I have a Citroen C4 1.4 with 1360cc 16V KFU / ET3J4 petrol engine with 120.000 km. This problem appeared 3 - 4 years ago at around 40.000 km.
When the engine is cold / hot there is a flat spot between 1000 - 2500 RPM's, and very low power. Ocassionaly when starting the car cold or hot, it would shake without wanting to rev up before shutting down. Smell from exhaust is best described like fault fuel smell. When driving and pressing the gas pedal instantly, there's a half second lag before the power actually kicks in.
There's a constant Antipollution Fault error on the dash, and scans showed that there is a P0171 (Richness Adjustment: Bottom Limit) code stored in memory, although after cleaning the code it didn't showed up again. (The ECU is Magnetti Marelli).
So far I've checked the plugs, coil pack, EGR valve and MAP sensor. I've been to a Citroen service center and all they did was clean the throttle body, but this doesn't really cured the problem, it only maked the power curve of the car a bit strange. (I've also noticed that the mechanics in the Citroen dealership drilled a small hole in the throttle flap because the flap apparently only oppened 80%?)
Recently I've also been to a friend of my who makes some readings and concluded that the front lambda sensor is faulty, but after disconnecting the sensor from the car, the simptoms were there and the car feeled exactly the same!
Do you have any experience with this engine and this kind of problem? What should be the next step I should take in finding the core of this problem?