Water pumps: Explained in super detail

Steady yourselves guys.

We are about to go into mad, mad detail

as we enter the thrilling, high-stakes world

of the water pump.

And it should really be called the coolant pump.

Because if your water pump is pumping water,

then you are a very bad person and you best check yourself,

because this pump should be pumping a mixture

of 50% coolant and 50% water

or thereabouts depending on your climate.

We'll talk a lot more about cooling later

when we get to the rest of the system,

but for now let's just briefly explain the coolant system.

The engine runs hot, obviously.

We've literally got a controlled petrol fire going on

under the hood.

And the ideal operating temperature for this engine is

about 200 Fahrenheit, or 90 Celsius,

That's the ideal operating temperature

for any engine really.

That's hot enough for a lovely flow of oil,

and nice combustion in the cylinders

but it's not so hot that the engine is damaged by the heat.

So the engine parts which are close

to the combustion process need to be cooled down

and that's why we need the coolant system.

Coolant, which is a mix of water and ethylene glycol,

is used to carry heat away from the hottest parts

of the engine and out to the radiator

where it's cooled down.

Now this car which we're gonna build, the Mazda X5 Miata,

has an absolutely typical cooling system.

There isn't much variation in cooling systems,

so this probably applies to every vehicle

that you're going to come across.

Now, when the engine is at operating temperature,

cold coolant is pulled out of the bottom of the radiator

by the pump, and it's pumped into the front

of the engine block.

It travels around the cylinders,

up into the head where it cools the valves

and then comes back out of the cylinder head

and over to the radiator to be cooled down.

Now there's a thermostat

which is like a temperature controlled valve.

That controls the flow of coolant to the radiator.

When the engine is cold,

we want it to get it up to temperature as fast as possible

so the thermostat is closed

and the water stays inside the engine system

until it's warmed up.

Once that coolant is up to temperature,

we want to start cooling it down

and so the thermostat opens,

the coolant flows all the way around the radiator

where it's cooled by the airflow

of driving along in a vehicle.

Now, we're going to explain this whole cooling system later

in much more detail as we get on with the build

and we come across the parts that need to go on there.

But for now that's basically all you need to know

so that we can dig into the water pump.

Now a water pump has a very simple job.

It keeps the coolant in the engine circulating.

If coolant doesn't circulate then it's going to boil next

to the hottest parts of the engine

and elsewhere it'll be cold.

This is a totally average water pump.

It fits onto the front of the engine here,

and this pulley is driven by a belt from the crankshaft.

The same belt drives the alternator.

Now some water pumps are driven by the timing belt,

or directly off the camshaft or crankshaft.

It really doesn't matter

but there's an important point here.

Can you see that the water pump is connected directly

from the crankshaft by a belt?

That means that when the engine is running

then the water pump is also running.

And when the engine is cold, the thermostat is closed

and the coolant isn't passing through the radiator

but we still need to circulate

that coolant within the engine

so that there's an even heating going on inside.

And so, the water pump is always pumping.

If the engine is running, the water pump's going.

In terms of parts, we've got the pump housing made

from cast aluminum and there's nothing special there.

This one has a couple of mounting points

for the timing system,

so the timing idlers fit onto these two here.

A shaft runs through the body

and on one end we have a flange.

Just a second while I take these nuts off.

Okay, so what we have is a shaft running

through the housing here.

On one end, we have this flange which mounts to the pulley.

And this pulley is obviously attached to the belt

which runs on the crankshaft.

That's what drives the pump.

On the other side of the pump we have the impeller.

And the impeller sits inside this hole on the block.

Coolant comes into the pump here,

through the water pump inlet which is connected

to the bottom of the radiator.

It's pulled up and into the center

of the impeller along this channel.

Now the impeller has blades which spin the fluid around,

flinging it outwards and creating a lower pressure area

in the middle that pulls in more coolant.

Now if you want to be technical

then this is a centrifugal, impeller pump.

Now centrifugal force is the same one

that you get on a roundabout, an outwards force

that acts when something is rotated.

