Rfo,
Just did a rad change on my V8, I've got a few tips for an expansion tank system after you have to change or refill the coolant.
Firstly, run the car up, leave it for about 10 mins, it may take longer in a diesel because of the lower running temp. On mine I was up to temp in about 10 though. The danger you have is the thermostat being sat in an air pocket and not being able to open. A thermostat requires the wax cylinder part to be covered in coolant to release the spring, to help this along, fast idle will help get the water pump to fill up the block.
At this point I had the radiator cap off and could see the level had dropped, I was refilling as I went. When I felt the top hose get hot I knew the thermostat had opened. It was then time to put the cap on and fill up the overflow tank. A tip I got off the web was to over fill this beyond the hot mark.
Now, take the car out for a spin and do some turns, believe it or not you'll still have some air pockets, a good way to get rid of them is round a roundabout then turning the opposite way. This will help slosh the water in the block around.
Hopefully now the car will be at operating temperature, and should be bleed of air, stop and let it cool down. A neat bit of science means and deficit of water in the cooling system will be sucked out of the expansion tank. Come back the next day and see if the levels dropped, top up to cold if necessary. If its about the hot when max mark, you can syphon it out.
Hope that helps!
The trouble is that probably won't work on a Mondeo diesel because the engine/ coolant does not get hot enough to open the thermostat by running it at tickover (especially on a cold day) - so you think the system is bled untill you take the car for a long (or hard) run and it overheats when the coolant level drops when the thermostat finally opens
Revving the engine may help to warm the engine up, but a run may be nessesary to fully warm the engine up carefully monitering the temp gauge and having a top up of coolant in the car (these cars / engines can take some time to warm up!)
Over - filling the level past the "max" mark can be bad practice/ a bad idea too, because the system may be bled at this point (for all you know) and the level may stay overfilled, this could cause other problems like burst seals etc - so i would only top it up to the "max" mark maximum, at any time (remembering it may expand when hot, and should never go past the "max" level in use)
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