problems with k reg mondeo SI - el tel
i think i have more than 1 problem with the car and they hav all come at once. the car is overheating after only a few miles of driving and the revs drop to below 500, the heater sometimes blows cold air, and there is sometimes a knocking sound from the engine when the revs drop. the was also a lot of smoke/steam coming out of the back. the oil levil is fine and its not leaking water, the only water loss is in the form of steam from the bottle.

any ideas?
problems with k reg mondeo SI - TurboD
Head gasket?, always assume the worse and be surprised if it's only a thermostat or airlock.
What miles?, at K reg it's pretty well done it's stint anyway .
But steam out the back is a head gasket to me, sorry
problems with k reg mondeo SI - sine
Does the radiator get hot and cooling fan come on?
Could also be water pump which might cause the heater to go cold.

Steam out the exhaust is not that unusual in winter depending on temp & humitidty etc, and does not automatically signify HG failure but combined with the other symptoms is doesn't look good.

Assuming the radiator is getting hot then Purhaps start with a compression test?
problems with k reg mondeo SI - Screwloose
el tel

Change the thermostat first. Don't drive it like this. Even if it's [was?] just the thermostat sticking, driving it with the symptoms you describe will quickly make sure that it becomes a lot of nastier things too.