Fuel rail pressure and pump concern
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Joined: Apr 2020
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From: Lille, France
2009 E350 CDI Bluefficiency 4matic
LP pumps should be giving 70+/-5 psi pressure so either there's a leak somewhere (injector or rail valve) or the pump is restricted. I don't know too much about this pump but usually there should be a strainer before the pump so it might be clogged with debris if you have in the system.
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2012 ML350 Bluetec, 2015 GL350 Bluetec
is this strainer in the tank? it seems like a big difference 20 psi. I purchased a new LP pump to make sure this side is good. I will have to check this strainer as well. IMO it may be the LP pump or HPFP. Do you know if the seal kit for the HPFP would help with pressure?? What I understand is they use it to stop leaking.
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From: Lille, France
2009 E350 CDI Bluefficiency 4matic
I'm not sure if this pump indeed has a strainer, but if there's one it should be in the tank attached to the inlet of the LP pump. When you lift the pump out you should be seeing it if there's one. When you disassemble the HPFP and replace the seals you will be also checking the measurements of the moving parts and if everything is within spec it should be working properly.
Something interesting today. I went to take some fuel out of the tank. The level was showing almost 3/4 tank full. I used the LP pump to pump it out and I took about 8 gallons and then a lot of bubbles started coming out until no more fuel was leaking. The level now shows 1/4 full tank. The pump wasn't pumping at a constant speed, it was like taking a brief 1/2 second break. At this point, I think the LP pump may be bad.
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2012 ML350 Bluetec, 2015 GL350 Bluetec
Something interesting today. I went to take some fuel out of the tank. The level was showing almost 3/4 tank full. I used the LP pump to pump it out and I took about 8 gallons and then a lot of bubbles started coming out until no more fuel was leaking. The level now shows 1/4 full tank. The pump wasn't pumping at a constant speed, it was like taking a brief 1/2 second break. At this point, I think the LP pump may be bad.
The symptom can simply be a faulty fuel level sensor though.
Thread Starter
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From: Lille, France
2009 E350 CDI Bluefficiency 4matic
That's a clear sign that there's something happening in your tank. However it might not simply be the LP pump is bad. You will need to do a more thorough check to prevent future problem happening again. Take the LP pump out and check carefully both on the pump and inside the tank and let us know what you see.
The symptom can simply be a faulty fuel level sensor though.
The symptom can simply be a faulty fuel level sensor though.
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Joined: Apr 2012
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2012 ML350 Bluetec, 2015 GL350 Bluetec
I agree geniushanbiao, the level sensor may be bad. Earlier before the fuel pump out, I was hearing this noise from the pump while pumping when there is not enough fuel in the line, but air too. You know what I am talking about. I will open the tank and see what is happening and let you know. Should I open both sides or just the passenger side under the back seat?
Thread Starter
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From: Lille, France
2009 E350 CDI Bluefficiency 4matic
Will do in a few days, the new pump is on the way. I don't want to leave the tank open for so long. I will let you know.
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From: Lawrence, Kansas
GL320
The fuel tank is shaped like a saddlebag with a siphon hose that transfers fuel from one side to the other. If it read 1/2 a tank and now reads 1/4 but one side is dry, its because the other 1/4 tank of fuel is held in the opposite side. There are 2 sending units that average the level.
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Thread Starter
Junior Member
Joined: Apr 2020
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From: Lille, France
2009 E350 CDI Bluefficiency 4matic
The fuel tank is shaped like a saddlebag with a siphon hose that transfers fuel from one side to the other. If it read 1/2 a tank and now reads 1/4 but one side is dry, its because the other 1/4 tank of fuel is held in the opposite side. There are 2 sending units that average the level.
when you say 2 sending units, do you mean 2 electric pumps or it is something else?
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From: Lille, France
2009 E350 CDI Bluefficiency 4matic
Got it. I think I watched a video where the in-tank hoses were broken and there was no transfer of fuel. It seems this is common problem.
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R320, Astro, JSW, ST4
I've not actually tried to pump the tank all the way down with the LP pump.
But I have (mistakenly) replaced mine. From what I've been told by my mechanic, these very seldom fail, unlike those in the early PD VW TDIs.
