407 2.7 Hdi - Diag, DPF & turbos boost readings

[407]

My 407 2.7 Hdi has twin turbo's but Diag only shows one reading and all it says is "Air Pressure at the intake measured by the turbo pressure sensor".

Some data / assumptions are

a) both turbo's feed into the inlet manifold
b) there is only one turbo pressure sensor (eg not one for each turbo)
c) at idle (672 rpm) there is no turbo boost and so atmospheric pressure and turbo boost are the same and so Airflow 1 (rear) and Airflow 2 (front) should be roughly the same
d) there is only a single DPF (eg both sides of the V6 come into a single exhaust pipe
e) in my case, both EGR's have been blocked off so I assume no leaks

I did another Diag test yesterday and results are as below

at idle Atmospheric pressure was 1023 millbars, Turbo pressure was 980 millibars (sort of close to being the same and so no boost) but airflow 1 (rear) was 305 mg/impulse and Airflow 2 (front) was 496 mg/impulse, rear is around 200 mg/impulse more or around 40% higher

At 3136 rpm, Atmospheric pressure was still 1023 millibars, Turbo pressure was 1236 millibars and airflow 1 (rear) was 480 mg/impulse and Airflow 2 (front) was 610 mg/impulse, eg the rear is still around 130 mg/impulse more that the front or around 21% higher

Diag now throws up a P0401 (air circuit amount of air measured higher than the local setting) error plus a P0102 (Flow meter signal (rear) coherence local)

I suspect my air flow issues are with the rear Turbo eg the one not easy to get to - its not spinning as a fast as the front one so maybe leaking exhaust pressure via the wastegate ?

Some questions (given the car is otherwise running fine and no MIL lights etc)

a) at idle, (eg no turbo boost), why is the rear airflow a lot less than the front one
b) does it matter given both turbo's feed into the inlet manifold (eg does the ECU use the airflow readings or only the turbo boost pressure readings) - I read somewhere the Landrover with the same engine only has one turbo
c) how do I test the rear turbo for wastegate leaks ?

[407]

I'm still chasing down the low rear MAF reading and suspect I have an air leak after the MAF.

Looking at the pipe work it seems there is a pipe (#2 in the pic) loff the air intake pipe ( 9653096580) leading to the oil seperator breather (#3 in the pic)

How do you get to the pipe to check if it is still connected as I've tried from the top (removed steel turbo pipe), side (diesel filder in the way) and from the bottom (exhaust pipes in the way) and there in no hand access.

[407]

I'm resigned to taking the diesel fuel filter off as that seems to be the only access to that area.

I did throw my endoscope down but could not spot the rubber hoses between the intake pipe and the oil seperator box

In my travels I found some C6 turbo boost data for the 2.7 hdi

2500 rpm = 0.3 +/- 0.05 bar
full throttle = 1.3 +/- 0.3 bar

[407]

For the record, it seems there was a missing clamp (courtesy prior owner/ mechanic I guess) on the intake tube between the air box and the rear turbo (thanks Marc per parts diagram) .

A stainless steel clamp later and the readings are a lot better, albiet still get a OBD P0102 error about the rear airflow but no MIL lights on the dash.

Missing clamp location

Both clamps on rear turbo intake pipe