Dont know if anyone can help, i've had a running problem with the frst, when on idle the engine seems to hunt and runs a bit lumpy but as soon as you come off idle it runs fine then when driving the car as long as it is not on boost it runs fine however as soon as you start to increase the revs and she starts to hit boost she holds back like she is missing,someone said this could be the cam and lifters which i have just changed as they where damaged but this has made little difference to the running problem, i have also tried another coilpack which also made no difference has anyone got any idear what it could be?

 

Try a new map sensor. It can make the car feel like it has a bad missfire when it comes onto boost.

 

I had a similar problem with mine years ago.if accelerating hard it would go then hold back or jerk so you'd back off the throttle.If you accelerate slowly it would seem fine.
The problem with mine was the fuel pressure was way too low.The c.o was adjusted but didnt make any difference,so what was done is the fuel pump in the tank was swapped for an escos one(apparently the fiesta ones have a rubber pipe that can fail)
Then the fuel pressure and c.o was set and if never missed a beat since then whilst i owned it.
Its worth looking into this from the symptoms you describe.

 

thanks mate just gotta find one now lol

 

Check the throttle position sensor as it went on my frst many years ago and turned out to be that

 

Map sensor.