What causes overboost? BKD 2.0tdi

Yesturday I went into a layby at around 20mph but they had put a curb(?) over the whole layby. It was raining so I didn't see it. Nasty bang from under the front. Drove from that to destination fine, was in that place for 10 mins, came out, car in limp mode all the way home, was like there was no turbo.

Took to garage today, car works fine now, got ecu scanned, and it said "overboost caused limp mode"

My question is it is very coincedental this happened right at this moment of impacting the bottom of the car (undertray gearbox area) with what they assure me "happens a lot with these". I would like to ask if anyone has anymore details so I can make sure it is just coincedence?

Thanks

We checked the whole car from underneath on a ramp first, all hoses etc were as they should be. That was my thought on the way home over the Welsh mountains at 20mph with a stream of traffic behind me...intercooler hose, but nada all good.

Car has been great since and much faster than usual so some good has come from it!!

Ok to follow up car is running great BUT is being terrible on MPG. £10 of diesel is getting me around 20 miles, used to get me around 70-80 miles.

Thoughts?

Right some updates.

Booked it into Audi which cost me £60.

They checked the whole car, told me they needed it overnight to try a "cold start" then half an hour later they said a pipe underneath was twisted and intermittently rubbing on some wires, causing limp mode.

They fixed that then they test drove it, reported fixed.

I went the next day, paid, picked it up, drove it home and it went back into limp mode.

The next day I was theorizing and realised with Vpower diesel in (which I believe is normal diesel + 2 stroke engine oil) it does it less, so I put a full tank of vpower in.

Went down to wales down the a55, it went into limp a few times, then the EMS light came on (and has stayed on ever since). In fact coming back from wales on the a55 it went into limp mode about 10 times, including some dangerous overtakes.

Parked on my drive and didn't touch it until last night.

Went out last night, roads empty, long 30 mile drive, didn't go into limp mode, tried ragging it to get it to do it, nada, eventually it did, at around 4krpm in third.

I notice most of the time it dumps air as usual but occasionally it flutters, then I get a little police siren noise and it goes into limp mode. If I put it in N and turn off/on it turns off limp mode though the EMS stays on.

I am not sure what to do now. I can send it back to Audi but they want permission for "tech to run it around for a week" (on a full tank of vpower no less) and they checked all the hoses, vacuum system, claimed to have checked everything...their tech department seem a bit useless to me especially as it went into limp 5 mins after I picked it up...what kind of test drive is that then?

Should I just buy a new turbo?

It also loses coolant, they had to top it up, I wonder if it is something like coolant being burnt off - picked up by EGR system - too much vapor - clogs up turbo? So I need a new head?

I am keeping the car it is only worth £1500 which is a bit ridiculous for a 90k 2007 in mint condition!

There are no stain marks anywhere which is indeed wierd.

Pretty sure my overboost issue is stuck vanes at open.

Been to a garage today, they tested everything, car needs a bit of work

- needs a new turbo - I ordered a brand new GT1749V - After a chat with them about hybrids and larger turbos they just recommended OEM and a stage 1 remap, then bolt ons like exhaust etc, for 100% reliability. £800 later it was ordered along with uprated oil feed/returns.

- sump nut leaking oil - will replace.

- intercooler pipe leaking oil - will probably buy and fit a hard pipe kit and an S3 intercooler with metal end plates and do all this in one go before remap so I don't have hoses popping off etc.

As for a remap I am not sure what to do, the garage do their own but I think I would rather go with Evolution Remaps I had them on my PD130 passat and it was brilliant, they road-map it to your car.

- Exhaust: I already have backbox delete pipe from my 1.8tFSI which will go on so just need to get rid of the cat and centre silencer, should sound ok on a diesel.

Being an SE it will be a total sleeper and pretty quick with 200hp and around 450NM with s-tronic full throttle gearshifts!