Td5 rough acceleration and surging under load

Hi everyone, I have a problem with surging under load in my 1999 Discovery Td5 manual. Have had the car since last October and first noticed the problem when we decided to ‘see what she’s got’ so we ran it up to about 100kph in 5th and floored it which is when we noticed that it surged, accelerated roughly, as if it was starving for fuel. We then noticed that around town when we ran the rpm up (say past 2250) under some decent acceleration (although not even floored) the same problem would occur. Another thing we noticed is that when we were running it up through 4th gear, it would not accelerate past about 3200rpm (which is around 100kph) despite flooring the throttle, and was rough to achieve that speed. Also for what it’s worth, the car feels a bit down on power compared to our 2001 Defender Td5 (which in comparison has almost twice the k’s at nearly half a million). Also when started completely cold, it will run a bit rough for about 5 seconds or so before smoothing out. Other than that though it otherwise drives really nicely and smoothly, just a bit down on power.
It’s just had a major service (less that 1000k’s ago) and among other things the turbo was replaced with a new unit which would lead me to believe that it shouldn’t be overboosting and that the wastegate should be able to move freely.
Have tried driving around with MAF sensor plug disconnected but this made no difference.
I believe that the ecu red plug and MAP sensor have been cleaned in the service.
The fuel pump doesn’t make any noise indicating a failure in the near future.
The only modifications to engine are the silicone hoses (though that wouldn’t make a difference), egr blank off and mid pipe to straight pipe replacement. So, so far I believe it is most likely an injector issue or less likely, the air filter to turbo hose collapsing under load. I don’t think it would be a vacuum leak or diesel leak, so if you’ve made it through all that, any help would be greatly appreciated. Cheers!

it sounds like overboost, which on the discovery td5 isnt controlled by the turbo, but from a boost modulator, a cheap part around £30,
if you can get it plugged into a diagnostic have a look the map sensor reading ,should be around 230kpa at max boost

Ahh okay that makes sense, thanks for that. Ill try and get it code read asap and find out how much a boost modulator is over here in oz. If i'm not driving it to the point of surging, am I right in assuming that it would still be fine to drive daily?

yeah it wont do any harm to drive it, if it overboosts it goes into limp mode with reduced performace,
turning ignition off then restarting it resets it

Okay thanks for that mate, will try and see f I can get it code read then.
This wouldn't be linked to the fact its a bit down on power and idles rough for the first 5 seconds or so on a cold start though right?

generally all td5 are a little rough on cold starting as the engine has only 4 glow plugs but 5 cylinders,
so the cylinder without a glow plug takes a couple of seconds to perform as it should, although some seem to suffer more than others

also it's not enough to clean the red plug if it's oil there... you must replace the injector loom asap otherwise it wo'nt be better

you can bypass the modulator for test by taking the pipe which is coming from the intercooler and connect it to the wastegate valve...then it will run like a defender...but replace the modulator anyway if it's faulty cos it throws a fault code in the ECU and disturbes the management, also with it bypassed you need a defender fuel map loaded in the ECM to make the engine run at full capacity

Finally resolved the problem, turns out that the little pipe that goes into the boost modulator had simply come off. Put the pipe back on and she's no longer surging or carrying on, I think I have regained power as well. Car goes really well and runs smooth as even when floored, so thanks for your help john and sierrafery, much appreciated!