Smoking Diesel Freelander TD4 54 PLATE UK

[AMO123]

Can anybody help, I am a caravaner and was towing my carravan when the exhaust started to throw out black smoke when I coasted but disapeared on aceleration. But if I stopped or slowed down the smoke would virtually disapear. I have now detached the cararvan to find that the smoking has gone. The engine sounded ok and started ok and ticks over as it has always done. I have checked all the vehicles oil and water all ok. Has anybody any ideas on this. Thanks. I have spoken to a few mechanics and they have said that it may be a faulty sensor or a split pipe on the air flow. I checked the engine and found a split in the intake air pipe just after the air flow sensor which could have caused the rich mixture as the air flow sensor would have been bypassed by air leaking into the air pipe. Hopefully this has solved the problem I hope this is of use to anybody else with the same situation. Thanks