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fuel pressure too low woes

27K views 19 replies 8 participants last post by  DieselBound 
#1 ·
a couple of weeks ago my car went into limp mode

was halfway to my indie so went there first, he scanned it quickly with his snap on and thought it was probably the fuel pump but was not sure

he was super busy and my thinking was we would probably end up needing to the star machine anyway so made a huge mistake and took it to the dealer

they did a bunch of testing on it and determined it was the fuel rail
was showing fuel rail pressure top low and that the fuel pressure regulator valve inside the fuel rail was bad

i questioned this, but the mechanic they had on it was their best diesel mechanic, he is very good, my indie felt he was right and i thought if by some chance they were wrong the dealer would step up and do the right thing

NOPE

get the car back and 1 day later limp home again
dealer had it towed back and 3-4 days later have diagnosed it as having had a bad fuel rail AND a bad fuel pump

do not believe this for a second
they want another 1000 to repair it

i am yanking the car out of the dealer
ordered my own parts and am having the indie do it

have been toying with the idea of buying one of the star machines on ebay and in the middle of all this decided that i should have done this 2 years ago, it would have paid for itself twice over by now
i have 2 mercedes and my best friend has one

definitely would have been worth it and think a lot of us should consider doing that

tucson dealer just sent another email standing by their story that i had both major components go out

i am not a happy customer, really upset
 
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#2 ·
What did they do the 1st time, replace the pressure regulator valve? Did they replace the rail?
Its highly unlikely that the valve, the rail and the pump went bad at the same time.
I would guess the pressure was low, they replaced the valve, it didn't work, they decided if they replace the rail and the pump then there is nothing else to replace, problem fixed.
I'm not sure that your indie would have done any better, sometimes you have to eliminate a couple of symptoms before you cure the patient.
One thing I would do is go back to the service manager and ask him to eat the valve repair cost, this is assuming that the rail and pump do actually fix the problem. But it sounds like you already bought the parts and the indie is going to take over.
Let us know how it works out
 
#3 ·
sorry i should have been clearer

the fuel pressure regulator valve is inside the fuel rail so you have to replace the entire fuel rail

so entire fuel rail was replaced just to get that valve and now they are saying it is the fuel pump

this car was running probably 97% before it went into limp mode
mpg were down a little but other than that car was really strong and running smooth, probably no one but me would be able to tell there was an issues at all

then limp mode, once limp mode was cleared car runs strong and no problems until you step on it hard and then limp mode again

i think the fuel pump was weak and it caused the fuel pressure valve problems......
maybe i am wrong but that makes more sense to me

i know sometimes we have to throw parts at these things, but 1500 for the fuel rail and labor is a lot and now it needs another 1000 in fuel pump etc

my experience with the dealer have all been ok up to this point, other than anytime i roll a car in for anything it is there a week to 2 weeks

my indie and i would have had this done in a week and probably started with the fuel pump rather than the rail
 
#4 ·
Officially they sell only whole rail and pressure regulator assembly, but AFAIK regulator is changeable like earlier, you just cannot bought new regulator only.

There are couple of more possible culprits for low fuel pressure, blocked fuel filter, fuel quantity control valve, worn injectors, primer pump...

I would go for refurbished fuel pump...?
 
#5 ·
i got a new mercedes fuel pump from husker for about 400

it will get put in the car thursday or friday

i certainly hope this is the problem and not something else lol

is it even possible to check fuel rail pressures correctly if the pump in the tank is not putting out enough pressure?

the car has 200,000 on it but i am still not convinced that both this sensor and the fuel pump went out at the same time
 
#6 ·
Like Marseum said, could be a few things. Obviously make sure the fuel filter is in good shape. From there I would put a "T" fitting in the larger diameter fuel line that runs from the firewall to the stainless steel hose barb just down from the aluminum intake manifold on the drivers side. The smaller diameter tube is a fuel return line, so don't bother with that one. You need at least 23-25 psi from the in-tank fuel lift pump to allow the high pressure pump to make enough pressure to start the vehicle. Obviously you can start the vehicle, so I know for sure that you are getting the 23-25 minimum. The question is if your in-tank fuel pump is putting out enough pressure when you stomp on the accelerator. If your in-tank fuel pump is weak then you can be starved of fuel when you romp on the accelerator. Leaking injectors, and perhaps the quantity adjustment valve on the high pressure pump might also be culprits, but I would check the fuel pressure from your tank first like I mentioned since it's easy to do and will at least rule that pump out of the equation.
 
