May be a dumb question but everything I've read says healthy fuel pressure should be around 80. My 04 idles around 78. But I also read that the internal pump regulator is 5 bar (72.5psi). If the regulator is 72.5 psi, shouldn't anything above that be bleeding back into the tank?

If I actuate the pump in STAR with ignition off it primes and holds a steady 75. But once I turn the car on it holds 78 at idle. How is it possible for this to be higher than the regulator? And higher when the car is running? Is it the heat expanding fuel in the rail and raising pressure?

Asking because I am having an issue with pressure rapidly fluctuating between low 70's and low 80's during certain low rpm high load conditions, but holds 78 psi on highway or higher rpm. Trying to see if it could potentially point to regulator or maybe the damper (but everything I've read about damper failures just talk about them leaking)

 

The dampeners fail and cause +- 10 psi fluctuations mostly at idle. Mine did, and it wasn't leaking. The fluctuations are caused by all 8 injectors opening amd closing, obviously slower at idle speed, so the fluctuations are worse. Raising rpm smooths out the pulses as the cycles are happening faster.

 

+1 on the dampener. Regulator is stamped 5.1 Bar (74 psi).
I would hypothesize pressure change is due to difference between voltage engine off (12.3v) versus engine on (13.9 v). Regulator looks like stamped steel. Good enough for government worwork... something to discuss over beer.

 

Is the damper on the fuel rail? I'll have to check mine at idle....

 

Yes, on the back section of the rail.

 

So i guess its a maintenance part. I will be replacing soon than.

 

Interesting. Can you buy this dampener separately at stealership and just install and replace the one? Never looked at this

 

Following up, damper fixed the issue for me. The old one was not leaking and not problematic at idle, only during certain circumstances, particularly at higher speeds 50+ with very little, <10% throttle input, or for a few seconds just after a completed shift. Very easy to replace, make sure you let out fuel rail pressure and stuff some rags underneath the damper before you pull it out. A surprising amount of fuel dripped out of mine. Pull airbox and throttle body and it's right there. Comparing the two the new one looks to have some type of mesh inside, on the old one I could not see this mesh. Not sure if it was damaged or disintegrated , or just completely covered with gunk. Definitely a maintenance item. PN 1120780092

 

Benzparts.com doesn't think that fits my 04 E55 but FCP Euro does. Is it right for all years?

Did you replace your TB gasket?

 

Not sure if it is the same for all years but I got mine from FCP also 04. I reused the gasket, did not seem to have any issues. Cleaned the TB while it was off. There is also a single retaining clip that holds the damper to the rail, not sure what the P/N is for that yet but I am looking. It is pretty beefy but would feel more comfortable replacing. It looks to be the only thing holding the damper to the fuel rail. If it were to fail the damper and loads of gasoline would be shot all over a hot engine. I am sure the clip has fatigued to some degree after 16 years of heat cycles

https://www.fcpeuro.com/products/mer...des-1120780092

 

I'm thinking that Mercedes really wants to sell/replace the entire fuel rail rather than this part. I can't find a diagram anywhere that lists it separately. So the chances of getting that retainer seem pretty slim.

Did you have any drivability issues (surging perhaps) or STFT symptoms to go with it? Just trying to find any piece of evidence that I should do this...I've got maybe a little idle roughness on a cold start.

 

Im sorry guys but I am having a real hard time with this Dampner being a pressure regulator. It is not. But feel free to tell me I am full of sheet .
What it does is "dampen" the fuel pressure pulse's each time the injectors fire. This helps to keep pressures consistant in the rails and against each injector.
Mine has been removed and showed no change at all in data log at the time of just removing it. Fuel pressure is basically controlled by the PCM with voltage and will slightly go up when supercharger is on.
A dampner that has failed may show fluctuations in pressure due to it not doing it's job but as far as controlling the actual pressure it self...no.,as It has no where to bleed off and excess or increase.

 

So with a fresh dampener then, I shouldn't see my fuel pressure dropping on WOT pulls? As my fuel pressure drops currently under WOT and have always wondered about that.

