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Engine guides and mods

Air Locks - Bleeding

An air lock in the coolant system can cause overheating of the engine, a cooked turbo or just lack of heat from the heater. Lets get rid of it.

Modification Details

This information will show you how to force the airlocks from a:
Fortwo 450 600cc,
Fortwo 450 700cc,
 452 Roadster,
Fortwo 451 turbo,
Fortwo 451 non turbo,
Fortwo 451 Diesel.

How Do I Know I Have An Airlock?

Generally you will get no hot air from the heater even when the heater blobs apear on the speedo.

The big indication is if it reaches 3 blobs too quickly or reaches 4 blobs during continuous driving.
In my experience, it takes about 7 minutes to reach 3 blobs with normal driving conditions.

If the engine is ticking over it will heat up very slowly.

If the car reaches 5 blobs then you will hear a warning sound and you will know you have a problem.
Pull up and stop the car at the first safe place and turn off the engine.

What Temperature Will The Alarm Sound At?

120 Degrees Celcius is the temperature required to show maximum temperature on the speedo.
The alarm will sound moments before the last last temperature indicator comes on.

Doesn't Water Boil At 100 Degrees Celcius?

Indeed it does but there are 2 things that increase the boiling point.
Firstly the additive brings the fluid's boiling point up to about 110.
Secondly the heating of the water causes expansion which in effect causes pressure.
Pressure increases the boiling point to about 120 degrees C.

That is why there is a minimum and maximum mark on the header tank. Too little water and the
pressure won't build up properly plus there is more of a chance of pulling air into the system. Too
much water and there won't be enough air in the header tank to create the pressure.

If the coolant exceeds its boiling point the molecular bonds break down and bubbles appear in the same way they do when you boil a kettle. The air can get trapped causing an air lock and the engine can misread the temperature.

Airlock Removal Fortwo 450

To banish any airlocks that occur, jack up the front of the car nearest the coolant tank.

Leave the cap off and turn on the engine. Keep a good eye on the coolant level and make sure it never falls below the minimum mark. When the coolant level appears to stop going down and bubbles stop surfacing in the coolant tank, give the engine 5 quick revs up to 4000 rpm. Check the coolant level, top up as necessary and replace the cap tightly.

Let the car off the jack and go for a short drive, try and keep the revs nice and high.
Ensure that heat is coming from the heaters and drive home.
Allow the car to fully cool down before checking the coolant level one last time.

Forcing Airlocks Out

This is another method that is effective against airlocks that the above won't shift but you
have to be very careful when performing it. You have to bring the car up to almost full
temperature and with a rag over the expansion tank cap, slightly twist it to release the
pressure before tightening it back down. This should force the airlock out but again, be
careful, this can allow very hot coolant to be released from the expansion tank.

The Correct (Better) Way - Roadster & 450 Fortwo

If you have an airlock or the level got so low there was nothing
in the expansion tank you will need to refill and bleed the system to free any trapped air.

Firstly, access the expansion tank as shown above. Fill it all the way to the maximum line.

Now go to the engine and find the coolant temperature sensor and disconnect the electrical connector.

Just underneath the electrical connector is a metal clip. You'll need to pull this clip out.
The problem is that this clip will spring out never to be seen again.

So, it's a good idea to secure it to something so it doesn't get lost. I placed a zip tie around it...

...and then secured it through the fuel pressure regulator clip.

Lever the clip out with a screwdriver, pliers, knife or very narrow dildo.

Grab the coolant sensor, twist and pull the sensor until it comes out.
You'll hear a rush of air and then coolant will start to piss out. Quickly replace the sensor.

Refit the clip to hold the sensor in place.

Check the level of coolant in the expansion tank and top it up as necessary. Replace the cap.
Start the car and keep an eye on the temperature gauge. If the temperature goes up
too quickly or too high, you'll need to repeat the process to remove more trapped air.

Airlock Removal Fortwo 451

Refit the lower front radiator hose with a new clamp and refit the drain plug in the engine.
Fill the coolant expansion tank up at the front of the car. Fill it slowly to allow the coolant to get into the system. Don't worry about going over the maximum mark on the coolant tank, any excess can be drained or syphoned.

When the coolant stops being taken from the coolant expansion tank and the level remains steady, you'll have to
briefly pop a pipe off in the engine to get rid of an air lock. The pipe is in the same position for both the turbo and
non turbo petrol engine. It comes out of the side of the engine and is held onto a barbed connector with a reuseable
spring clip. Wide pliers are usually suitable to push the tabs together on the clip to allow the pipe to come off.

999cc Petrol Turbo

999cc Petrol Non Turbo

Thanks to Jerry B for this photo.

You should get a short rush of air and then coolant. As soon as the coolant starts to flow from this pipe, reconnect
it to the engine and refit the spring clip. Check the drain plug in the engine is sealed and not leaking at all.

Recheck the coolant level and top it up to maximum. Leave the coolant cap off and start the car.
Allow the car to tick over and keep an eye on the coolant level and the dashboard warning lights.
Keep the coolant tank topped up as air bubbles will be pushed out of the system which will bring the level down.

After about 5 minutes, rev the engine a few times up to 5,000rpm, check the coolant level, top up as required,
fit the cap to the tank and go for a short drive (take the spare coolant with you). On your return, check and
top up the coolant again. It's a good idea to regularly check the coolant level daily for the first few days.

If the engine temperature warning light comes on, turn the car off immediately. If you have a 2007 or 2008 model then you should have a coolant temperature gauge on the dash. Make sure the car doesn't get very hot very quickly.

Airlock Removal 451 Fortwo Diesel

You won't have any air locks because you have to vacuum fill the diesel model.

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