I couldn’t find any existing thread that described what I’m experiencing so I’m making one. About two months ago I driving to work when I felt a massive loss of power and a rough idle. I had misfires and a system lean code, and a bad MAF. Somehow the spark plug became unplugged (don’t ask how it still confuses me) and I replaced the MAF. This fixed the issue... or so I thought. Yesterday my 8 (S2 6speed) threw the p0171 code again. I pulled the intake and checked all the hoses but everything seems to be connected without leaks. Any ideas what it could be? I have no power loss but it will idle a bit low if I start it hot just after shutting it off for a couple of seconds.

 

Bad injector possibly?

 

I wouldn’t think so. I don’t have misfires or loss of power. It honestly seems to be running perfectly normal

 

Kinda weird that a MAF would suddenly go bad.
On a warm idle can you get the STFT/LTFT values from OBD? If it's getting air somewhere without going through the MAF you would get high fuel trims there. Possible that installing a new MAF left a small air leak in its mount, for example.

 

I’m not sure if my OBD will read it. I have a MaxiSys MS906TS. I’d have to play around with it to see. What STFT/LTFT levels should I be getting?

 

Here are my active STFT/LTFT

 

I also did a smoke test but found nothing. Everything I find on the internet mentions the VFAD solenoid. Would that throw a separate CEL? Or is the P0171 the code it throws?

 

So it looks like you have a significant enough air leak that the computer can't always compensate for it. Maximum fuel trim is 25% and you're at 21 in that screenshot, so definitely possible for it to get too lean.

There is no CEL for the VFAD itself, but if the vacuum line (or any vacuum line) is not connected or cracked, this is what you would see. The VFAD line is out of the way under the UIM and intake that it's often overlooked.

 

There are two right? One that goes under the air box to a metal line and one under the intake to a green and white bauble by the throttle body. I checked the latter for leaks with the smoke machine but I didn’t find anything. If the lines aren’t bad, what’s the likelihood the solenoid is bad?

 

It should all connect to a nipple under the UIM that you can't really see, have to go by feel. The solenoid wouldn't really leak itself. It takes a pretty big leak to get to +25%, so the VFAD *could* be it, but the real answer is to eliminate every possible connection.

Also for the MAF, what is the OBD airflow g/sec at idle rpm fully warmed up?

 

It varied from 4.1 - 4.26 which is normal right? You typically want 2-7 g/s at an idle

 

You typically want 5.5

 

So my MAF readings are low? Wouldn’t I expect them to be higher seeing as it’s running lean...

 

Would using a cold air intake fix this issue?

 

Most air intakes will make this worse.
The MAF is reading low because some air is not going through the MAF and getting in somewhere else instead. The computer is trying to compensate with fuel trim but maxes out the possible trim and goes lean. 4.1 is too low, the car needs about 5 to hold 800rpm idle.

I know you haven't found it yet, but that air is getting in somewhere...

 

So when further inspecting, can I rule out anything past the throttle body? Or the everything suspect

 

You can't rule anything out without examining. There's a bunch of vacuum lines and solenoids behind the UIM that may be damaged.

 

So I’ve taken the entire intake system apart and haven’t found anything. I’ve driven it about 8 or so times trying to get the CEL back but haven’t been able to do so. Which seems good but my trim and MAF readings really haven’t changed. Trims down to 18 ish and MAF is around 4.7

 

Without being there it's hard to be more specific, but the low MAF rate and high fuel trim point to unmetered air getting in. Are tou working with a vacuum diagram that covers all possible connections?

 

I went ahead and set up an appointment with a local rotary mechanic. I know when I’m beat. Hopefully he can figure it out. They’re even going to compression test it for me

 

I’m also getting a too lean code. Started after a recent track day. I’ll check the obd2 readings tomorrow. Car doesn’t feel like it’s lost any power but it idle has gotten rough and will occasionally die when I come to a stop at the idle rpms hunt up and down about 300 rpm.



I cleared these and only the “Too lean” keeps coming back