The other day my 2000 Xreg MK5 Fiesta 1.6l Zetec-S lost a lot of it's acceleration power. I knew this because I was being overtaken by a 1.2l Corsa with my foot flat on the floor. It was misfiring.

So, since then I've:

- Changed the coil pack
- Changed the HT leads
- Changed the spark plugs
- Changed the oil (including an engine flush) - this did help a little as I think the wrong oil was in it before

Other symtoms include:

- Really heavy engine shaking when cold and at low revs (i.e. pulling away)
- Air filter chugging though removing it didn't stop that
- Exhaust chugging
- No power until about 3000rpm in all gears - the chugging seems to calm down then

Other car details:

- About 82k on the clock
- PiperX air filter (no other mods)
- There's a plug floating from a sensor attached to the piping between the air filter and the engine that looks clean but I can't find where that should be plugged in

Any help at all would be greatly appreciated!

Mark.