Hey,

Car: Audi a4 b6 1.8t (2003)

I'm getting a P0171 fault code on the car with loss of power. All the information online points to one of these three things:

1. MAF

- I cleaned the MAF with the cleaning solution from autozone. No difference.
- Bought a brand new sensor and put it on, no change.

2. Vacuum hoses

- Almost 100% sure that there is a vacuum leak. But not sure where to start.

3. O2 Sensor

-Could be the pre-cat O2

Looking for any advice or tips/insight.

Thank you!!

 

Yeah there's some good information on the DV, ill be replacing that next weekend for sure.

Yesterday I was inspecting all the vacuum lines I could see and found this piece disconnected from the tubing:



Here is a picture of the tubing and the valve



That piece was just hanging open, so I got some super glue and stuck them together.

Now it seems like I have more power, except the CEL came back on after being reset.

 

The broken part is the brake booster suction jet valve. The come apart frequently.
On the 1.8L any one of the many vacuum check valves can fail and cause a lean condition (P0171). Also look at the entire PCV system for mushy or split hoses and to be sure the crankcase breather elbow is intact (not cracked). It's under the intake right behind the oil filter. The PCV valve itself or metered orifice could have failed as well. It's in the rubber "T" that hooks to the breather elbow I mentioned.
The 1.8L is one of the most frustrating engines there is to find the actual cause of a lean condition, i.e. "vacuum leak" and there's usually more than one culprit. Really it's an unmetered air leak (a vacuum can't leak) but that's just getting nit-picky! lol
Oh yeah, I would highly doubt a bad O2 unless you have very high miles. 95% of the "O2 codes" are caused by something other than the O2 sensor UNLESS it's an O2 heater circuit code.

 

Thanks for the detailed response, this weekend I am going to crack down on this thing and try to find and fix the leak for good.

Yesterday I was going through the vacuum lines and tightening up the clamps on them and found a hose that was completely detatched. I couldn't find anything nearby to connect it to.

Here is a picture of that loose hose and where I found it:

Hose (Below right side of the engine)


Heres the spot where I found it


There's a bar that runs up and over the engine with studs sticking out of it, to seal the hose off temporarily, I connected it to the one of those studs and zip tied it.

Any idea where to connect it to?