bryce jeffords

Hello everyone, my car is having an issue that I can't seem to figure out.

2003 Mercedes SL55 w/ Brabus K8 package

First let me start off with a bit of history. The car was involved in an accident and I acquired it. I repaired it and made it what it is today. Before I acquired it, it had sat for a number of years. From what I know, at least 4 years. We got the car running, new fuel pump, filter, etc., and as it was running for the first time we let it idle. We noticed the fuel pump started whining pretty loudly. It wasn't a consistent whine, more of a surging. We let the car run to see if it would sort itself out, and it essentially killed the pump in about 10-15 minutes of running. We tore out the pump, and filter, replaced it with a fresh one. All from dealer, no aftermarket. Again, once we got the car running, it started doing it again, but we noticed it took a bit longer this time.

At this point, we figured we would take it out and drive it to see what it would do, and the noise would go away! We actually noticed just by revving it in neutral, or say holding a 2000 rpm, it wouldn't do it. But whenever you came to a red light, or sat in traffic, it will whine/surge. So we ignored it, and have driven about 5k miles now without incident, except at idle. So today we have set out to finally fix this problem.

We threw a fuel pressure gauge on it, and I was astounded. It seemed the idle pressure was about 70-75 psi. When we revved the engine, it would drop about 2-5 psi, and immediately bounce back. Seems pretty normal.

This is when things got weird. We let it idle. Once the pump started to make the dreadful whining/surging, we noticed the fuel pump would jump from
75 to 80 and drop back to 75. Constantly. Until you revved the car, it would keep doing this. So we turned it off and restarted it. And the fuel pressure gauge went NUTS. It was bouncing from 40 to 75 faster than I could focus on it. It was really intense. So we killed it again. And the pressure would stabilize right around low 70s.

At this point, I have a few friends that work at dealers, and they were absolutely stunned. They had asked if we changed the fuel pump or filter lately. I told them we had, and it does it almost instantly after each time. They were dumbfounded. They told me they would have to diagnose it, of course, but I really don't want to take it to the stealership and just be annihilated in my pocket.

So, any help is appreciated, any direction. Maybe someone has experience this problem?

Thank you in advance for any knowledge you might have.

cnterline

Check connections at fuse and relay behind pass seat, also there is a fuel pressure regulater of some sort above the rear diff ,unknown if that could be your problem but it's a possibility ,did you drain the tank ? 4 yrs of sitting can't be good for fuel ,if you have scan tool see what your long term fuel trims are this will give you an indication if computer is trying to add fuel ,any ck eng light/ codes ?
If there s crap in the tank that will kill that pump and filter quick

bryce jeffords

I will double check connections today, I checked them before changing the pump and they looked perfect.

I did see the pressure regulator and maybe thought that could be the problem also.

We did flush the tank about 3-4 times before putting new fuel and pump in.

I do have access to a snap on scanner and will take a look at fuel trims today.

Only check engine light we had was a faulty gas pedal, which we replaced.

bryce jeffords

I checked O2 sensors, because i could not find fuel trim numbers on the Verus. O2 sensors look beautiful, but the car does run rich once the fuel pump starts over pressurizing. i ordered the Fuel pressure regulator that sits below the tank. I will probably change it on monday and give an update.

cnterline

I know this may sound retarded but anyway that fuel filter Is backwards ?

bryce jeffords

fuel filter is correct haha. even tried changing that and it blew thru just fine. definitely not clogged in anyway. also, you can see the fuel pressure rise at the actual fuel rail, so i know it's getting passed the filter fine.

cnterline

It's either that funky regulater or that 2nd new pump is shot,I put a new pump in and a week later it was shot .

bryce jeffords

Fixed the problem. the regulator was completely clogged from the gas that sat in the car for 4 years.

cnterline

Good find

hlmpk

Hi Bryce,
My fuel pump whined exactly the same way in the last 2-3 years, and it is getting worse now, so I need to fix it. The noise is really annoying at the red light.
Can you tell me the part # of the fuel regulator and where to find it (website...)

Thanks,
hlm

bryce jeffords

I don't have the part number, however, the regulator is on the bottom of the tank. if you follow the fuel pump lines, you will find it. it's hard to see and is buried under some hoses that go back into the tank.

hlmpk

Hi Bryce,
I bought the part (see picture), but my mechanic says that it will take him 3 hours to lower the fuel tank, and may be also to move the rear axle in order to access the part.
I thought this part could be fairly access from underneath the car and it does not take much time to swap.
(or maybe my mechanic was checking a different part -- missed communication with me -- to come up with such a longer time).

