I had both high and low pressure error codes on my fuel rail. I changed out the fuel rail (my rookie self) and now the high pressure code persists.

Is there a common culprit that causes this?
Occasional stutter at idle may be related

Backstory: I had the shop replace the valve gasket because I could not guarantee I'd do it without contaminating the fuel system. The engine light came on soon after that repair with those codes but the shop claimed innocent / coincidence.

The shop suggested the new fuel rail but their price was insane. Did it myself. They charged me to clear the codes and we found the high pressure code comes back. They suggested a fuel filter and low pressure pump 🤔. That made me question life considering it's a pressure too high code.

So I've come to the true experts. Any ideas? Please help. I can't afford these repairs.
*I have a scan tool now that can read fuel rail pressure and reset the codes.
*Sometimes the code takes a minute to come back or even have to drive a up my driveway.

 

Should I try:
- Leak off test
- replace leak off line
- replace high pressure pump
- replace fuel filter
- consider I got a bad fuel rail ( was new Bosch part )
- rebuild injector ( can I narrow down the culprit?)

 

I would do a diagnose first, as reading Fault codes it's just the opportunity to start the step by step tests suggested by the Diagnose System like Xentry. As it's infinite more cheap to use a diagnose system ( yoursef or with help of a technician ) compared with random parts exchange.

The pressure buildup is made by the valve on the high pressure pump and after some time the valve from rail begin to regulate.

 

There is a bunch of stuff that we should be able to rule out based on their function within the fuel delivery system. As the previous poster suggested, diagnostics software would help.
injectors are not likely the problem. If one or more were bad, you would run pretty rough, and likely have a CEL and accompanying misfire code.
Fuel rails themselves tend to not fail, since they are really just tubes that get fuel from the high pressure pump to the injectors. I would not look here.
Fuel filter and pump assembly. This is all within the fuel tank, feeds fuel at lower pressure but higher volume to the high pressure fuel pump. When this pump and filter assembly is failing, it tends to result in difficult starting, because it is not getting enough fuel to the high pressure pump. I would not rule this out, but would also not consider it the most likely culprit.
High pressure fuel pumps can fail, but you want to be super-duper sure before that one. They are expensive, and don’t typically fail.
What you refer to as the “leak off line” I assume is the fuel return line. This sends excess fuel from the rail back to the tank. This is controlled by a valve. If this valve fails, you may experience either high or low rail pressure, depending on how it fails. This is definitely a possible culprit. Not 100% sure, but this valve likely has an electrical connection. I would verify that this is actually connected properly. It’s a simple thing that can cause problems, and if not connected properly would likely result in consistently high pressure.
You did not mention the pressure sensor. These can fail, and cause error codes like what you are seeing. These are relatively cheap, and I have seen them fail on other cars.
Fuel delivery problems can be hard to diagnose without the right software. If you don’t have the software yourself, you should find a mechanic who does, or buy it yourself.
good luck and keep us posted.

 

Hi @Adam Gallant ...

I'd have to agree to narrow down the possible issue.

One odd habit I have (among many) is when I'm in this forum, and reading a thread, I always scroll down to the last Reply ... then I scroll down a little more and will see a section named, "Related Thread".

There's one for your post. And although their problem is "P0087 Low Fuel Pressure", there are some great Replies in there (started Jul 2021 thru Jan 2025!), to include some in-depth troubleshooting.

Also, I HIGHLY recommend that you first go to Reply #2 by "konigstiger" ... in their Reply is a PDF document:
"Introduction of the M276 and M278 Engines", produced by MB ... lots of great detail, to include nice detailed images. Here's the thread:
.
https://mbworld.org/forums/e-class-w...re-sensor.html

 

Backstory: I had the shop replace the valve gasket because I could not guarantee I'd do it without contaminating the fuel system. The engine light came on soon after that repair with those codes but the shop claimed innocent

Which “valve gasket” did you have them replace?

 

Valve cover gasket. Are there other gaskets in there I'm not aware of? Anyway, the fuel rail had to come off in order to replace it

 

Thanks for your time and reply. I have read a lot of threads but nothing seems to fit. The suggested pdf is for a different engine (mine is 651).

 

Huge thank you for the detailed response. I have had both Mercedes and a third-party mechanic shop scan the car for me. Mercedes spent an hour on it and told me they wanted two more to narrow it down. I don't have much time to work on it but I'll update as I go

The fuel rail in the om651 includes the pressure regulator and sensor. That's why it was suggested to change it out in the first place.

I was able to confirm that the stutter I have at idle does coincide with an increase in fuel pressure on the fuel rail. Thoughts on what might cause the stutter?

 

Like a misfire, on an average (random though) of about twice a minute the vehicle will stutter. I was able to catch the fuel pressure go from 270bar to 400 for a second.

My scanner only updates about once a second so I don't know the true value of how high it goes.

Thoughts on what would cause this

Captured spike in pressure. Normally it's 270bar. The RPM drops too

 

Are you suggesting the pressure fluctuation is caused by the stutter? I'd guess it was the other way around.

 

The pressure increase too much and the management force instantly decrease and when the pressure abruptly drops the rpm shudder.

 

That seems more probable.. I was thinking something along the lines that an injector wouldn't spray due to something wrong during the last cycle; leaving pressure in the rail.

 

The rail pressure is not cyclic, in fact, the volume of the rail acts as a reservoir of fuel at a constant high pressure. The individual injection events don't have a significant effect on rail pressure. OTOH, a momentary drop in rail pressure (in response to the spike) would be accompanied by momentary loss of power.

 

Million dollar question then: what would cause the spike? The pressure regulator is new included in the rail, so? High pressure pump? Any other suspects

 

Sorry, I can't afford million dollar answers! Can you put an actual pressure gauge on it and find out if any of the sensors are lying?

Also, are you certain of the ordering of spike and stutter?

 

Hi Adam, Any updates on this issue?

 

Hello everyone, Please help, deleted 13 GLK 250 codes 0087, P062A. Low power but sometimes gets better, so not consistent, 8 liter to 100km consumption. No limp mode. I bought it this way.
Replaced lift pump assembly, lift pump drive module, High Pressure fuel pump, no difference. Along the way testing air charge boost leaks, turbos wear. Codes persistent and 0087 show up at just revving it. No xentry diag. to dig dipper.
Any help is much Appreciated

 

TLDR: some modules had to be trained into the new fuel rail by Mercedes.

After talking to dozens of mechanics a shop finally told me they didn't know what was wrong (didn't charge me hours thankfully) and asked that I try Mercedes again and see if there's a Learn In software that had to be run.

I had asked Mercedes before but this time I got somebody new and they told me that it was a thing. Unfortunately when I brought it to Mercedes, who I hate, they ran the computer for nearly 2 hours of me sitting there and told me it didn't work. I want to sell this car. I've had so many problems, so when they asked if they could look further into it I reluctantly said yes. They drove me home and by the end of the day they finally called and said you can come get your GLK. IT WAS FIXED THE CODE WAS GONE. What did they do? Apparently "found more modules that needed to be trained". What BS.

Now I have a light for my coolant. I replaced The water pump and I'm not quite sure why I'm getting the code because it comes on so rarely, but I think there's a small air bubble in the system. And I can't just run the car in the driveway because the diesel doesn't warm up enough to burp the system unless I'm out driving. This isn't a great winter Court for that reason. My toddler's car seat is always freezing no matter how long I idle the car before we go.