There are several possibilities for a P0172. Probably the most common is one or more stuck/dirty fuel injectors on bank 1. If you have or can get Chevron Techron on your side, try a little heavier concentration in the next fuel fillup. A couple of Techron treatments might clean up injectors. And, yes, bad fuel somewhere could contribute. Don't go crazy with an overdose though. maybe a full tank bottle on 3/4 tank of fuel a couple of times.

There are other reasons for P0172
An air leak past the MAF on Bank 1.
Bad fuel pressure regulator
Even something as simple as a poor seal at the oil dipstick. On a personal note, I've replaced the O-ring on the dipstick and I have added a larger O-Ring to fit right under the shoulder below the handle. Finally, I use a simple tie wrap to keep the dipstick solidly seated on the added O-Ring. Overkill without a doubt, but makes me feel better.

All the Best, and luck solving it.

 

 

If the MAF measures a certain volume of air that the ECU is using to properly fuel, but air is lost from a leak past the maf, that would cause an overfueling. This is especially true with turbo or SC setups as, at a certain point, vacuum crosses over to past one atmosphere and pressure exists, not vacuum. You're assuming that an air leak past the MAF is creating a low pressure situation and drawing in air, but can also be the other way.

In closed loop the ECU uses MAF Frequencies (in Hz), to fuel from a table, then an adder (or a subtract) based on the pre-cat O2 FB from the previous combustion cycle. Normal O2 cycles are equal continuous lean/rich. LTFTs continue to adjust over time to compensate for small normal changes to maintain a Stoich AFR, changes such as air filters getting dirty over time, etc. But, in the case of an air leak or vacuum leak (such as a P0171/74) certain normal changes are exceeded (LTFT adjustments made too fast and large), P017Xs will be set. Lean is the worse condition and can be damaging to the engine so most engine management programs I've seen are limited to a 25% lean correction before a P0171/74 are set. On the rich side the 25% rule doesn't apply.

Sorry for getting too wordy.

 

 

I don't particularly care for the MAF placement on the W12. I've built some FI engines on US V8 cars and I've always located the MAF right before the TB and after all piping to and from the boost and air inlet. That way, even with a leak somewhere in the inlet, I have more accurate air intake. The only issue is turbulence around the MAF in case of a leak, or where it is placed in proximity to bends.

This is a good discussion, but I think a reasonable approach for you to take is simplest first. I also think you're on the right track to try the injector cleaner. If after a couple of tries, clearing the code, seeing if it resets, then move onto more complex possibles. At least running rich, you're not in danger of engine damage. However, if you're dumping large amounts of fuel into the exhaust, the Cats won't like that.

 

3 bottles of Chevron ordered

 

This:

 

That's it. Just don't overdo it. OK to go stronger, but probably want to stay away from double dose.

 

So I put the first bottle in, and at the same time cleared codes. This time, using VAGCOM i got a bit more detail than I did with the generic reader.

I also used measuring block 32 to see current fuel trim. The first value is the additive value, so what's happening at idle. -ve means its running rich and the system is trying to lean it out. RossTech Wiki says this indicates a vacuum leak, but then a video by RossTech on Youtube says that a vacuum leak (dipstick not seated) causes a lean condition. So i'm confused.

Wiki: http://wiki.ross-tech.com/wiki/index.php/Fuel_Trim_Info
Video (skip to 2:10):


The fault codes point to potential MAF problem, as it was the dodgy MAF signal that turned on the MIL light this time.

 

Hi, any luck diagnosing this? I have the same issue with my 991 and the same code coming up