My ford ka duratec engine in mis firing at times,can anyone tell me why!?

i have a ford ka 2003 1.3 duratec engine,its done 15000 miles.as i go up through the gears,i can feel a slight hesitation.some days it will hesitate more than others although its not appearing to cause a major problem with fuel economy and driving.ive had it put on a diagnostic machine but no fault was found.the spark plugs have been changed,redex put in the petrol and a new ignition pack put in,all leads and wiring has been checked,i have no air leaks but i still have this irritating problem,can anyone tell me what they think it might be as nobody seems to have any idea,not even my mechanic who has been doing my cars for years,thanks.

Answers:
my wife had a Duratec ford Ka with roughly the same problem and it turned out to be a slight blockage in a crushed fuel pipe that had been crushed and was restricting the fuel pressure to the fuel rail
It's a Ford..breaking down is what they do.
Their truck engines are built on 1995 technology. All they do is put a cool looking body on it and charge you up the butt for it.
You may want to get the throttle body cleaned. The plate inside may have carbon build up inside. Ford throttle bodies have a thin film on the plate. Only ford safe throttle body cleaner should be used. If regular carberatorcleaner has been used in the past,there is a possibility that the film has been dissolved and is the cause of the hesitation.
Deposits or even a thin layer of carbon can cause hesitation. See how that works. An upper end cleaning can be done by a professional for under 100$. They don't always do the throttle body cleaning when this service is done so request it. They will usually put fuel injection cleaner in the tank, Go under the hood and find a vacuum line.Disconnect it and attach a bottle with a hose and metering tube to it and run this through your system until the bottle is empty. If you have these deposits a scanner won't usually show a code unless the computer detects a misfire.Hesitation isn't a misfire all the time so.hence no codes
Check your crank position sensor this is a usual fault on duratech engines Regards mick.
My Duratec had the similar story..it ended up being emission control related items..about 3 of them.if your "service engine soon" light is coming on, that means emissions only.I agree too with the other guy about the throttle body deposits.
The problem is with improper connection of the spark plug lead to their respective plug on the distributor, you might have made a mistake there. Check the firing order and connect the spark plug lead accordingly.

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