Previously after applied some fuel cleaner on my car, the P0191 error code was temporally gone.
However, unfortunately it came back several months later, this time with obvious symptom of poor ignition, which I highly suspect has something to do with fuel system obstruction.

Therefore, I removed the sending unit from the car, and after disassembled it, I found it was composed of a fuel filter and a fuel pressure regulator.
Since I was unable to find a fitting fuel filter at that time, I simply asked my mechanics friend to change a new fuel pressure regulator for me, and put the old fuel filter back.
After the fix, the poor ignition was solved, the P0191 CEL was gone for about 1 month, then unfortunately it came back again.

This time, I did tried to find a fuel filter which was the same size as the genuine one, and changed it onto the sending unit to replace the old filter.
Then I applied a bottle of fuel cleaner, then 2 weeks had passed, so far so good. The P0191 CEL is gone, at least for now.

Therefore, my guess is:
1. The genuine fuel filter wasn't long lasting as Mercedes said.
It will break down as time goes by.
And as it breaks down, there will be some deposition which runs from the fuel filter to the fuel pressure regulator, then even obstruct the fuel pressure regulator.

2. If we apply some fuel cleaner, the deposition in the fuel pressure regulator will be cleaned temporally.
However since the fuel filter is still breaking down, it's only a matter of time that the error code pops up again.

3. Therefore, my current suggestion is:
If the P0191 CEL popped up, change the fuel filter as soon as possible, then apply a bottle of fuel cleaner afterwards.
This way we may prevent the fuel pressure regulator to suffer from further damage.

The fuel filter I used was febi bilstein 39831.
Everyone can simply buy one via google search with a cheap price compared with an entire sending unit, not to mention an entire fuel pump set.
We simply have to enlarge the central hole of the new fuel filter to the dimention of the old fuel filter, then put the old o-ring back.
It's a easy job even for DIYers.

Hope this information helps and we can all get rid of the annoying P0191 error code for good.

 

Thanks for taking the time to share and document your findings. Great info!

 

still gone?

 

So far so good.
However currently I'm having my wheels refurbishing so I didn't have much miles on my car.

 

Update:

After previously changed the fuel filter, the P0191 still presents from time to time but was with significantly decreased frequency.

Weeks ago I also had my intake manifold repaired, in the process I also asked the fuel rail to be cleaned and changed some of the o-rings with signs of leakage, also had the PCV valve changed.
So far the P0191 never popped up again.

 

Thanks for sharing the update. I have had the same issue since purchasing my car. No driving issue, just the CEL. I will be experimenting with fuel injection cleaner over the next 2 - 3 fill ups to see what my results are.

 

Bumping this thread in case it helps someone.
Different car, same engine. I've had the code P0191 (NO CEL) since 2016. Idle and performance off-the line were excellent. Only issue was the pesky code always appears after reset.
Dealer reported fuel pressure 4.0-4.5 bar (395 kPa- 450 kPa), and we didn't get low fuel pressure codes and the pumps passed the SDS test.
Here are the things that were replaced to fix the issue (in-order):
1- Fuel filter
2- intake manifold
3- PCV
4- Fuel rail
5- Fuel injectors
6- Both MAF sensors
7‐ Upstream O² sensors

None of these cleared the code. Today I am happy to report that replacing the fuel pump finally cleared the code and the car is better than ever. Looking at the old VS new pressures, it appears that the old pump was underperforming by 0.5 bar ( ~ 50 kPa) causing the code to appear. Please note that WIS specifies a constant pressure of (4.5 - 5.5 bar) for the M156 (except the W221).
Previously, the scanner showed the PWM correction at 25.0%. Leaving the ECU to increase injector timing instead as it reached the limit for the pumps modulation.

Good luck

 

What was this repair?

 

When filling your fuel tank avoid the habit of keep adding fuel to round up to the next dollar or to fill car to the max, once the fuel nozzle stops its done, don’t keep squeezing more fuel into the tank.
This creates an overpressure condition which can cause a lot of this problems, first time getting this code, clear the code and stop topping your fuel and you might never see the code again, if you keep getting the code then the damage is done.

 

Guys....

This is in response to Post # 13 regarding corroded gas lines, etc. Braided stainless steel gas lines would be the way to go provided you can get the correct fittings.

Just a thought.....

Rob