Mercedes Fuel System Diagnosis

Andy Technician Lutherville-Timonium, Maryland Posted   Latest   Edited  
Question
Driveability
Emissions
2008 Mercedes-Benz ML63 AMG 6.2L (M 156 E63) 7-spd
P0191 - Fuel Rail Pressure Sensor Circuit Range/Performance Bank 1
3699: Fuel Pressure Control: The Upper Limit Is Reached

Hey Fellas,

History: I have a 2008 ML63 giving me some trouble. It originally came in a year ago for the same code. It needed other things and it apparently wasn’t a priority to them at that time. From what I understand it has been on and off since. Suffice to say it has absolutely no effect on the driveability of the car.

Now the truck is back to specifically have this issue fixed. (I assume due to our bi-annual emissions testing in Maryland). Looking at the freeze frame the fuel rail pressure was 52.6 psi, Open Loop, 4 seconds run time. FP definitely doesn’t seem high? Due to the code description I immediately went after the fuel rail pressure sensor to see if anything was obvious there. The Verus Edge with 17.4 shows very poor factory level data. Global honestly has a vastly better data set. Taking the car for a drive I never saw more than 62.2 psi and less than 49.1 psi. Numbers that definitely do not draw any suspicion. I then verified clean reference and ground at the sensor and scoped the output voltage (screenshot included during code set). I found that if I clear the code and start the car cold, or partially warm, I can get the code to reset in 4-5 seconds after 2 to 10 starts. Always in park, no throttle necessary. The pressure has never been more or less than 50.2 psi or 52.6 psi in those conditions. Hardly enough to elicit a Upper Limit Reached code being set.

ProDemand doesn’t have much information on this truck other than a fuel spec of approximately 3.8bar (55.11psi). Absolutely no help on the DTC index. I’m almost ashamed to say it but I have never purchased OEM information before, but this truck seemed like a great candidate so I took the leap. We spent $60 on a Mercedes one day subscription and after a lot of digging and familiarizing myself the best I could, I found their DTC index. It was as helpful as ProDemand with no information. Basically the trouble tree is to see if the data is plausible and that was it. No mention of tests or specs whatsoever. I played around in their WIS program looking for fuel information and the like. Maybe it was my frustration of the situation, or that I know time is limited, as well as other cars that needed to be fixed. Either way, I am here for any and all insight that my colleagues may have. The only things I could find on Google and AMG forums were saying the dealers were recommending pumps and those that went that route had success. I know dealers have the techline protection and after time spent on the phone running specs are given the grace of installing parts without the worry of financial burden. This gentleman, however, has a extended warranty and I need to be a lot more sure before we go down a potentially $2000 dollar path for parts/labor/&diag.

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Raymond Diagnostician
Phoenix, Arizona
Raymond
   

If the engine runs normally, Based on those codes alone i would Replace fuel pump assembly, found some information related to the code 3699 but cannot post pdf here, is not an accepted file. I would also flash and SCN the engine module.

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Andy Technician
Lutherville-Timonium, Maryland
Andy
 

I would feel a lot better about the situation if I could find some set criteria. What could the computer be looking at considering the pressures look good but it sets the code like clockwork? I don't have a PID for pump duty cycle but that doesn't mean it doesn't exist in the computers logic. Also you would think if the duty cycle was higher to produce appropriate pressure then it would hesitate

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Raymond Diagnostician
Phoenix, Arizona
Raymond
 

Having the factory scanner we rarely have to get involved with such drawn out technical diagnostics. based on information i have read the normal fuel pump Idle voltage should be 5-6 Volts and current draw is 7 Amps, but this is actual values from the Mercedes scanner, not sure if yours will show this value. Does your fuel pressure bleed off or go up when static?

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Andy Technician
Lutherville-Timonium, Maryland
Andy
 

I do not have the voltage and amperage PIDs in the Verus OBD2 but I could check another tool that may. I just checked the pressure like you asked and it rises.

