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.
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.
Airlock Removal Fortwo
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.