#7 ·
thanks guys

will give this to the mechanic

unless i got a bad fuel filter all my filters are changed ahead of miles recommended
guessing in the almost 2 weeks the dealer has had it that was checked...

the dealer has been testing stuff on it for 3 more days and this is their diagnosis
so hope this time it was definitely the pump and not another sensor
although the sensor might be cheaper lol

gets worked on friday
hoping to drive it some this weekend and will keep this thread up to date on what happens etc
 
#8 ·
one question i have is that if the all along the fuel pump was a little weak and not putting out correct amounts of fuel, wouldnt all the sensors that come after that in the fuel system be likely to show some issues?

to me since that is the first thing in the loop , it just seems some of the sensors after it would think something was wrong somewhere....
 
#10 ·
when i first took it to the dealer they spent $300 just getting it diagnosed and determined it was that sensor in the fuel rail, it was showing a fault code

and that the in tank fuel pump tested fine using the star machine

they had pages and pages of diagnostics/graphs etc

then get the car back and it immediately does the same thing in exactly the same driving situation as before

have it towed back to the dealer and now they say the star is saying the fuel pump is weak, supposedly have another bunch of star printouts showing this....

insist both things were wrong which i find hard to believe

either way they want 630 for the fuel pump and seals
and 500 labor to install it - said they gave me a break on the parts lol

and they want to keep the car until next monday or tues to get it done, retest it etc - the car has basically been down 2.5-3 weeks with all this


no way i am paying them another 1100

pump should be here tomorrow from husker and my indie will hopefully get it installed tomorrow afternoon

will keep this up to date on whether this solves my problems or not
 
#11 ·
Star will only be able to check the voltage and current going to that pump, not it's outlet pressure and flow. Star is probably looking for open circuit, short circuit, and normal operating current for that particular fuel pump. Problem is that just because it's drawing the correct amount of electrical energy doesn't mean it's producing the flow and pressure needed. That's why from the beginning I mentioned to hook up a "T" on the fuel line going to the engine. Some diagnostics need more than just looking at a computer screen and self checks. Glad things are progressing on the fix for you.
 
#12 · (Edited)
I agree, and you were 100% right and I should've listened to you

Biggest problem was it was in the shop at the dealers and I get nowhere with them compared to dealing with my indie

So lesson learned and i plan to buy a star and only deal with my indie from now on

Josh i really appreciate all the advice you gave me and think you were right on with everything
 
#13 ·
I am just keeping my fingers crossed that it works out for you. I think your right, the Indie would be the only person willing to step outside the box and use "alternate" trouble shooting methods vs just running diagnostics from the computer. Best wishes, and looking forward to stellar results!

BTW, I would be interested in the old pump assembly if it becomes available :)
 
#15 ·
i hope the pump is the problem

according to the dealer it is....

will suck if it is something else, especially like an injector

fedex did not get the fuel pump here until almost 4pm today, so we are putting that in monday

will keep this up to date
 
#20 · (Edited)
I'm working on this very thing on my car (an up-to-date thread that I WILL follow up on*).

* I've been across the entire Internet and run across WAY too many cliff-hangers- I find it horribly rude. For reference note that this is pretty much all about basic common rail stuff. I run across a ton of such things on, in addition to Sprinters [with om647s]. Cummins, Duramax and Fords- focus on ones with CP3s and you'll get a good feel for all of it.

om648 low rail pressure errors
 
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