There is a simple screen on the bottom of the pump assembly, I don't think it's intended to be removed.
The same pump is responsible for supplying the LP fuel to the HP pump and balancing the fuel levels between the sides of the saddle tank.
It sounds like the fuel system design expects the pump to be running only as the motor is running (,or getting ready to start - priming).
I suspect the bubbles are a side effect of no return fuel from injector overflow lines, and one side of tank goes low.
I don't think replacing the pump will get the other side of the tank emptied - I think it will bubble just the same as it is now.
You will need to open up both sides and get a hand pump to empty both sides completely.
But first - You've got most of the fuel out.
Can you get someone else to sniff it for a gasoline smell?
I worry about you buying all these parts and doing unnecessary things like I did.
Also, MB design for the fuel pump sealing ring at the tank is overly complex and often requires replacement (x2) sides, in case you don't have them on the way.
If you are very careful you may be able to reuse, I needed to replace 1.
(Sorry, don't have part number handy)
But I have (mistakenly) replaced mine. From what I've been told by my mechanic, these very seldom fail, unlike those in the early PD VW TDIs.
There is a simple screen on the bottom of the pump assembly, I don't think it's intended to be removed.
The same pump is responsible for supplying the LP fuel to the HP pump and balancing the fuel levels between the sides of the saddle tank.
It sounds like the fuel system design expects the pump to be running only as the motor is running (,or getting ready to start - priming).
I suspect the bubbles are a side effect of no return fuel from injector overflow lines, and one side of tank goes low.
I don't think replacing the pump will get the other side of the tank emptied - I think it will bubble just the same as it is now.
You will need to open up both sides and get a hand pump to empty both sides completely.
But first - You've got most of the fuel out.
Can you get someone else to sniff it for a gasoline smell?
I worry about you buying all these parts and doing unnecessary things like I did.
Also, MB design for the fuel pump sealing ring at the tank is overly complex and often requires replacement (x2) sides, in case you don't have them on the way.
If you are very careful you may be able to reuse, I needed to replace 1.
(Sorry, don't have part number handy)
Thread Starter
Junior Member
Joined: Apr 2020
Posts: 52
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From: Lille, France
2009 E350 CDI Bluefficiency 4matic
I've not actually tried to pump the tank all the way down with the LP pump.
But I have (mistakenly) replaced mine. From what I've been told by my mechanic, these very seldom fail, unlike those in the early PD VW TDIs.
There is a simple screen on the bottom of the pump assembly, I don't think it's intended to be removed.
The same pump is responsible for supplying the LP fuel to the HP pump and balancing the fuel levels between the sides of the saddle tank.
It sounds like the fuel system design expects the pump to be running only as the motor is running (,or getting ready to start - priming).
I suspect the bubbles are a side effect of no return fuel from injector overflow lines, and one side of tank goes low.
I don't think replacing the pump will get the other side of the tank emptied - I think it will bubble just the same as it is now.
You will need to open up both sides and get a hand pump to empty both sides completely.
But first - You've got most of the fuel out.
Can you get someone else to sniff it for a gasoline smell?
I worry about you buying all these parts and doing unnecessary things like I did.
Also, MB design for the fuel pump sealing ring at the tank is overly complex and often requires replacement (x2) sides, in case you don't have them on the way.
If you are very careful you may be able to reuse, I needed to replace 1.
(Sorry, don't have part number handy)
But I have (mistakenly) replaced mine. From what I've been told by my mechanic, these very seldom fail, unlike those in the early PD VW TDIs.
There is a simple screen on the bottom of the pump assembly, I don't think it's intended to be removed.
The same pump is responsible for supplying the LP fuel to the HP pump and balancing the fuel levels between the sides of the saddle tank.
It sounds like the fuel system design expects the pump to be running only as the motor is running (,or getting ready to start - priming).
I suspect the bubbles are a side effect of no return fuel from injector overflow lines, and one side of tank goes low.
I don't think replacing the pump will get the other side of the tank emptied - I think it will bubble just the same as it is now.
You will need to open up both sides and get a hand pump to empty both sides completely.
But first - You've got most of the fuel out.
Can you get someone else to sniff it for a gasoline smell?
