Hi. You could also have leaking boost at any one (or more) of the clamps from the turbo baffle hotpipe connector (the most common spot) which is under alot of pressure.
If u see oily residue on or around or under the baffle (i did) its a sure fire sign.
I replaced my baffle with a mild steel pipe i had laying around that was right size & a silicon joiner on the turbo outlet side.. as the baffle had oil residue and alot under it & the clamp at back was a proper 2cm approx turbo clamp, however one at front was a normal autospares type clamp & i had residue back soon & the steel pipe was getting oil get out of it & over its outside under the joiner making the front clamp able to leak just a slight amount of boost.
Being smooth steel without ridges at front & back wasn't a help.
Before i sorted that i tightened/replaced every intercooler & hot pipe clamp.
Some are behind the grille etc which u can get to with bit difficulty or easier if u take the grille out.
I noticed a few were not even very tight! Thatd cause limp modes.
And did!
Prior to that i did the EGR block and that helped with power loss ALOT & stopped me blowing black smoke at all & restored alot of lost power.
I also noticed my fuel filter plastic round front bleeder knob dial was not that tight, so i tightened it up a fair few more hand turns until it didnt seem to move.
I reckon i was getting limps from that too. Seemed to have now zero limps for a week or so. Plus so much restored power and engine performance & Excellent better turbo leak free performance.
It only has to leak the slightest bit of boost to cause all sorts of drama plus lose a couple psi.
With the boost guage i fitted from the boost sensor on the d/s back of intercooler behind the grille i can conveniently watch the boost all the time. Seemed great maybe up to 18-20.
I noticed my limp modes come back & had about 20 of them!
On motorways going 100 then pressing to accelerate uphill anynore than a light feather causing it, at 80, anytime i stabbed the throttle,
Even getting so bad the other week as random limo mode without accelerating at all while rolling at 60!
Last couple times i couldnt even restart the car immediately and clear it as usual. Never had CEL on dash at any point either by way, never.
I had to wait a min, prime the fuel primer in engine bay (a trick i notived worked if the car wouldn't turn over right away that made it start)
I was quite a perplexed what could be still doing it now & why it was getting worse.
I had a realisation back to the turbo outlet hot pipe baffle clamps.
I was correct & lucky i thought of that! As im good now
( hope it stays that way!)
I cleaned the steel baffle pipe n silicon joiner up of all oily residue which had seeped to the outside at the front not very super heavy duty standard clamp.
I put not 1 but 2 big turbo clamps at the front side of the silicon joiner.
Did them up super tight over the baffle (in my case mild steel pipe).
So now the turbo outlet hot pipe baffle area is 'major' done up right & heavily sealed.
I now have no limp modes,
Turbos boosting to about 21psi,
And i can stamp on the pedal to the floor if i like any gear, any time anywhere & am getting zero limp mode, zero breakdown..
And i am making epic power & turbo performance.
So pays to check & replace all those hot pipe and intercooler pipes & vacuums hoses on solid .. no cracks, and doing egr & the fuel filter drain dial clockwise turn a few careful hand turns without breaking something by an overtorque..
As i think air can get into the fuel system and cause problems like yours as well.
Probably good idea do these 1 by 1 so u can rule them out, but the hotpipe & intercooler ones seemed the biggest cause on mine.
Could always be the scv too.
For sure. Worth replacing,
Then doing the ecu & fuel reset after wards by disconnecting the neg battery terminal & holding down the brake 10-30secs. Then do battery neg back up. Then do a couple modest/light drives at least for 10 mins a time, then turn car right off after each 10 min.
That has the ecu and fuel sys relearn the new scv has been installed,
etc etc & fuel and driving habits and with a bit of luck, youll not have that problem any more.
Good luck