Problem with juddering 2011 C4 Picasso 1.6Hdi e-110

[rmunns]

Juddering is not really the right word, but I was unsure how else to briefly summarise what happens. A few minutes after a cold start (i.e. first thing in morning, or after car has been left for enough time to have cooled down) the car misfires. On a neutral or slightly positive throttle it is definitely missing. From where I live this is when I am entering the local town, speed limited to 50km/h (30mph). Maybe a couple of miles further on, the problem goes away.
The car was bought 3 months ago here in France from (best described as) an intermediary salesroom. In Europe they are common; for a commission these people sell cars on behalf of private sellers who don't want to bother to sell it themselves. It carried a guarantee to me and, to be fair, the sales people have tried (using their repairs garage) to fix the problem, but to no avail. Twice they have replaced the diesel filter, claiming 'that's fixed it'. But it hasn't.
The problem happens as the engine reaches operating temperature, then seems to clear after a few more minutes. Any ideas?

[pauljordanie]

Did they run diagnostic check? Find and fault codes? Mine's rough when it first starts, that's down to a couple of failed heater plugs.

[rmunns]

No diagnostic check done - A 'diagnostique' will be at my cost. I hoped to find someone with similar problem who had figured it out. The problem of misfiring/fuel delivery only happens when the engine has reached temperature and goes away a couple of minutes later. I am unhappy at taking the car out in case the problem suddenly gets worse and I end up stranded. The company I bought the car from have an arrangement with a local 'Halfords' sort of place to have things done and I've got little confidence in them. The main Citroen dealer is near to the salesroom, so I guess it'll have to go there if I can't get some suggestion. I'm baulking at that because my 'main dealer' experience rarely ends in total satisfaction for the money I am charged.

[rmunns]

I am suspecting the coolant temperature sensor. Where will I find this for this engine?

[GiveMeABreak]

Unfortunately there are just too many variables - that's the whole point of having engine diagnostics to help narrow down faults. For the price of a couple of dealer diagnostics you could buy your own Lexia clone and a cheap laptop and do it yourself.

As for the water temperature sensor, item 2.