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Raymond Diagnostician
Phoenix, Arizona
Raymond
   

rises when off? found this in my notes from training Check engine light concern, no drivability concern. If you encounter a ME 3698 checkThe correction factor: PWM signal fuel pump via the path shown below.A 3698 will show this factor at a -15% or more--check for a restriction in the return,check return flow to tank from fuel filter pressure regulator. If there is no returnflow all available

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Andrew Technician
Commack, New York
Andrew
 

Here's what you need to do: Start and idle the engine, go to Engine > Actual Values > Further Actual Values > Fuel System Here you can see actual and target fuel pressure, as well as "Correction Factor". 3699 will set if the correction factor exceeds 15%, a normal value is 0% to -5.0%. This problem can be caused by a bad fuel pump but it is much more commonly a vacuum leak. You also

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Andy Technician
Lutherville-Timonium, Maryland
Andy
   

Thank you very much for the excellent response. I am going to have to see if my other tool will give me those correction factor PIDs. I did smoke test the intake system the end of last week. I made discs to fit into the inlet ovals so I could get a good positive pressure seal. The only thing I found was the seal at the base of the two to one inlet. I would like to see if I can see that

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Chris Diagnostician
Commack, New York
Chris
 

Generally what I do is put a block off on the throttle body and go into a vacuum hose. I do love your custom block off plates though.

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Andrew Technician
Commack, New York
Andrew
 

Mercedes has a bulletin for this where they tell you to like take a piece of sheet metal and trace the intake plumbing onto it, then run it through a band saw and glue it together!

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Andrew Technician
Commack, New York
Andrew
 

Andy make sure you check for leaks with propane/carb cleaner, not all leaks show up under pressure. Did you check the vacuum lines for the secondary air valves and the valves themselves? If they are leaking they may only cause a vacuum leak when secondary air is active so make appropriate tests. Check the mixture adaptations you should have "idle speed range, left cylinder bank" and "idle speed

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Andy Technician
Lutherville-Timonium, Maryland
Andy
 

Thanks, Andrew. Unfortunately I am feeling very electronically under tooled for this. As I said the Snap-on data for this car as of 17.4 is literally worthless. I tried my Auto Enginuity to no avail as well. That had much more data, but oddly enough didn't even have fuel rail pressure, much less the correction factor that I would really like to be able to see. The only other thing we have is an

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Andy Technician
Lutherville-Timonium, Maryland
Andy
 

So today I installed the Y outlet/throttle inlet boot and re-smoked the intake manifold. Even wiggling the Y around it did not leak any smoke whatsoever (Although I found the MAF g/s to be extremely close to before). I then taped over the small holes under the filter housings, and around the filter inlet necks. After that I felt like the tract was as sealed as it is going to get. I then removed

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Chris Diagnostician
Commack, New York
Chris
 

Ok, so let's go with what you have and what you know. I'm not seeing anything in the fuel pump wave form to warrant replacement just yet. Did you get an rpm on those pumps? You may not have a "factory" scan tool, but you do have a damn good global OBD2 scan tool. We can fix this using obd. If you aren't intimately familiar with all the different make's correction rates and fuel strategies, then

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Andy Technician
Lutherville-Timonium, Maryland
Andy
 

I'm not sure how many commutators the pumps have. If it's 8 then I'm seeing about 12.5 seconds of the left pump waveform = 4800 rpm. Each segment in the screenshots is 2 ms. I have been using the global OBD2 of the Verus through the whole diagnostic. The Mercedes data on this software revision/vehicle is far too poor to do anything with. I have provided a cold start data stream, idling then

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Andy Technician
Lutherville-Timonium, Maryland
Andy
   

I'm calling this one fixed. Well after a lot of testing I decided it was time to pull the trigger on the pumps. I replaced the fuel filter unit on the left side of the tank, and both pumps on the right side of the tank (also if you do this job remember to replaced the sealing ring kits. They come with tank threads, lock rings and sealing rings). Also the pumps do indeed have 8 commutators.

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Andrew Technician
Commack, New York
Andrew
 

Well, hopefully that did the job. Mercedes is great about most things and I love them, but they are not great about code criteria. The actual monitor criteria are a secret that only the engineering teams have documentation on. In general, guided paths in the test tool and experience are what directs atypical engine performance diagnostics. If you have a subscription to Startekinfo even for a

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