That's the thing with CRD3 + Pedal Box you never need to "floor it". I myself have never done this as it's just not necessary.
When I say floored it I don't mean pedal to the floor(yes I know it sounds strange 
) and I have found that if you are on a motorway say in 6th @ around 70-80 and you do push the pedal to the floor and as soon as the boost is in then slowly lift your foot you will feel the car surge more as you get to the 3/4 pedal travel point.
Do you mean surge as in the power is erratic or surge as in vigorous acceleration?
Sorry, surge as in acceleration. To me it feels like I loose power if I put my foot to the floor. I find the CRD3+ box to be very smooth.
I think a lot of people don't get the best out of their Diesels is because they try and drive them like a petrol engine.
Part of my job is trying to get professional drivers to drive more professionally which means in commercial terms getting them to use less fuel by not driving like Neanderthals, and using anticipation rather than just watching the arse end of the vehicle in front.
The downside of this is also one of the upsides: technology.
The spy in the cab software penalises them for using foot to the floor driving which it deems as inefficient, which is fair enough, but it is also flawed in that it wants you to use cruise control as often as possible as that fools the system into thinking it's being more fuel efficient, the vehicle ECU software likes to think it knows better than a person, which unfortunately only works in lab conditions or on a steady cruise. A bit like DSG in auto mode, it's ok for what it can do but there's no substitute for a pair of trained eyes and ears than can anticipate traffic flow, road camber on bends, corner entry speeds etc.
If you're used to driving a petrol engine there's no denying it's more fun on the days you're going at it hell for leather using the full revs.
But for the rest of the time?
Diesels run at 100% airflow unlike a petrol that has a metered mix that constantly adjusts the air volume as well the the fuel (hence being called a throttle as that's what it does).
Back in the early days of TDI's I was bored of the wet lettuce hot hatch offerings of VW and my VR6's were ok but a bit flat in the all important mid-range. So before the low pressure turbo 1.8t GTI's arrived on scene I decided to have a play with TDI's which were all the rage in Europe at the time but not very popular in the UK until the early noughties apart from Peugeots which seemed to have got the right recipe at the time.
Having come from a background of pre turbo GTI's I was used to tuning involving improving gas flow, changing cam profiles and increasing the capacity of the engine.
When I moved to Diesels first of all it was a bit of a culture shock so I did a lot of research. I spoke to Van Aaken who were at the forefront of Diesel tuning at the time and then experimented with a few things myself.
I learnt that the equivalent car to the modern Golf GTD at the time was the Peugeot 306 90bhp Diesel that could instantly get 20% more power just by winding up the fuel pump. This was a very popular mod on the continent where Diesels were a lot more prolific and once the new wave of direct injection Diesels hit the market the equivalent but dearer alternative to this mod was the Tuning Box.
So I bought my first tuning box as I was curious to see what it was all about.
Just like the old Peugeot 1.9TD tuning it worked by making the most of the fact the Diesel engine runs at 100% 'throttle' airflow (as it has no throttle) so just needs to throw in extra fuel to make more power. That's why tuning boxes have a crude reputation, all the tuners had to do was throw in extra fuel until it got to the point where excess smoke was made (this being unburnt fuel) then wind it back to the point where EGT's stayed at a safe level and it didn't smoke excessively. Hey presto - 10 to 20% more power instantly.
There was no direct means of upping the turbo boost and there didn't really need to be, the turbo boost was increased by the fact the exhaust gas flow was higher due to the extra combustion so the turbo worked harder and increased boost that way.
The cheap and nasty end of the market tuning boxes just tricked the engine into thinking it was in warm up cycle constantly so the engine continued to overfuel which meant it produced more power.
Nowadays there's lots of emissions gear on Diesels so tuning boxes, or at least the better ones, have to be a bit more sophisticated. But underneath all of that they still basically work the same in that they just throw in extra fuel to up the power under load as they don't have to worry about ignition timing and incoming air flow like tuning a petrol engine.
The reason these cars go into limp mode is that they're overfuelling at a level that the ECU isn't happy with, quite possibly it could be exhaust gas temps or any number of other sensors detecting something being different to what it is expecting. These sensors are mass produced so some will be more sensitive than others and Andrew from DTUK will be quick to tell you some cars will suffer problems and others will be ok.
This is why I was asking if you meant it had become a bit jerky which would likely be excessive overfuelling.
The surge you describe is the turbo building boost rapidly which is what gives tuned Diesels their unique charm. Once you hit the carpet with the accelerator pedal the turbo is pretty maxed out so it has stopped building boost and has hit its ceiling of efficiency.
I've always found that despite modern Diesels being able to rev almost like a petrol engine that they perform best at less than foot to the floor driving, but rather using the massive torque put out as the turbo goes up through its sweet spot of boosting.
Once your foot is buried into the carpet the turbo is already doing all it can so I think (just guessing here) it leans off the fuelling a bit and hence doesn't feel like a petrol turbo car does when you max out the throttle. Mind you an awful lot of modern petrol cars feel incredibly flat unless they're tuned so they can meet emissions regs. Thankfully Diesel engines still have a bit of turbo surge and give you that characteristic kick in the back under part load which is their forte.
Apologies to those that have a better understanding of it all than I do or anyone I've bored to death but that's just my understanding of the situation reported and why tuning boxes can give issues.
