C270 fuel supply issue.

[Anchor96]

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.

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.

 

[mersum1es]

You don't have pumps in tank. Aux pump (match box size front of the engine) draws fuel from tank. You may pop out aux pump delivery line (going to hp-pump located front of the engine underneath it) when someone cranks engine. If fuel starts flowing out just push it back. It is relatively safe because no high pressure but use eye protectors at least.

If hp pump is getting fuel you have left leaky injector/pressure regulator/hp-pump. You can also measure injecors tank leak yourself, did they do that in test garage? And leak is usually temp sensitive and worse when cold, I bet they tested injectors at room temperature.

Diagnostic tool would give a rail pressure measurement. Rail pressure sensor may (rarely) be giving wrong value to ecu and it refuses to start. By unplugging sensor car should start (in limp mode) if pressure actually is sufficient to fire. If you still have air bubbles in lines, it may well be preventing rail pressure to rise enough.

 

[Anchor96]

Thank you, thats alot of useful information ,
Diagnostics showed that rail pressure was normal with the engine idling, when it was running there were 0 problems, no performance loss, stutters,cut-outs - all good only start was hard and now it wont even do that .

Fuel rail sensor was replaced after the last diagnostics so I should have another look at rail pressure.

So the actual fuel pump is in the engine bay just above the HP pump, and the ones in the tank are lifters?

 

[mersum1es]

On idle everything may be ok but during cranking problem escalates because low cranking RPM, pump cannot compensate even moderate leak in system. Pressure should rise above 150-250bar (cant recall exact value) during crank, before ecu gives permission to inject fuel. Low pressure does not trigger any fault code before engine runs.

In tank there are only level sensors and venturi type transfer pump between saddle tank sides. Primer pump comes when fuel system is upgraded in newer MY cars.

Is your car outside, what is temperature? You may try warm injectors and hp pump, if their inner leak is culprit.

Ensure hp pump is getting fuel. Measure rail pressure if you can.

 

[Anchor96]

Yes the car is sitting outside, once I get home I'll try warming it, the oil I have might slow down the crank speed aswell when cold, not ideal for winter, but isn't the culprit sice it started earlier even at -25.

I will check the hp pump and rail pressure definitely, thank you for all the good information thus far

 

[Anchor96]

On idle everything may be ok but during cranking problem escalates because low cranking RPM, pump cannot compensate even moderate leak in system. Pressure should rise above 150-250bar (cant recall exact value) during crank, before ecu gives permission to inject fuel. Low pressure does not trigger any fault code before engine runs.

In tank there are only level sensors and venturi type transfer pump between saddle tank sides. Primer pump comes when fuel system is upgraded in newer MY cars.

Is your car outside, what is temperature? You may try warm injectors and hp pump, if their inner leak is culprit.

Ensure hp pump is getting fuel. Measure rail pressure if you can.

Oh one more thing, how prone are the fuel lines to corrode?
The lines that run underneath the left side of the car look brand new, but the place where they run ontop of the fuel tank look a bit like 50/50.

 

[mersum1es]

I remember few cases where pipes to fuel heat exchanger near rear axle has been leaking, just cannot recall which one W203 or W210 has matrix there (another model it is located in engine bay)... but answer; it is pretty rare.

 

[Anchor96]

I remember few cases where pipes to fuel heat exchanger near rear axle has been leaking, just cannot recall which one W203 or W210 has matrix there (another model it is located in engine bay)... but answer; it is pretty rare.

Alright, I'll perform the other tests first and if it still has issues I quess I'll drop the fueltank and have a closer look at the piping.

Thanks again, I'll report back in 2 weeks when I get home