Anchor

Hi all,

I'm well aware that there are a handful of threads regarding this issue around the place, but the ones that were solved hasn't been of help for me sadly.

So, the issue from the really start.
I drove about 200-250km's that day, everything was nice and smooth, I did have less diesel in the tank then I usually do, was slightly under 1/4 (barely in fuel light range), as I was going to the store.

When I got out and tried to start the car, nothing.
Tried cranking it for 10 seconds-ish before it finally got running, went home, tried again, same problem cranking long as hell before fires up.

Was quite baffled so I contacted a few mechanics I know, they all said excess injector leak-off.
Sent all 5 injectors to be tested - all green and good to go.
Next thing I tackled was the fuel rail pressure regulator, engine started running smoother (it was hunting for idle earlier which was shaking the whole car) so that wasnt a complete waste, but still, long crank.
After that I replaced the fuel filter, no joy.
All the fuel lines and o-rings in the engine bay are brand spanking new, no leaks everything nice and dry.
Having run out of ideas, I decided to top up the tank until diesel was flowing out of the fuel cap, quess what? Few cranks and fired right up.

Now, to me that looked pretty obvious at start, that there must be air leaking into the fuel system from somewhere.

Since once i turn off the engine, fuel won't flow and its just finding its way back to the tank instead of staying still, which means I need to crank the sucker until its back up to the engine and reaches the pressure needed to fire up. (As in if tank is full, it isnt able to flow back, since there is nowhere to go, and if tank emptier, then there is?)

Blind as a bat as I usually am, a good few days later I finally discovered an actual fuel leak under the car, there are 2 rubber hoses going from lefthand side fuel pump/sender?? to the fuel lines whoch are heading to the engine, anyway there was fuel on that so I popped open the rear seat.

I'll try to attach a picture aswell, but the sight wasn't pretty. The fueltank top where the pump/sender??? sits, had a puddle of diesel on it.
The rubber hoses I replaced didn't look that bad at all, but fearing that there might be an evil micro crack or something I binned them anyway.

After fitting new hoses , clamping all up nicely, the car wont do anything, wont fire if its life depended on it.

Im aware that the Mercs fuel system bleeds itself if you crank it long enough, but after flattening a 80amp battery twice , I also bled it from the injectors slightly.

Fuel was coming with air bubbles still, but it looked like it barely had any flow whatsoever.

Now I assumed that my in-tank fuel pump was weak, talked with a mechanic and he said that a fuel pump cant just go weak?

No clue how much sense all this story makes, i'm confused as hell aswell, not a professional spanner so thats all I got for now.

Fuel pump relay in rear SAM looked a little bit fishy, ill get back home in 2 weeks then I'll put a brand new one in it, but thats just swinging blindfolded.

Oh the car has had multiple diagnostics, once the engine was running, everything pressure ,compression wise and so on was good
No engine codes, only swirl flaps but those are shut off dont matter.

So the overall question, would be, if I should drop the tank out of it and check the lines underneath or does someone with more experience here have a little idea what else usually might create an issue such as this on the W203 platform.

Edit:
And could someone explain what and where are the pumps/lifters/senders or however the in-tank system is, so much controversial stuff on the internet don't even know which is the right one.

As far as the fuel levels go on both sides of the tank, it is equal ,so no issue with the fuel transfer side-to-side.

RedGray

Check what the fuel pressure is. There should be a test port on/near the fuel rail. The fuel testers are cheap and easy to use.

I don't know for sure with diesel vehicles, but I think that all W203 cars have dual fuel pumps in their tank. That's because the tank has a two hump design, to allow for drive-shaft underneath.

Anchor

Alright thank you for the suggestion, will give it a shot once I get home, but that'll be in a while so some more possible causes would be nice to hear aswell, if I finally get my hands on it I got a little checklist to follow 😃

Anchor

Apparently my fuel pump is in the engine bay near the HP pump , and those two in the back are to show fuel level if I understood correctly 🤔

I'll measure the pressure soon though and if thats no bueno, I got a hunch that the fuel lines ontop the tank have thrown in the towel and is letting air into the system 😐

ItalianJoe1

There's still a low pressure pump in the tank that feeds the high pressure pump in the engine bay. They all have to work and can't have leaks. There should be a port on the low side before the high pressure pump.

Anchor

A little bit of a bad pic but its the closest one on my phone, but I was told that the matchbox size thing above the HP pump, is responsible for getting fuel out of the tank, getting controversial again idk what to believe 😂

Thanks for your answer though, once I get home I'll dive into the fueltank and see whats there.

Reinhardt

Hi Bud,

Still having problems with fuel supply?

Anchor

Hey, the 2 fuel lines ontop of the fuel tank (starting from the front of rear left wheel well up to the fuel sender pump) were leaking.
I cut through the old lines at the bottom and ran 2 new lines and she is going strong till today.

Now I problaly have an issue with the MAF sensor, sometimes it dosent want to rev over 3.5k

Reinhardt

Bud,

Did you check your egr sensor?

Anchor

Diagnostics gave me a MAF error so based on that I quess it needs to be replaced/cleaned somehow.

Or the car is just confused as I read the errors but didnt delete them yet.(was using brake cleaner sometimes etc to start it before the pipeworks got really leaky)
Last time I checked with diagnostics it gave me the crank/camshaft position sensor etc. Replaced those aswell ,but I havent wiped the errors to this day... Should do it

Will be cleaning the EGR anyhow when I get time to work on it. Thanks for the idea.