Air in cooling system - what's the solution?

Every time I start my Mondeo 2.2 TDCi air bubbles can be seen in the expansion bottle (It is air there are no hydrocarbons present as the garage as tested for hydrocarbons and the test was negative. The solution was tested with exhaust gasrs and turned colour immediately). The bubbles are present even when the engine is cold. The bubbles increase when the engine is reved. The engine does not use any water. The cooling system has been completely stripped down, every pipe has been cleaned and checked but still air is getting into the cooling system. The garage set up a pressure guage to measure the pressure in the expansion bottle the pressure rose steadily to 25psi before they turned the engine off. 48 hours later the guage was still reading 20 psi. Have you ever encountered anything like this before or can you advise on where the air could possibly coming from?

Asked on 12 June 2015 by Steve864

Answered by Honest John
Very frustrating. Was the system pressure tested with the heater set to maximum heat? Could be a problem with the heater matrix. Or may be due to an airlock from suddenly not using the heater at all once the warmer weather arrived. You might be able to purge the airlock by taking off the pressure cap and running the engine from cold to hot with the cap off. Squeezing the main coolant pipes may help but be careful to avoid any moving parts catching a shirtsleeve or finger and be aware that the electric fan may suddenly start turning. If that doesn't work then maybe you have a problem with the coolant thermostat, so the next step is to replace that.
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