I had an issue with my 2002 2.0 TDDI 115 Mondeo which I solved and thought I would post here in case anyone might find it useful. I googled around and didn't really find a solution that worked.
Mondeo 2002 TDDI 2.0. 170k miles. Standard.
The cars performance slowly got worse over the period of about 6 months, nothing major, I just noticed that pulling onto the motorway it struggled to pull around 70 mph. Once it was going about 80 it would have normal performance, putting the foot down was met with a nice pull etc but from about 65 to 75 flooring the throttle didn't really do much.
This was actually so gradual that it was only when I borrowed a Picasso (boo hiss) 1.6 TDI I thought, hang on this is faster than my 2.0...
I took a look at the air filter and saw that it had been sucked up from the seating position, so I put a new one in thinking that it was dirty. It made no difference.
After another month or so it started to produce lots of black smoke when setting off. This is when the real trouble started.
Occasionally, when the engine was cold it would suddenly loose all performance, as if a turbo pipe had popped off. It wouldn't rev and produced finger-pointing levels of black smoke. The engine wouldn't rev cleanly, it would jump and burble throughout the rev range, at least the smoke shrouded my red face as buses etc had to manoeuvre around the stranded car. It would then miraculously clear and run fine for the day(s) that followed.
This then started to happen more regularly and you would even feel it doing this when driving along, sudden loss of power, stuttering, smoke. The car was now smoking like a trooper at any time unless you were coasting.
Googling I found various things it could be and with happy heart and a spring in my step I lifted the bonnet on each occasion...
I had replaced the fuel filter (the contents of the old one was very dirty so I thought that was the cause).
Cleaned the EGR, which wasn't really dirty (shoulders slumped at that one)
Checked the turbo pipes etc.
Checked the turbo, didn't feel or look worn/damaged.
Put some injector cleaner in the fuel tank.
I started to think (from forum posts etc) that it was the fuel pump, but I did notice that there was a feint smell of fuel when I got out of the car after a run. This made me think its not the pump as it is oversupplying it. Black smoke = too much fuel, but a lot of forums suggested that it is also caused by under-fuelling.
I found a post from a guy who had similar issues with a Fiesta Diesel, someone had mentioned that it might be the air flow sensor. Mine doesn't have one but does have a Manifold Absolute Pressure sensor.
I took that out (oh isn't it conveniently placed) and cleaned it, it was jet black when I took it out, after cleaning it is bright red
Fired it up not really expecting much, car runs perfectly. I was surprised at that.
Forum threads with similar symptoms had people saying how they had spend money on new pumps, turbos, injectors etc and still and the problem after all that expense this worked for me so deffinately worth a shot.
Intermittently poor performance (like the turbo was not connected at times)
Engine cutting out.
Engine wont rev, stuttering.
Black smoke when accelerating.
Semi-constant smoking when driving.
Remove the MAP sensor, clean it, re-fit.
Obviously this isn't a catch all fix, it might work for you it might not but in this case it fixed it. It might even help other engine/cars with similar issues. For the time it took to clean the MAP sensor (which took about 20 mins from start to finish) its worth a shot. It should take 5 mins really but due to the rusted bolts holding the sensor to the pipe meant I needed to remove the pipe to get proper access.
Hopefully this will help someone out and save them wasted cash/efforts.