I worry about you buying all these parts and doing unnecessary things like I did.
Also, MB design for the fuel pump sealing ring at the tank is overly complex and often requires replacement (x2) sides, in case you don't have them on the way.
If you are very careful you may be able to reuse, I needed to replace 1.
(Sorry, don't have part number handy)
I have opened the tank today, but only the pump side. I took out the pump. I was only able to inspect this side of the tank. There was about 2" of fuel left. I did not see any debris. I will open the other side tomorrow and inspect the other element and the tank as well. I did disassemble the pump, there was not much garbage on the filter either. It is not much of a filter but a piece of plastic net. It looks like this is the original pump and it went long-distance - 236K miles. I noticed another thing when pumping out the fuel - the pump did not start to pump 1 or 2 times and it cut it short 1 time. Also, while I was measuring the pressure at the filter I noticed it went back to zero as soon as I turned the key back. I would assume the pressure should stay for a while. Is there a one-way valve or something which maybe releases the pressure and needs to be replaced?
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2012 ML350 Bluetec, 2015 GL350 Bluetec
I have opened the tank today, but only the pump side. I took out the pump. I was only able to inspect this side of the tank. There was about 2" of fuel left. I did not see any debris. I will open the other side tomorrow and inspect the other element and the tank as well. I did disassemble the pump, there was not much garbage on the filter either. It is not much of a filter but a piece of plastic net. It looks like this is the original pump and it went long-distance - 236K miles. I noticed another thing when pumping out the fuel - the pump did not start to pump 1 or 2 times and it cut it short 1 time. Also, while I was measuring the pressure at the filter I noticed it went back to zero as soon as I turned the key back. I would assume the pressure should stay for a while. Is there a one-way valve or something which maybe releases the pressure and needs to be replaced?
What I'll suggest you for the next is to first give the tank a good cleaning make sure there's no debris or anything inside the tank. An inspection camera might help you inspect the interior of the tank better. Drop in the new pump and connect the feed line, but don't connect the return line at this moment. Instead, connect the return line to a large clean bucket while plug the return line connector on the pump. Put fresh clean fuel into the tank and switch ignition on that will use the fresh fuel to flush the lines. Keep doing this for like a minute or so and then connect the return line back, switch ignition ON and then check the fuel line pressure.
Last edited by geniushanbiao; May 12, 2020 at 04:49 PM.
Thread Starter
Junior Member
Joined: Apr 2020
Posts: 52
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From: Lille, France
2009 E350 CDI Bluefficiency 4matic
That's good to hear that there's no debris otherwise you are in big trouble. The little plastic net is the strainer I talked about. If it's not clogged that's good. If the pump intermittently does not start you might want to check the connectors and wires. If they are all good then the pump is on its way. At this mileage, I'm not surprised to see an LP pump fail although it doesn't happen that often. Things do break.
What I'll suggest to you for the next is to first give the tank a good cleaning make sure there's no debris or anything inside the tank. An inspection camera might help you inspect the interior of the tank better. Drop in the new pump and connect the feed line, but don't connect the return line at this moment. Instead, connect the return line to a large clean bucket while plug the return line connector on the pump. Put fresh clean fuel into the tank and switch ignition on that will use the fresh fuel to flush the lines. Keep doing this for like a minute or so and then connect the return line back, switch ignition ON and then check the fuel line pressure.
What I'll suggest to you for the next is to first give the tank a good cleaning make sure there's no debris or anything inside the tank. An inspection camera might help you inspect the interior of the tank better. Drop in the new pump and connect the feed line, but don't connect the return line at this moment. Instead, connect the return line to a large clean bucket while plug the return line connector on the pump. Put fresh clean fuel into the tank and switch ignition on that will use the fresh fuel to flush the lines. Keep doing this for like a minute or so and then connect the return line back, switch ignition ON and then check the fuel line pressure.
it starts around 7:00
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2012 ML350 Bluetec, 2015 GL350 Bluetec
I will do that. Only thing is, I got no car to get more fuel lol. I gotta get somebody to get me to the gas station. The fuel looks ok, clean and clear. You don't happen to know about this one-way valve though? I watched a clip where the guy was explaining something about that. The pressure after contact and 15 seconds pump run should stay the same - to allow the car to start immediately, not have to wait to build pressure again - hence some long crankings. Here is the video, watch it and let me know what you think.