So just around the impeller here, on the pump housing,

there's a spiral shape cast

into the water pump and that's called a volute

or a volute, I don't know how it's pronounced.

The shape of this volute is what creates the pressure

that pulls water into the pump.

Now I don't want to go into the complex fluid dynamics

because I don't understand them.

But we need to know that the combination of the volute

and this plate that closes the impeller,

creates a closed route

for the coolant instead of just throwing it out randomly.

You can more clearly see a volute

on one of a more typical bigger water pumps

this is the same thing,

on a car it's just cast into the housing itself.

Now these little water pumps are incredibly powerful.

A pump this size will empty a small swimming pool

in about an hour, and at high engine speeds

this is going to circulate all the coolant

right around the engine 20 times every minute.

Water pumps are sealed for life,

and they're replaced as a whole unit,

generally done at the same time

as the timing belt is replaced

because you've got to dismantle a lot of stuff

to get in here.

Water pumps are a part that are cheap to buy,

but very expensive in labor to replace

so it makes sense to do it while you're in here anyway.

That's the basics of a water pump,

but let's get stuck into some more geeky details.

Now first take a look at the pulley.

The size of this pulley, relative to the pulley

on the crankshaft, determines the speed

that the water pump is working.

Here we've got basically one to one.

Now the manufacturer is going

to size the pump and the pulley based

on the cooling requirements of the engine.

There's complex fluid dynamics and stuff at work there.

But can you see that the water pump turns

at a fixed ratio of the engine's RPM?

That's nice and simple, but it's not very flexible.

Because in an ideal world,

the rate of the coolant's flow should really be determined

by the temperature of the engine,

and the outside temperature.

Because with a hot engine in a hot climate,

the coolant needs to be circulating a lot faster

than a hot engine in a cold climate.

And this direct drive pulley doesn't account

for that requirement.

That's one reason why some manufacturers are starting

to fit electric water pumps.

Because they can be controlled much more flexibly

and there's another benefit,

which is that you can squeeze

an electric water pump virtually anywhere on the engine.

A limitation of this pulley-driven system is

the water pump has to be somewhere

that we can get a belt around it.

Which means that it's got to be really

at the front of the engine or possibly over the back.

Right now the BMW 3-series engine has

an electric water pump.

And VW's massive W12 engine

that goes into the Audi A8 also has an electric water pump

and it has that in addition to a mechanical pump.

So there's two pumps.

Why two pumps?

Well partly it's about that extra flexibility

to control the flow of coolant

but also there's something called heat soak.

So let's say you're driving hard

on the back roads of Bavaria,

rallying up the road in your lovely Audi.

You sweep into your tree-lined driveway

and floor it for those last three miles up to your castle.

You handbrake turn in front of the door,

kill the ignition and dash into the house for a bratwurst

and some kind of rock-hard, dark brown bread.

The second the engine stops,

the coolant stops circulating.

But just a microsecond before that,

the fuel was being burned inside the cylinders

and now the heat from that burning is not being carried away

by the coolant.

So it's not at all obvious

but the engine temperature actually goes up

after it's turned the engine is turned off

because there's no longer any cooling going on.

This is called heat soak

because the heat just kind of soaks through the engine

and eventually it cools down.

So in that W12 engine, the electric water pump keeps running

for up to 10 minutes after you turn the engine off.

The exact duration depends on the temperature of the engine,

the temperature of the oil and the outside temperature.

It's just a much more gentle shutdown.

Now heat soak wasn't that much of an issue

for smaller engines until recently.

You used to get in the car, start the engine,

drive to your destination, turn off the engine and dismount.

Two engine developments have changed this.

First there's stop-start technology,

where the engine turns off

while you're stationery instead of idling.

And second you've got hybrid engines,

where the engine might not be running at all

even when you're moving.

Now both these types of engines need an electric water pump,

partly to avoid this heat soak that we already talked about,

but mainly for your own comfort.

Because don't forget that the coolant is

what warms the air that blows out

of the blowers inside the car.

