed for the combustion to take place and when the combustion process should take place.


Air Filter

The black box contains an air filter. If you’ve ever walked down the street on a windy day you’ll have had all manner of things blown in to your face from leaves, crisp packets etc. As we’ve already seen air is essential to the engine working properly, but imagine what would happen if those leaves, or some large chunks of dirt managed to get in to the engine, the engine wouldn’t work as it should, but also you would be introducing a foreign object in to an engine which shouldn’t be there. Now imagine that dirt was a metal washer, or a screw that’s come loose or something else. Air filters stop that from happening, ensuring that the only thing that enters the engine through the air pipe, is clean air, nothing else. We’ll look at the air system later in the blog.


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