I've got a weird problem with my car, its a 1.4 HDi, same engine as the Ford 1.4 TDCi. I changed the air filter on it and upon driving it the EML came on and the further you push the pedal down the more the engine keeps refusing to rev whilst driving. It acts like it's either in limp mode or it's not getting any fuel if you try and accelerate quickly. It'll still do 70 but only if you don't push it, if I come to a hill it's hopeless as it just keeps revving then cutting back then revving some more (the clutch isn't slipping). It's as if it's getting fuel and then suddenly not getting any so cuts out but then gets more fuel and starts revving again.

However, if I put the old air filter back on and floor it, it'll just keep revving and I have no issues whatsoever. The old air filter is BLACK, it's absolutely filthy but it's the only way the car will drive properly. I've plugged it into OBD and it says there's a problem with the MAF circuit (P0100 IIRC) but like I say, put the old filter back in and it all goes away. You can feel that it sucks more air with a clean air filter just by putting a hand in front of the inlet pipe, in fact with the dirty one you can barely feel anything at idle.

Sadly the MAF sensors for these are quite expensive so I want to check I'm not missing anything before replacing that. I've checked all vacuum pipes but literally the only things I touched is the air filter cover and the 3 screws holding it on! I've also tried two different air filters to no avail, both good brands. I haven't tried cleaning the sensor yet as I'm not sure what to use? Anyone know if a dirty / dodgy MAF sensor can cause issues like this?