It a good thing it starts easily, and runs well/smoothly at 4k. Your high pressure pump sounds like it's in good health. Since you don't get any codes at these operating ranges I would tend to agree it's most likely a Y94(Quantity control valve) or a Y74(pressure regulator valve) issue. I'm thinking that perhaps some internal components in those particular valves are rather loose from age, and at certain RPMs they reach resonance with the engine vibration and refuse to respond properly to the N3/9 commands. Most likely your problem is in the Y74(Pressure regulator valve) which is on the rail, and would be subject to the highest vibration amplitude.

Keep in mind the N3/9 actively controls the engine smoothness up to 1200 RPM. Also the Y74 contains a spring, magnet, coil, and a ball seat. Like I mentioned earlier I can see these components(plunger and ball seat specifically) vibrating heavily at certain engine RPM's and refusing to close off properly causing the rail pressure to drop significantly.

 

I would check the rail pressure regulator O-ring, these are now available separately. It does not always help, so rail pressure diagnosis would be useful.

 

Thanks for the fast reply Diesel Benz and Josh8loop!

Actually I had a spare brand new Y74 (Pressure regulator valve), and changed it. No difference. Didn't notice any O-rings on this? Was it inside the rail or something? There are no signs of leakage. I guess that if there were a lekage diesel would come out, and not air in....?

I have ordered the Y94 Quantity Control valve today to see if there will be any difference.
It sounds right what you are saying about vibrating....this could for sure be the cause. The car is around 8 years now so it could be possible.

I will keep you posted on this one. Thanks again for your help.

 

Maybe a dumb question, how long has the fuel filter been in the car?

 

Not a dumb question at all
I change the fuel filter every spring, and I changed it recently with no better results..... So I've covered that one too

 

It may not be a bad idea to inspect the electrical pins on the connections to the Y74, Y94, and the N3/9. Perhaps you have a loose pin that is getting a little wonky at certain RPM's? Most likely not the problem, but couldn't hurt to check

 

Will check that if the replacement of the Y94 valve doesn't help....Thanks.

 

Update on this one......
I have replaced both the Y94 and the Y74 valves......Same shit.

But! today I noticed something else..... When i switch on the ignition I can hear the fuel pump in the rear start and pumping fuel...... I waited a while until the I knew fuel was filling the front fuel filter and that there were sufficent fuel for the high pressure fuel lines. Started the engine and no problem. The engine is running fine on idle.
I thought that I would try to press the pedal and it was stable at 2000 rpm too! At least for a while. Then the old shit started again.

This made me suspect the low pressure fuel line from the tank to the filter in the front. Maybe a faulty low pressure fuel pump or a clogged filter?

I started researching this and according to star diagnosis the fault code I got (P2021-001) may be caused by low pressure in the low pressure fuel lines.

I think this may be caused by the rear fuel filter... These filters hardly ever get replaced as far as I know....

But there is one thing I dont't understand on the W211... is the fuel filter on the left side, and the fuel pump on the right side? How does this work?

 

Bumping this one again.....

Today I tried to find the fuel filter in the tank, but no luck. I can't understand whre it can be hidden.. is it inside the pump or something?

This is getting frustrating....

 

Not sure about the diesels, but in the gassers there is a filter in the pump.

 

Meaning the whole pump has to be replaced....?

 

Unfortunately yes. They call for it at 60k miles on the gassers, but I have not done mine yet.
I am not sure what the diesel interval is suggested by MB. I would check with a dealer to
make sure there is a filter in the pump( I have a feeling there is) and what the interval is.
Then get a good aftermarket one if you think it is plugged up.
How many miles are on the car?

 

350.000 km (217479 miles...?)

 

If there is a filter in the pump it is due, or the pump my be getting weak at that mileage.

 

Update again.....

I've checked the lift-pump in the diesel tank and cleaned the "cup" where the pump is fitted. There is a filter below the pump with fairly big holes in it. There were some dust and some sand in the cup below the pump and I cleaned it.

I had the inspection hatch open and listened to the pump. Everything seems fine and plenty of diesel is getting through.

I refitted everything and started the engine. Everything sounded normal. When I tried to press the gaspedal it worked for a few seconds and then the problem was back again.

Okay I thought.... this was my last shot and I had nothing to loose, so I drove the car in a backyard behold a local school (local time was around 01.00 AM) =)
I opened the hood and I thought I would try to disconnect everything related to rail pressure.
I disconnected the B4/6 Rail Pressure sensor. Then I got into the car again and started to press the gaspedal again. Holding it steady on around 2000rpm. No problems at all! Wtf I thought..... So I decided to go for a little ride, and the car run just fine! I pressed the pedal to the medal, no problems at all!
The car seemed to go into limp-mode because of this of course and had a limit of 3000rpm, but I was able to drive the car in 70-80km/t (around 50mph) with no problems at all!

I guess I have to try to change this one.... The problem must be that it reports wrong values back to the ECU or something.....
Will update this thread after the B4/6 sensor.......

 

I guess I have to try to change this one.... The problem must be that it reports wrong values back to the ECU or something.....
Will update this thread after the B4/6 sensor.......

I'm guessing it isn't the B4/6 sensor but the car just runs when it is not able to detect incorrect rail pressure regulation. I hope I'm guessing wrong because I think you already changed most of the more likely parts that could cause this error.

 

Seems that you were right Diesel Benz.... It wasn't the B4/6 sensor either...
What would you suspect next? ........ offh.... I'm getting tired of this piece of crap

High Pressure Pump next?

 

Wow! What a battle you have had with this issue. Good job sticking it out! When you run the engine does it smoke alot? Do you have a DPF, and or CAT? Wondering if at certain RPM ranges one of your injectors is stuck open and dumping fuel into the cylinder unmetered? This could be approximately diagnosed by comparing exhaust manifold temperatures exiting the cylinders.

 

****** SOLVED! *******

Believe it or not, I had this one solved today!
The problem wasn't on the fuel-side, the problem was a leakage on the connection between the hose from the intercooler to the turbocharger.

T?umik dolotowy turbiny CDI | Silnik \ Przewody \ Przewody intercoolera | sklep-merc.pl

That's the best picture I could find of it (afaik I'm not allowed to post a picture of it from EPC)

The leakage probably was probably so large that the engine struggeled to charge air, and the motor got problems keeping up, the result was that the camshaft didn't run smooth enough to get the high pressure pump going. It's the only possible explanation I got for this.
StarDiagnose mentioned a charge air faultcode just once, and I ignored this
This is a fine example to show that you need some more knowledge of the system as a whole instead of just following diagnostic equipment step by step

But it was completely out of my mind that the problem COULD exist on the charge air side.

Anyway, just wanted you guys to know. Thanks for all your assistance, and I hope this thread will be helpful to someone else.

Case closed.

 

Found this interesting so I figured I would bump it.