Lean condition after test pipe

[Dr.B6Banter]

Would a test pipe cause a lean condition code? I tried searching around about this issue but could not find anything conclusive.

2 Faults Found:

17841 - Secondary Air Pump Relay (J299): Open Circuit

P1433 - 004 - No Signal/Communication

17519 - Oxygen (Lambda) Sensor Regulation: Bank 1: System too Lean

P1111 - 002 - Lower Limit Exceeded

My understanding was that the primary O2 sensor that is used for lambda correction is before the cat and should not be influenced by a test pipe. I did a log of measuring blocks 01 and 115 to see what the lambda correction values were at idle and under load. The results were a bit confusing because the lambda correction values were quite high (meaning fuel is having to be added IIRC) at idle and they fliped to negative values once I started driving. I did not notice much fuel having to be added or removed under full boost which in my mind would rule out a fuel pressure (pump/filter) issue.

I tried searching for a long term lambda measuring block to log that to see if the value averaged out somewhere 0 but I could not find any such measuring block. At least in the following log the average of all lambda values was around -1 which seems normal enough, but the extreme spikes seem odd to me.

Link to log:
https://drive.google.com/file/d/1sv4...ew?usp=sharing
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[Protection]

Fuel being added at idle (lean condition) then returning to normal under load is a definite sign of a vacuum leak. You are running lean at idle because the leak is allowing the intake to draw in more air than is required. However once you increase RPMs, because the leak is relatively small comparatively to what the engine is drawing in, the fuel correction returns to normal or near normal.

I would smoke test the intake. But in the absence of having a smoke machine, you can look for loose or damaged vacuum lines. It could be any line going into the intake after the MAF. I would specifically look for lines you may have bumped while installing the test pipe.


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[Dr.B6Banter]

Fuel being added at idle (lean condition) then returning to normal under load is a definite sign of a vacuum leak. You are running lean at idle because the leak is allowing the intake to draw in more air than is required. However once you increase RPMs, because the leak is relatively small comparatively to what the engine is drawing in, the fuel correction returns to normal or near normal.

I would smoke test the intake. But in the absence of having a smoke machine, you can look for loose or damaged vacuum lines. It could be any line going into the intake after the MAF. I would specifically look for lines you may have bumped while installing the test pipe.


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Makes sense. I disconnected the TIP and more importantly the IC piping going from the turbo to the metal crossover pipe while installing the test pipe so that lower IC pipe may have started to come loose.

Would that type of vacuum leak not cause a high idle?

[Protection]

It will not cause a high idle in my experience. I accidentally overlooked a vacuum line leading into the intake manifold while working on my 1.8 B6. The idle was normal. The only sign there was a problem was when the MIL light illuminated. (I got the same code as you). A smoke test quickly revealed the problem, as smoke was pouring out of the unplugged port on the manifold. (The port faces toward the ground on the underside, so it was not easily seen.)



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[EuroxS4]

Lean condition is not too much fuel,its more air than fuel at idle,ecu is adding fuel to compensate. If ecu was subtracting fuel at idle you would have a rich condition fault. Make sure all the gaskets you changed when the test pipe was installed are on correctly. o2 sensor can pickup outside air if gaskets are not changed or not properly installed. causing them to read extra air causing lean condition.

[Dr.B6Banter]

Looks like it ended up being a vacuum leak. I checked the TIP to look for any obvious vacuum lines that I forgot to reattach but couldn't find anything so I moved over to the driver's side and noticed one of the nylon vacuum lines that goes from the intake manifold to what I believe is the vacuum reservoir was broken near the quick connect fitting. I'm assuming there is a check valve before the break in the line since it was just a vacuum leak and not a vacuum and boost leak.

Used a bit of fuel line to patch it up and went for a drive and the fuel trims seem normal and no lean code yet

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