KylerAllen

Hey,

Super confuzzled here.

The car is a 2008 E350 4matic (W211). Was headed towards town one day and it died on me. Cranked it and it wouldn't start. Checked all the fuses, etc.
So, I towed it home and did some research, finding the most likely problem to be either the Crankshaft Position Sensor, or the Fuel Pump/Sending unit.
Ended up replacing the CPS with an O'reillys special, but later went to Mercedes OEM. Didn't work, so then I figured I'd try the next thing.
Pulled the schraeder valve off of the fuel rail and turned the key to position 3, or right before the cranking position (accesories, etc. on). It was supposed to shoot some fuel out, but didn't.
Okay, so then I replace the fuel pump and sending unit. It fixed the lack of fuel pressure for sure, but still no start.

No codes. Did have one for the CPS, but it went away when I replaced it.

By now, I'm a little desperate, willing to try anything. I spray some starting fluid into the intake, and it starts. I have to hold the throttle for a little bit (for the first minute it won't idle on it's own), and then it runs great for the rest of the time.
No lack of power, and idles perfect. Now here is the kicker. Every hour the engine is shut down after running, it gets harder and harder to start, until I'm cranking for hours to no avail.

Thank you in advance!

juanmor40

If you let it cool down, say a few hours w/o energizing the car, i.e key out, will it start?

KylerAllen

In about 4 hours w/o energizing the car, it will not start

KylerAllen

So I found the solution if anyone else falls upon this thread in search of a solution.

Replaced the fuel sending unit (left side of fuel tank), replacing an Amazon special previously installed. My proposed conclusion is that the fuel pressure regulator (part of the fuel sending unit) was of poor quality.

Thank you all who replied to this thread.

juanmor40

Glad to hear you are back on the road

Were you able to diagnose the lack of fuel pressure before replacing the unit? Did you ever connect a fuel pressure gauge to the measuring point in the engine?

These fuel-sending units seem to be a common failure after a certain mileage, @100K+ (sister's W211 failed @150K).

While traveling overseas this summer, I noticed nearly every car suffers from low fuel pressure. The common denominator there is fuel quality, extremely dirty fuel, which leads to clogging of the fuel filter and/or overheating of the fuel-sending component. I experienced it myself driving a Mitsubishi with nearly full fuel system service before I borrowed, they skipped the fuel pump and I got stranded . All cars available to me then: Toyota, Audi, VW, Honda, Mitsubishi had suffered from the same issue within a year.

I started wondering if the MB fuel-sending unit relies on a certain fuel depth in the tank for proper/minimal cooling.

KylerAllen

I did to a certain extent. I did not connect a fuel pressure gauge, but I did jump the relay while a hose ran into a container from the fuel rail. It had minimal pressure. I felt it was a reasonable to believe that fuel was loosing pressure back through the regulator. Not sure entirely, but it seemed reasonable