I own a y reg 1.8 TDDI focus 90bhp when I brought her she was fine but in need of a service and while I was there as there was no service history I got a new cambelt/timing kit put on then 2 days later my diesel pump failed (bad luck eh) I put on a reconditioned bosh pump with a new brain on it from PB Asher and it was fitted by First Diesel in Croydon (25 yrs experience!) but since it was fitted it no longer had a smooth idle but drove well throughout the rev range. I gave it back to First Diesel as I wasnt happy and they 1) checked the old pump in case it had any metal in it - it was clear 2) take out the injectors and clean them - they did this twice 3) re-checked the job and ensure eberything was as it should be. All to no avail the problem still happens the weired thing is if I put the aircon on the idle smooths out and it sounds fine??? The alternator recently failed as well so I have just fitted another unit but this has not changed the bad idle. Before any of this work was done the car had a full service all filters including the diesel filter was changed the bad idle problem only happened after the new pump was fitted - the new pump was checked by First diesel with no fault codes recorded. They said it might be the injectors or an electrical fault but they are not really sure as the injectors are 2 stage and they are only abl to check the first stage - if it was the injectors surely turning on the aircon would not alter the idle? It reacts exactly to turning the aircon on or off to good/bad idle - has anyone suffered with this problem before it cant be an isolated case surely - any ideas as I dont want to keep chucking money at the car it getting too expensive!!. HELP PLEASE.

i would first look at the EGR, give it a good clean and italian boost down the motorway. keep it in 4th and hold at 3000k revs for about 20 mins to clear any cobw

best stuff to buy, in my opinion is Wynns EGR 3 cleaner is best with a tank of Wynns fuel treatment.

if still rough i would also look at the fuel filter, i know the 1.8tdci gets very moody if the fuel filter hasnt been replaced at correct intervals.

both methods are the cheapest and easiest to begin with.,

since it happened after the pump was fitted to me thats where the issue lies either the timing is out a tooth or its the pump no offence but it doesnt matter how much experience a garage has they can still do it wrong and since it didnt go in like that they should have lookd into it

Seeing as it improves with air conditioning on maybe you could check the aircon pump clutch?

Might be wrong to reactivate such an old topic, but I'm having the same exact problem... Focus TDDi 1.8 (2001.) Had my fuel pump circiuit-board replaced, now it idles like an idiot... if I rev it up, it's fine.. if the A/C goes on (pumping some additional fuel) it's fine again, when it's still "cold" (again when gives some additional fuel) it's fine..... i'm so puzzled, I'm going to apply for a job at Ravensburger!!