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Diesel Power loss / EGR Fault

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Hi, sorry not sure where to post this as am after a bit of help on standard ecu hardware.

I have a Nissan X-Trail M9R engine, 2.0 dci 173bhp - Year 2007.

I've replaced, Turbo, EGR Valve, Boost Solenoid (replaced again a week ago), Manifold Pressure Sensor, MAF Sensor (replaced again a couple of weeks ago).

Story goes, I had it remapped when i first got it, 173 to 199bhp, car ran fine. At some point I felt like I was hearing a lot of knock usually putting foot down before turbo was boosting.

Eventually the turbo bearings gave in and I replaced it, the EGR valve gave in at some point and i replaced it but it never fixed the fault being logged, little did I realise that the EGR on this engine needs it's position learning, fast forward another year and DFP gave in and getting the MOT is difficult with the EGR fault being logged so decided to start trying to resolve it.

So I replaced the DPF and exhaust all through, then took it to a garage for them to do an ECU reset for the DPF asked them to relearn the EGR position, they reset the DPF but said when trying to reset the EGR it wouldn't work, they could tell it to relearn the position but as soon as they tried to command the EGR to open/close it errored. The fault code is P0409 which apparently means is a circuit problem.

I looked at having the EGR mapped out but the remapper said it's better to start with a clean ECU so I went back to my original mapper (reseller?) who didn't have his equipment so then had to go to the people who wrote the map (i'd say manufacturer?) who said he's put the original back.

Now the current situation....

I have the EGR fault (it's got a blanking plate in to stop it interfering), and a weird turbo fault and am wondering if someone can just say they know what's happening etc, I've checked the turbo and it's fine, spins and no play so no concerns there, as above replacing the boost solenoid has helped with this turbo issue, i've checked vacuum and turbo actuator moves as it should.

While accelerating i'd say not just full throttle but that's when you feel it the most, turbo will spool to 22psi stay steady and then drop to anywhere between 8 to 14psi, some times pressure recovers to full 22 sometimes it holds back to 18psi.

My thought it because the EGR is in an error state the ecu is backing off the turbo, I have booked in to Nissan for them to do an injector remap on it, this apparently will make sure the ECU is running Nissan Original but I don't want to waste £99 on this if someone can read what am describing and confirm it's the EGR causing the turbo to drop and a remap and egr delete should work.

Sorry for my ramble!

Wow that is a tricky situation. With no personal knowledge of that vehicle I can't give you any specifics unfortunately. I'd be inclined to see if you can have the EGR deleted in the ECU by whoever mapped the ECU in the first place but it sounds like they are just providing a generic map and may not be able to make specific changes like this. You also of course need to consider the legalities around doing this as in many countries it will make the car illegal. While there is a DTC present I think trying to diagnose your boost issue is probably not going to be too helpful as it's quite possibly due to the fault being present in the ECU and some kind of safety mode (as it sounds like you've concluded yourself). Beyond this I can't offer much more direction sorry but I'd start by investigating your options for a EGR delete through the tuner.