hi bit of an issue with focus 1.8 tdci mk2.5 has bad idle at low revs in any gear, its only does this with tuning box when disconnect its ok, i have had box on previous car with no problems at all, also had egr blank in and tries it out with tuning box and still same

Did your previous car have exactly the same type of engine? During time there were several different versions of the 1.8 TDCI. Next to the differences in power and torque there were also differences between engine management, fuel system and turbocharger.

Because the engine runs fine without the tuning box the problem must be the tuning box itself. A tuning box tricks the engine management by manipulating sensor values. As a result of the "incorrect" sensor values the engine management inject more fuel which increases the power output of a diesel engine. Cheap tuning boxes do only have a few cheap components like resistors and transistors. The more sophisticated power boxers are controlled by a micro processor and can be reprogrammed to a specific engine.

previous car was a nissan 1.5 dci the tuning box which i never mentioned was from bluepark was re-programmed for my focus. I have noticed there is a few different 1.8 tdci's but i trusted they put correct version on the box, will double check again they have used correct one, its a shame because it worked great on previous car i had

i think the engine in the focus remained the same throughut the facelift, there were a different batch of fuel injectors which did cause an issue on some models. the Galaxy and i think the mondeo too had different fuel injectors (from continental instead of Siemens) which were advertised as 125hp rather than the 115 on the focus.

there was also a change in the drive chain, using a wet belt instead of a timing chain from the fuel pump to the crankshaft.

i would remove the tuning box and also get the EGR looked at / possibly blanked off if it has not been done before, it will make the car run much smoother at low revs.