2005 Passat B5.5 BHW Chasing Down Limp Mode

I'll try to be brief .

Several months ago, my beloved Passat started choking and stuttering above 45MPH. CEL illuminated and "Emissions Workshop!" message displayed.

Mechanic recommended

-decarbonise the (VERY clogged) intake
-replace EGR
-replace boost pressure control valve (I think this = N75)
-replace MAF
-replace worn intercooler hose (lower passenger side - the expensive one)
-replace vacuum lines

Recently a friend of mine and I did all of the above except the vacuum lines (had trouble finding the right ones - see below). After cleaning and replacement of parts, the car would regularly go into Limp mode on hills. CEL came back on as well as the "Emissions Workshop" warning.

My mechanic's VCDS reported: 16683 Turbo Underboost-tbd,*
19558 Intake Flap Motor-V157, No signal-tbd,*
16791 Exhaust Gas Recirculat.Sensor B circuit, Low-tbd,*
19557 Intake Flap Motor-v157, Open circuit/short circuit to ground-tbd

Clearing the codes did not clear up the problem. However, restarting the engine cleared up the issue for a time (i.e. typical limp mode behavior).

I invested in an OBDEleven which confirmed my suspicion that the intake air flap motor was failing, so I replaced that, too. After that, the car would still go into limp mode, but less often, and now was only reporting Turbo Underboost.

I then finally found the right vacuum lines and replaced all the braided lines EXCEPT the line from N75 to (I believe) the vacuum reservoir (the big round ball attached to the motor just behind and under the N75). I lost my courage on this last one because the nipple on the reservoir is *right* up against some other piece of wizardry right below it, and I just couldn't see how I would ever be able to get the new hose on once the old one was off- -and anyway what I could see of it looked fine - and what I couldn't see felt fine. I may regret that decision... For all connections, I carefully cut off the old hose along the length so as not to damage the connectors, I only did one hose at a time so as not to mix them up, and as a further reassurance I used teeny zip ties around the base of each connection. I used 3mm and 5mm ID hose from IDParts.

Now after all that the car *still* goes into limp mode- - but much less frequently and much more randomly. Now the CEL does not light, and I don't get the "Emissions Workshop!" message. And now I can clear the problem just by clearing the fault code. So I'm driving down the road with my phone connected to the OBDEleven and regularly hitting the scan and clear buttons every 20min or so. Sub optimal for sure.

What to do?

I've checked around on this and other forums to make sure that I didn't need to run the adjustment for the Intake Flap Motor (and I can't anyway, as ODBEleven reports that this isn't supported via ch 060 or 098).

I should add that when the car is not in limp mode it is REALLY movin'! Power like I've not seen in the 5 years I've owned it. This leads me to believe that the turbo itself is probably fine.

Could the problem really lie in that one little hose I didn't replace? Or is there something else I ought to be looking at here?

BTW I'd also appreciate any tips or encouragements regarding the replacement of that last hose - just how do I get it back on properly?


Thanks all,

pc

Update: I just drove 2.5 hours through mountain country - my usual weekend commute. Limp mode only happened one time, and that close to the end. Air temp and also engine temp were much cooler during this drive than any other since I did the repairs mentioned above. Air temp was around 60ºF tonight vs 80-100ºF on previous daytime drives on the same route. I also didn't have the AC running as I have before. Oddly, the temp gauge stayed quite low (near the first 10% of the arc) right until late in the drive as well. Shortly after the only underboost incident, engine temp suddenly climbed back up to near mid-range.

So, now I'm wondering if the car's current limp mode troubles might be related to engine temp somehow?

pc

Drive it like you stole it. I babied mine until I started to get limp mode. After regularly getting on a bit more it has never come back.

Have you checked the vacuum produced by the tandem pump?

Can you log requested boost, and actual boost with OBDEleven?

@MEagearhead: Thanks! I've read elsewhere that a hard drive can help clear buildup from the vanes of turbocharger, and I intend to try this on my next drive.

I think I could probably log requested and actual boost with OBDEleven - but I'm not sure what channel I should be looking for. Any thoughts?

pc

It's under 08 measuring blocks, Group 011.

Well, with an underboost code, I would be looking for vacuum leaks or a boost leak.

So far I've seen 3 different hoses split on various BHWs. Turbo out let. Upper intercooler hose between pipe and intercooler. And the small elbow going to the intake manifold. You usually hear the hissing sound under acceleration when one of those splits, and it will smoke like crazy when you accelerate.

If it's not smoking like crazy when you accelerate, chances are better you have a vacuum leak.

Since you have OBDEleven, the next tool I would recommend investing in is a hand vacuum pump with a vacuum gauge. Remove the hose going to the turbo actuator, plug your vacuum pump w/gauge into that hose to get a reading of how much vacuum is there. Login to the engine computer using OBDEleven, go to basic settings. Enter 11 and activate the basic setting.

The engine RPM will rise to around 1400 RPM and the computer will switch the N75 from fully open to fully closed every several seconds. Your vacuum gauge should vary from somewhere north of 20" mg vacuum to ZERO in correspondance with what the computer is telling the N75 to do.

If you're getting 20" mg or less, you have a vacuum leak that should be investigated. The BHW turbo usually takes only 17" mg to pull the actuator to its stop screw/max boost position, but a healthy system will exceed 25" mg normally and the higher above 20" you are, the more likely you are to avoid an underboost code due to a vacuum leak.

I had trouble with limp mode caused by poor seal at the hose connector O rings. No smoke. I ended up making an adapter to supply continuous low pressure air to the charge air circuit to locate the leak.