Hey everyone so i've recently been having issues with my 2012 Mercedes ML350 it started having long starts and shaking alot. Scanned the vehicle and got P0087 FUEL PRESSURE IN SYSTEM TOO LOW, P0302 MISFIRE CYL 2, P0303 MISFIRE CYL 3 P0300 AND P1CCF HIGH PRESSURE COULD NOT BE BUILT UP DURING ENGINE START. After reading a few posts and checking the fuel pressure i decided to change out the HPFP & 6 SPARK PLUGS W/6 COILS.
After that the truck ran fin for 2 days then it started shaking worse that before. I checked the codes again and I got P0087 and now Cyl 1 & 2 misfire codes. i checked the rail pressure with the scanner and it reads 149-152bar but only while idling. koeo i wasnt getting any reading from the rail.
So I went ahead and change the fuel rail pressure sensor and the in tank fuel pump. Now with the koeo i get 49psi at the rail w/ the fuel gauge But im still getting p0087 and cyl 1,2 misfires at this point i dont know what else to do if anyone has any suggestions i'd greatly appreciate it.

You have covered pretty much all the right suspects:
HPFP
HPFSensor
LPFP
(tank Filter?)

You can disregard the misfirings. They are directly caused by the low pressure.

Let's not bother with fuel trims.... I'd focus on the low pressure fuel pump now.

What's the current mileage ?

What type of pump, 3-Ph right?
Make sure you have equal power delivered to each pump phase winding. Controller may be toast or need a better GND source.

Filter may be plugged up.

++++++ Tank level?
Tank level may be inbalanced L/R... fill up the fas tank to rule this one out.
🤞

Hey thanks for the reply and for the suggestions fortunately I finally figured it out. Reading through all these msg boards I read about one guy who was dealing with the same p0087 code and what worked for him was changing out the coolant temperature sensor. I figured I've already changed everything else short of the injectors so I gave it a shot and it worked. I drove the car hard afterwards to confirm and the misfires were gone along with that pesky check engine. Worst location for a coolant temp sensor too behind the cyl. head but it was worth it.

Good to know a marginal engine coolant temp sensor can lead to your P0087:
FUEL PRESSURE IN SYSTEM TOO LOW.

What mileage on the vehicle?

Yea I was surprised too, but since installing the sensor all the issues have ceased. The mileage is 129,854