Please, advise.
Thanks

bryce jeffords

Yep that's the part. The tank can't be lowered. It comes out the top of the car. As far as removing the axle, I just unbolted it from the center housing and kind of moved it to the side. As I said, it's accessible part to replace, but it is very difficult and will require patience.

hlmpk

My mechanic replaced the Fuel Pressure Regulator last Saturday.
It only took him 1 1/2 hour. He put the car on a lift that hanged 4 wheels free, so he had more space and did not have to move the axle. He only needs a long screw driver and a long pair of pliers (and some patience).
I was a little disappointed when I took the car out of his garage: that dreadful whining noise was still there! Then I drove another 50 miles that day.
The next day, I took the car out again, drove 30 miles around town but never hear the whine - weird!

I will keep an eye on it and let you guys know.

kennethcole79

did you have old gas in the tank or vehicle sat for awhile? if so mechanic should have at least drain the tank where the hose from the tank to fuel pump is, and flush out the old gas with some fresh gas.

Also if your fuel pump has been been loud for a while it could be the fuel pump on its last leg.

i think the noise bryce was describing was a whine and surging type noise that happens at the fuel pump after a few minutes from starting the car its not immediately after starting.

your problem maybe the fuel pump sound like it on its last leg.

steps i recommend

use chevron with techron gas or the concentrate additive (used it on vettes b4 to resolve fuel sender unit problem also mercedes safe)

1. drain and flush tank at the tank to fuel pump hose located on rear passenger side.
2. replace fuel filter
3. replace fuel pump (they have some used ones cheap on ebay $50-$80 make sure you have the right part number sl500 pump is different ) sl55 pump is the same as the bugatti pump but used 2 instead lol. pumps on the sl55 are pretty strong they tend to go bad from fuel filter clogs and fuel pressure regulator clogs which causes them to overheat.
if that doesnt work

4. change the fuel control unit located on the fuel pump (also can be found on ebay under 100 make sure part numbers match up)
if that doesnt work
5. only thing i can think off would be the evap canister located inside the rear passenger side quarter panel/wheel well. but i doubt thats your problem cause that would throw a code and check engine light. If you have a star unit it would tell you.
s

good luck keep us posted

hlmpk

This thing really puzzles me.
The whining noise seems to subside in frequency and noise level, but it is still there.
Past weekend I took the car to a different Benz mechanic and we could not hear the noise, so I went back home!
The mechanic said fuel pressure is normal, so the fuel pump could just be noisy when it gets hot.
I want to be sure it is the pump before changing it.
[Note: I started to hear that noise 3 years ago, it is just getting lounder and more annoyed; the car performance does not change or degrade so I do not think it is a fuel, tank or filter problem]

hlmpk

I took the car to a 3rd mechanic(!) to change to a new fuel pump.
The car runs more smooth and the new pump is quiet, but that dreadful whining noise still comes back!
But we think we know the problem, I am ordering the part and will replace as soon as I receive it, and will let you guys know what I think happened... - don't wanna talk to early if it does not fix the problem! :-(

kennethcole79

do you think its the fuel pressure regulator

hlmpk

If you look further up, I already changed the regulator.

I believe it is the sensor, will see.
I have problem ordering the part: Mercedes changes the part, now the fuel pressure sensor (for my car) become 2 parts: a replacement sensor and a repair harness (don't know what it really is - no picture) ==> more expensive and I might have to rewire the pump!@#

hlmpk

Let's recap my problem:

Car: 2003 SL55 AMG, 44k miles, 4k miles/year

Problem: What I have changed so far:Here is what I think happened (all numbers are assumptions):Bryce's and my problem might have a similar symptom but due to different causes.
Bryce had a clogged regulator, its pressure threshold is like infinity, so the sensor kicked in to pulse (stop/slow down then restart) the pump.
I think I have a defective sensor, its threshold becomes lower (than the regulator), so the sensor keeps pulsing the pump (to save it from overpressure/overheat), before any excess fuel could flow back to the tank.

The fuel flows through and cools the pump down, but it also packs some heat to the sensor and might change its behavior, that's why I don't hear any noise at start-up or when it is cold outside, but after driving a while, the noise comes up at traffic light.

Past weekend, I changed the sensor and it fixed the problem, voila!

Looking back, since I do not have any pressure tester, I should change the sensor first, because it is the weak point in the system (more easy to fail than other parts)
We did a very quick test by disconnecting the sensor cable and putting it back. When the pump stayed quiet, I knew right away the sensor was the culprit.
[Caution: DO NOT do this test if you are not sure your fuel regulator is new, because if it is clogged, this test does not tell anything and the pump will run unchecked at full pressure ==> dangerous]

At the end, I have changed the filter, regulator, pump and sensor, but not the fuel control unit, yet! (last item in the system)
The regulator and pump seem to be wasted of money, but they are old (14 years) so I hope not to worry about them in the future.