I know there's a pressure limiting valve (the part 1 on the photo) on the fuel rail that's controlled by the computer. Its function is simply to release the pressure after engine shutdown. That's the only valve I can find in the document.
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From: Lille, France
2009 E350 CDI Bluefficiency 4matic
I see. So what is this "fuel check valve at the fuel pump" in the tank this guy was talking about? As I understand, there will be a one valve to hold the fuel not to go back to the tank when you turn off the key.
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2012 ML350 Bluetec, 2015 GL350 Bluetec
Usually fuel pumps are designed with a check valve in it. It's meant to help maintain the pressure in the feed line so if your feed line doesn't have enough pressure you replace the pump then the check valve is replaced at the same time. No need to worry too much about it.
Thread Starter
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Joined: Apr 2020
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From: Lille, France
2009 E350 CDI Bluefficiency 4matic
Usually fuel pumps are designed with a check valve in it. It's meant to help maintain the pressure in the feed line so if your feed line doesn't have enough pressure you replace the pump then the check valve is replaced at the same time. No need to worry too much about it.
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W211 320CDI -04, W210 270CDI -01
WIS document also says how much HP pump should return flow to tank in order to get siphon system work and transferring fuel in saddle tank from side to another.
If you read function of the system more carefully both 1 and 8 are controlling the pressure during engine run. True that rail valve releases pressure when de-energized.
If you read function of the system more carefully both 1 and 8 are controlling the pressure during engine run. True that rail valve releases pressure when de-energized.
Thread Starter
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Joined: Apr 2020
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From: Lille, France
2009 E350 CDI Bluefficiency 4matic
WIS document also says how much HP pump should return flow to tank in HP order to get siphon system work and transferring fuel in saddle tank from side to another.
If you read function of the system more carefully both 1 and 8 are controlling the pressure during engine run. True that rail valve releases pressure when de-energized.
If you read function of the system more carefully both 1 and 8 are controlling the pressure during engine run. True that rail valve releases pressure when de-energized.
So I got the new pump, replaced it and the pressure was exactly the same as the old one - 49 PSI. I think the gauge may not be accurate. So the old pump is good. Now, it turns out the fuel was indeed Gasoline, as B34chBum said. I think the ****ers from the gas station did something because I definitely did not pump gas in the tank. So I emptied the tank, flushed the lines, put 10 gallons of premium diesel. Tried to crank the engine. No luck, the rail pressure did not go above 43 bar. I used some starting fluid and the engine tried to run, but it died quickly. The pressure did not go over 43 bars. I Unplugged the control valve and the pressure sensor, still no luck. Interesting fact - after I used the starting fluid, the engine started cranking at 230-250 RPM. It was cranking at 185 RPMs before. So, what is next? Injector leak off test? I got a better scanner, I can see a lot of stuff now. Not sure what all numbers mean. I suspect one or both MAFS may be bad. But that does not explain the low pressure. Any suggestions will be appreciated.
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From: Lille, France
2009 E350 CDI Bluefficiency 4matic
++ , on being sure you put only diesel in tank - Especially since your troubles began after fill-up.
Slow down, be sure of basics first. You say you just dumped the filter into container, then strained it. So you are looking at things from Both sides of the filter media.
That is to say, you are looking at what the filter trapped and did Not allow into the high-pressure pump, etc.
I see it has been 2 days, hopefully you haven't yet torn everything apart.
You could attach more fuel line to the one at the filter inlet, and route it to a clean container.
Turn key to On not Start and allow the tank lift pump to push some to your container.
Smell it carefully, is it diesel Only?
If you're not absolutely sure, or if you think it smells at all like gasoline, empty the fuel tank and refill with good fuel.
Put a fresh fuel filter in, and ...
Slow down, be sure of basics first. You say you just dumped the filter into container, then strained it. So you are looking at things from Both sides of the filter media.
That is to say, you are looking at what the filter trapped and did Not allow into the high-pressure pump, etc.
