BKP Underboost Issues!

[Chris01243]

Hi all,

Been having a few little issues with my 2007 BKP 88k Passat B6 Estate.

I gave the car a good “clear out” the other week, came off the power and then accelerated again but had hardly any power. I can only rev the car up to 3000-4000 RPM and if I hold the accelerator down for 5 secs or so, it’ll flag up with P0299 00665 for underboost. Stop and restart the car, fault clears but will still only rev upto 3500-4000 rpm.
I have noticed my fuel economy has dropped a hell of a lot and the emissions being pumped out the back is shocking.

Around 6 months ago I replaced my tandem pump as I had a little oil in my fuel. I flushed the system (didn’t drop the tank for lift pump), changed the filter and have changed the filter regularly to keep things tip top. I have changed the filter since this fault has occurred just incase this was blocked etc. the filter that came out was a little bit black, fuel was clean and in the bottom of the housing was small black particles. I did take a picture of this but assume it’s just general crud in the fuel.

When the vehicle is revved upto 4000rpm when stationary I get a bit of blue smoke currently but this could potentially be a separate fault?

I carried out all my relevant checks and found that the inlet manifold actuator was not holding a vacuum, so I replaced this for the aluminium inlet from Darkside developments and blanked off the vacuum pipes. Went out on test, car was still the same.

So, I had another Passat which I was lucky enough to borrow some bits from that were accessible, I tried another N75 and MAF, made no difference.
I have replaced the MAP sensor with a Bosch one in the hope this would be my fault, still the same.

I used the smoke machine to see if I had any traces of intake leaks / boost leaks, I was unable to see any signs of smoke escaping.

I used a Mitivac on my vac pipe to the actuator on the turbo and this seemed to be ok. So I decided I would take off the turbo this week to inspect it for oil residue and check the vein operation. There are no signs of within the turbo, veins had a little bit of carbon on but were certainly moving ok. I still took the time to clean them up etc and get them moving a bit smoother. Sadly, the car is still the same.

Unfortunately I do not have access to VCDS, I only have access to Snap On Modus and also a WOW (Wurth) diag laptop. So far, on the Snap On I cannot actuate the EGR etc as there is no option to do so, so trying to source something for me to do this.

I have checked live data on my injectors (always paranoid about the injectors failing) , the control deviation seems to be very similar on all 4, am I right in thinking that if I had a blocked/faulty/leaking injector the reading would be high?

I plan to remove the EGR and inspect this closer to see if the valve isn’t seated correctly.

But what else is there I can check? I feel like I’m starting to go round in circles. I am hoping someone has had a similar fault to this and found a silly fault or fix.

Any help would be appreciated as I am running out of options!

[Crasher]

Under boost is nearly always a fault in the vacuum control system, take off the vacuum pipe to the servo pipe on the tandem pump and use the Mitty to apply a vacuum to the system, you should be able to draw a vacuum quite clearly and it must hold it.

[Chris01243]

Under boost is nearly always a fault in the vacuum control system, take off the vacuum pipe to the servo pipe on the tandem pump and use the Mitty to apply a vacuum to the system, you should be able to draw a vacuum quite clearly and it must hold it.

Thanks for this advice. I have done this tonight after work, it was slowly dropping off vacuum. So tried to eliminate where the leak was. However, it doesn’t seem to be a leak as such and assume it is normal for this to slowly drop off? Or do you mean test this with the engine running?

I whittled it down to the N75 valve causing the drop in vacuum which I assume it’s meant to do? It wouldn’t hold a vacuum on the bottom nozzle. Checked it against another VAG car and that one does the same. Removed the N75 from mine, pulled a vacuum repeatedly, had another gauge on the vac side for turbo actuator on a bench and then used a power probe to actuate the valve to make sure the vac transfers. All seemed to work ok? Or should this drop in vacuum not be happening with engine off etc on the bottom?

[mickeybo]

The n75 3 port valve the only time it losing its vacuum is when its not being energised by the ecu. When energised vacuum goes through the top 2 ports. When over boost happens the ecu dumps the vacuum via the lower port 3 The vac reading you should be getting approx 26/27 inhg

[Chris01243]

The n75 3 port valve the only time it losing its vacuum is when its not being energised by the ecu. When energised vacuum goes through the top 2 ports. When over boost happens the ecu dumps the vacuum via the lower port 3 The vac reading you should be getting approx 26/27 inhg

Thanks for confirming this. In this case it seems to be ok then.

Really starting to get frustrated with the car now.

I have just had a thought… are the PCV valves in the cam covers prone to failure? Could make me lose pressure, burn oil and have a lack of power.

When I took my inlet manifold off to replace the broken plastic one, there was a bit of oil residue and a lot of oil coming out of the bolt hole that sits in the middle and you remove it from inside the manifold itself.. is that fairly normal to see?

[Crasher]

You should be able to pull and hold a vacuum indefinitely, I have had to replace many N75 valves due to internal leakage.

[mickeybo]

On re-reading your problem i was not aware 0f the fault code po299 which points at turbo pressure control solenoid & the exhaust gas cooler solenoid are inter connected & share the same ecu switched earth at ecu pin no b29. I would advise you to check this out

[Chris01243]

You should be able to pull and hold a vacuum indefinitely, I have had to replace many N75 valves due to internal leakage.

Is this with engine/ignition on? As with engine off, with the mitivac it doesn’t seem to hold, I’ll see if I can get another N75 to test this on.

I checked mine then tested this against an old Vauxhall solenoid and that one also wasn’t holding a vacuum on the same side so assumed this was normal for it to not hold. Will try and get another solenoid to test.

[Crasher]

Engine or ignition off or on, doesn’t matter but engine off and a big leak can be heard.

[Chris01243]

Engine or ignition off or on, doesn’t matter but engine off and a big leak can be heard.

Got the car built up back together. So with the N75 off the vehicle, I cannot pull a vacuum on it unless I blank off the outlet to the turbo.

With it back on the car, running, I put my vac gauge inline, I had initially 25 hg . When revved, this went down and when off throttle it was then stuck at 10-11 hg. Put the new N75 on, exactly the same. How do these solenoids work? Are they a switched Earth etc? I cannot find any details on autodata regarding it.

With the vacuum feed pipe disconnected I have nearly 29hg vacuum coming through at a constant and seems to hold it fine. However… with this pipe not doing anything else, when revved my car sounds very deep and lumpy.. the blue smoke was then pouring out the back more so. Why would this be?

This car is truly stumping me… as soon as I think I’m getting somewhere I’m hitting a big brick wall!

Is there anyway I could test for injector issues without removing them? On a cold start it puffs a bit of blue smoke and then when revved.

Tried disconnecting the EGR as someone else mentioned to me and driving it, less smell out the exhaust but still boosting upto 3500-4000 and running out of juice. Have you know an EGR to get stuck in such a way it could cause this?

I’m now starting to think this could be wiring / ECU related but unsure.