Hopefully someone can help out...

My 2010 xjl 3.0d portfolio has developed a strange fault, idle is fine and holding any rpm while there is no load there's not a single problem. Once under load the rpm fluctuate's by around 3-400rpm at any speed in any gear even while cruise control is used the car surges with each change. It's constant under load. This fault started after its MOT 2 weeks ago and I cannot find the cause. I have checked intake pipework hot and cold side all o2 sensors MAF sensors and I have had the vehicle scanned. I fitted a gearbox over a month ago which has been absolutely faultless. For MOT it received new rear upper arms both sides new brembo front disks and pads then a full set of Dunlop tyres. Any help or advice would be welcomed with open arms

Thanks,
Ben

 

Hmm.. If you don´t have any codes, all sensors are working, but some propably are giving wrong info. I would try to swap places of the MAF sensors together. Upper one are used only for 2nd turbo, over 3000rpm (or somethig), therefore it is much less used. They are identical parts, so you can swap them freely.
Second easy check comes in my mind are the MAP sensor, right at top of the engine, on Y - intage.(throtle body)
I would say that the EGR are stuck partly open for the symthons, but you should have bright light on dash and codes on scanner... Exept, if your vehicle are "softed them out"?

Then more difficult task: You wrote that it started after MOT. Does MOT include the smoke test with full throtle? (in my country it does) There are change for timing belt (or high pressure pump belt) to jump on tooths in sudden "free full throtle" runup to max. If i understand right: one tooth on timing do not cause mechanical harm, but more than one can cause valves to kiss pistons. (not as romantic as it headrs)

You might also have stuck or faulty injector. It might work Ok on low volume, but when more capasity are needed it might just be sqiurting fuel, instead of spraying. An professional shop are needed to test them.