I see it has been 2 days, hopefully you haven't yet torn everything apart.
You could attach more fuel line to the one at the filter inlet, and route it to a clean container.
Turn key to On not Start and allow the tank lift pump to push some to your container.
Smell it carefully, is it diesel Only?
If you're not absolutely sure, or if you think it smells at all like gasoline, empty the fuel tank and refill with good fuel.
Put a fresh fuel filter in, and ...
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Joined: Apr 2012
Posts: 356
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2012 ML350 Bluetec, 2015 GL350 Bluetec
I don't have the WIS, I can't see how much HP should spit out. Now here is what happened today:
So I got the new pump, replaced it and the pressure was exactly the same as the old one - 49 PSI. I think the gauge may not be accurate. So the old pump is good. Now, it turns out the fuel was indeed Gasoline, as B34chBum said. I think the ****ers from the gas station did something because I definitely did not pump gas in the tank. So I emptied the tank, flushed the lines, put 10 gallons of premium diesel. Tried to crank the engine. No luck, the rail pressure did not go above 43 bar. I used some starting fluid and the engine tried to run, but it died quickly. The pressure did not go over 43 bars. I Unplugged the control valve and the pressure sensor, still no luck. Interesting fact - after I used the starting fluid, the engine started cranking at 230-250 RPM. It was cranking at 185 RPMs before. So, what is next? Injector leak off test? I got a better scanner, I can see a lot of stuff now. Not sure what all numbers mean. I suspect one or both MAFS may be bad. But that does not explain the low pressure. Any suggestions will be appreciated.
So I got the new pump, replaced it and the pressure was exactly the same as the old one - 49 PSI. I think the gauge may not be accurate. So the old pump is good. Now, it turns out the fuel was indeed Gasoline, as B34chBum said. I think the ****ers from the gas station did something because I definitely did not pump gas in the tank. So I emptied the tank, flushed the lines, put 10 gallons of premium diesel. Tried to crank the engine. No luck, the rail pressure did not go above 43 bar. I used some starting fluid and the engine tried to run, but it died quickly. The pressure did not go over 43 bars. I Unplugged the control valve and the pressure sensor, still no luck. Interesting fact - after I used the starting fluid, the engine started cranking at 230-250 RPM. It was cranking at 185 RPMs before. So, what is next? Injector leak off test? I got a better scanner, I can see a lot of stuff now. Not sure what all numbers mean. I suspect one or both MAFS may be bad. But that does not explain the low pressure. Any suggestions will be appreciated.
It might be injector leak. This agrees with both that your LP and HP pump cannot build enough pressure. If there's leakage this could happen. The leak test can be done by disconnecting the return line from the injector and hook up a clear tygon tube and hang it vertical to the hood. Check this video
Thread Starter
Junior Member
Joined: Apr 2020
Posts: 52
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From: Lille, France
2009 E350 CDI Bluefficiency 4matic
First of all, I'd suggest not putting new parts until you are sure the whole system is clear of gas. If you pumped out some gas then lots of parts can be ruined.
It might be injector leak. This agrees with both that your LP and HP pump cannot build enough pressure. If there's leakage this could happen. The leak test can be done by disconnecting the return line from the injector and hook up a clear tygon tube and hang it vertical to the hood. Check this video
https://www.youtube.com/watch?v=iSp6I3OiaeQ
For your case you do not need the tube with marks. Just a clear tygon tube from hardware store will work. See if there's any leak with ignition ON but do not crank.
It might be injector leak. This agrees with both that your LP and HP pump cannot build enough pressure. If there's leakage this could happen. The leak test can be done by disconnecting the return line from the injector and hook up a clear tygon tube and hang it vertical to the hood. Check this video
https://www.youtube.com/watch?v=iSp6I3OiaeQ
For your case you do not need the tube with marks. Just a clear tygon tube from hardware store will work. See if there's any leak with ignition ON but do not crank.
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2012 ML350 Bluetec, 2015 GL350 Bluetec
Theoretically the leak down test should be done when engine is running, so you have the normal rail pressure (~23000psi) and that will tell you how the injectors are working. For now what you need to make sure is just there's no injector leaking badly that's causing the rail pressure not building up.