 

I don't think we're confused, other than how the thread started. Later on he tried a new damper and that fixed it, but not because it is a regulator. Its job is in the name. Dampens rail pressure due to injector pulses.

So...for TB gasket....#7 or #5 of below??

https://www.benzparts.com/v-2004-mer...and-components

 

Looks like 7.
5 looks like the round seal where the Y pipe attaches to the TB

 

I ordered one of each and the damper. FCP Euro was priced like everyone else and now I can throw this in as a tuneup item as I please. I mean, I probably won't, but I could...

What I should do is connect my fuel pressure gauge and check the pressure at idle before and after just to see.

 

Pretty sure it will drop a little under full rpm and boost. I really can't remember and my system is boost referenced now so a little different .

 

That may be and is a function of the regulator being in the tank vs at the rail (call it another curse of the returnless system). It's the electrical equivalent of having the voltage regulator being at the alternator or sensing remotely after all the voltage drops. If you measure pressure at the outlet of the fuel pump it likely would be rock solid but still show a slight drop at the rail, due to pressure drop from flow under heavy demand (like resistance in a wire causing voltage drop during current flow). That's my attempt to explain. How much it should drop is the real question. I'd say less than 5 psi would be acceptable but would be much better to have a spec from Mercedes...

 

Thanks. I don't think it's dangerous as a few tuners have been through my logs and no one has said anything about it. So I'm sure it's in spec. Just something I noticed. But I am not going to scratch an itch, if it isn't there...

 

I see more than a 5psi from start of a pull to the rev limit. However my AFR still sits perfectly, so I am not to bothered over it.

 

Who cares where the pressure is..."with in reason" as long as the wide band says what you want ....your good.

 

Just wanted to update this thread since I participated.

I checked my fuel pressure off of a cold start just to see what the "before" looks like. I do want to say that my rail was at ZERO when I connected the gauge, so it clearly bled down overnight or whatever. I assume that's not normal? Though it doesn't seem to cause any issues. The pressure seemed to hold after shutoff but again apparently not after hours and hours of sitting. Anyway, pressure was 77 psi and wasn't fluctuating too bad but then as the idle came down a little, the needle started bouncing more. Maybe +/- 5 psi? Enough to warrant replacement of the damper.

Side note, if you have a pressure gauge with a relief as mine has, you can bleed down pressure very easily and safely. Of course fuel does dribble out of the rail even starting at zero and for some reason I wasn't prepared for this so it made a small mess under the car.

It's not a particularly hard job. Pulling the TB is super easy. Hardest part is that retaining clip. It did not make me nervous as I was expecting something made out of thin wire based on some other descriptions in this thread. Just a little fiddly to remove and install but vice grips helped the install (couldn't push it with my thumb, I tried). No need to disconnect the wiring harness of the TB, just set out of the way. I had cleaned it previously but I was able to get it 100% with it in my hands obviously.

New damper in, primed with ignition on, no leaks. Yay.

New TB gasket. I have no idea where that O-ring is supposed to go that I ordered, so I didn't use it. Took my time cleaning the surfaces to get all old gasket material off, per any usual repair.

Everything reinstalled, checked pressure again with the gauge. Same 77 psi of course but only the smallest needle bouncing and it never got any larger unlike before. So I'd say this was a success.

Driveability seems....the same? Maybe the transition into the blower is a little smoother (makes sense)? Certainly worthwhile as mine needed to be done. Nice thing is since this is a tuneup part (easy enough to check annually with a gauge but maybe also just put on a ~2 year schedule or something), I will never have to buy another since I bought from FCP Euro.

 

part number for the clip is 0009848161, about $3 at the dealer

 

My car is a 2000 E55 amg, the fuel pressure is 58 psi, is this correct? I saw comments of 55 - 60 psi and others of 70 - 77 psi, which is correct?

 

Completely different vehicles. W210 E55 M113 normally aspirated and one external fuel pump. W211 E55 M113K supercharged with two internal fuel pumps. Apples and oranges. Consult WIS for confirmation as I do not feel like firing up the computer tonight.
If I feel like it tomorrow, I'll hook up a fuel pressure gauge to my W210 E55 and see what the pressure is.