So you'd be pretty unhappy

if you had to sit in a traffic jam

with cold air blowing at you while the engine's turned off.

So many hybrids, including the Lexus

and the Prius use electric water pumps.

So, sorry, we went on a little detour

about electric water pumps there.

But now you know, electric pumps, more flexible

in terms of flow rate and mounting and where they fit.

But, the downside, they're more expensive.

However, they're essential for hybrids.

Anyway, let's get back to our little mechanical pump.

Now, you'll never work on the insides of a water pump

because it's sealed for life

and it's treated as a consumable,

That is, it's not expected

to last the lifetime of the vehicle.

You replace this whole unit except the pulley.

But knowing what's inside will help you to diagnose faults

and there's actually some interesting stuff hidden in here.

So, I'll pull this off, we'll have a look.

So the shaft, or spindle, is being turned by the pulley.

And at the front here is the water pump bearing.

And that's kind of a special bearing

because it's incorporated directly into the shaft

and that's the main reason why this thing is replaced

as a whole unit.

And the bearing is lubricated at the factory with a grease.

The bearing doesn't stop coolant

from leaking along the shaft

in fact any water getting into the bearing is terrible news.

So further back along the shaft,

we have the mechanical seal, towards the impellerant.

Sealing a rotating shaft

from pressurized liquid has always been a challenge.

And the mechanical seal in here is pretty smart.

It consists of two faces

that are pressed very close together by a spring.

And they're separated and lubricated

by a thin film of coolant.

The gap between them which is about a micron,

which is a thousandth of a millimeter,

just wide enough to contain a static film of lubricant,

but not so wide that lubricant can flow through.

Now inevitably friction will cause the seal to get hot,

and some steam will is gonna created

when this tiny film of liquid boils up.

And we definitely don't want

to get any coolant into the bearing.

It breaks down the grease

and that's gonna cause a huge problem for us later.

So between the mechanical seal

and the bearing is a small hole, called the weep hole,

and this little bit of liquid that gets created

from boiling of the film and the mechanical seal can escape

through that hole down the channel here

and then on this particular pump it's channeled out

to the back of the pump and then it runs

down the fronts of the engine block.

Now it's totally normal for there

to be some liquid escaping there.

But if there's a lot of liquid

and crystallized coolant around here,

particularly if you've got coolant dripping down

off the oil pan which is what's underneath here,

then you might have a leaky water pump.

Every now and then,

manufacturers send around technical bulletins

telling technicians to stop replacing water pumps

every time they spot a bit of coolant around the weep hole

and that it's perfectly normal.

Right, leaking pumps brings us

onto the last thing you need to know,

what goes wrong with water pumps?

Now basically a water pump is gonna fail

in one of three ways.

It can leak coolant because the seal isn't working,

and that's almost always caused by a bearing failure

which puts extra strain on the seal itself.

And the solution there, replace the water pump.

It can become noisy.

So the pump can become noisy and difficult to turn.

And that would be a worn bearing.

So pull the belt off the engine, turn the pulley by hand,

and it should turn easily and smoothly.

If you've got a growling noise from the water pump

then it's likely a bearing issue.

The solution there, replace the water pump.

And finally, the impeller can fail.

Now this is a tricky one because from the outside

there's nothing wrong with the water pump.

But the blades can break off the impeller if it's plastic,

or with this one it's steel

which means the blades can corrode away

and we're left with no blades at all.

Now one sign of a failed impeller is

that the engine is overheating

but you're getting no heat through the blower.

You can check for a failed impeller

by getting the engine up to temperature

so that the thermostat's open, turning off the engine

and then having someone start the engine

while you squeeze the top radiator hose.

And you should feel the coolant

immediately start pulsing through.

If you don't feel that, suspect the impeller.

Now, if the impeller is destroyed

then guess what the solution is?

Replace the water pump.

Guys, if you want this amount of detail

on everypart of a car

then come on over to the how a car works site

I've got a whole series of these videos

that tell you everything

you could possibly want to know about cars.

And we'll take you step-by-step through car mechanics