Turbo only boosts when engine is cold - Passat 2007 2.0 TDI 170 BMR (AO57SDU)

[nick.king]

Evening all,

6 months since my last issue so I'm due a post :-)

Passat 2007 2.0 TDI 170 BMR (AO57SDU)

Over the last month I've noticed a decrease in performance, I thought I was imaging it at first but I think I've finally realised what's happening.

Just took the car out about an hour ago, first time I've driven it since Sunday morning. As you all know it's freezing in the UK at the moment and the car seemed fine when the engine was cold, not as much power as I remember but the turbo seemed to be boosting. As soon as everything is warmed up I don't seem to be getting any boost, on the motorway it feels like there is nothing there after 70mph. I've had this car a very long time so it might be my imagination but I'm sure it performed a lot better than it is right now, even when the engine is cold.

So basically what I think I'm experiencing a massive decrease in power the warmer the engine gets. Drove it for about 2 hours the other week, 90 mins of that journey was on the A1, as soon as I got on to normal roads it had no power and was like driving a diesel with no turbo.

Any suggestions on what to check to prove what I'm saying is right?

Many thanks as always for taking the time to read and reply.

Nick.

[DMitch16]

A fault code scan would be the first suggestion as codes can be stored without triggering the fault warning light on the dash.

I always advocate that cleaning makes a huge difference to the way a TDI runs and that an intake, EGR and a fuel system clean every 50k keeps the BMR lively. I have cleaned my injectors and replaced injectors seals etc. too and after I replaced the timing belt, I reset the rocker / injector strike clearances for optimum fuel delivery.

Occasionally the DPF when reaching a full ash capacity can cause the engine to become sluggish as the exhaust pressure starts to increase. My 232k BMR is still pretty punchy for the mileage but I have fitted a new DPF in the last 10k.

[nick.king]

Hi Mitch,

Thank you for the reply as always.

I'll get a code scan done this weekend.

With regards to the other points I had a lot of work done during lockdown in 2020 and I've done 7,500 miles since then, mostly motorway miles. I had a new EGR, DPF (VAG part), turbo (recon unit), timing belt, inlet manifold (BKD aluminium conversion), various sensors and other parts (my mates garage and I was only billed for the parts and it was near on £4k). Since I had that work done I've not had the DPF warning on the dash but I assumed that's because I do mainly motorway miles (between 75-100 miles week) and hardly do any stop/start journeys anymore now I don't do the school run, in fact I only really use the car the 3 days I need to go in the office every week unless we are going away for the weekend etc.

I do have some REDEX DPF cleaner/regenerator I bought a couple of years ago I never used, would it be worth running that through as well then going on a good run?

Thanks,

Nick.

[mickeybo]

The problem your having at the moment most likely down to lack of vacuum at the turbo pressure control control solenoid needs to be checked out vac pressure should be approx 26/27 inhg

[DMitch16]

New recon turbo does not guarantee a good turbo actuator.

Check actuator vacuum holds steady when drawn (needs a handheld vac pump or similar kit). Check the movement of the rod from actuator to turbo, it should move back and forth by around a centimetre to open or close the VNT mechanism that increases decreases exhaust flow and therefore how fast the cartridge spins (pushing more air into the intercooler system and therefore increasing "boost")

Also check the layout of the vacuum hoses to and from the N75 solenoid valve on the bulkhead across to the turbo, across to the back of the cam cover or to what is now a closed off vacuum line to the front vacuum solenoid that used to actuate the intake swirl flap rail on the old BMR intake manifold, unless your BKD intake still has flaps (I recall not whether it does however). Make sure there is no leak from that modification or deletion if the vacuum line and solenoid was removed from in front of the alternator.

It is easy to accidentally have the hoses themselves connected up incorrectly especially at the N75 or at the "crossroads" in the line at the top right of the cam cover.

Thereafter it is trying to trace a leak in them or testing the N75 itself. Tedious unless you have a smoker machine.