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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,
Roadster,
Fortwo 451 turbo,
Fortwo 451 non turbo.

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.

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 Way

According to a few people 'in the know', the air lock will almost always occur in one place.
Mark (SmartsRcool) explained how to dislodge the airlock.

At the top of the engine at the back, there is a heat sensor which is fitted in with a u clip.
Red arrow shows the sensor, green arrow shows the clip.



Pull the clip out then fill the car up with water through the expansion bottle.
Lift the sensor out and the water will start to travel from the expansion tank to the hole. When the water runs clearly from the hole with no bubbles, replace the heat sensor and U clip. Taking the clip out is easy with a pair of pliers, putting it back in isn't as easy.

It has been noted by some that it is a good idea to tie the clip to a part of the engine
with string or fishing line as it has the ability to spring out and totally disappear
during its removal and refitting. It saves the hassle of getting replacements.

The best way is to hold the clip in place, push the sensor into the hole and
hold it in and use long nose pliers to lever the sprung clip back in place.
Be careful that you don't damage anything with your leverage.

Airlock Removal Roadster

Thanks to Damo for the info and Bjorn for the extended info and picture.

The Roadster is apparently less likely to get an airlock as mentioned above due to the position
of the coolant tank (at the back and not the front). In the Roadster's case there is a bleed point
in the engine bay that can be used to purge an excess air from the coolant system.

Open the bonnet and over to the right you will see a rubber bung with a slot in the top.
Prise the bung off and you will see the bleed valve underneath.



Again, jacking the front of the car up on this side can aid in the removal of blocked 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.

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