Hesitating at full throttle ?

[marcjero]

Problem solved (it seems to be) !
Fuel filter was clogged. In fact we think that it was fitted the wrong way. So it didn't last a lot of miles.
Could you please confirm that the upper fuel line is the one coming from the tank ?
Really happy to find a rational reason for this mess. Please note that the filter was fitted the wrong way in a Mercedes shop. Shame on them !

[marcjero]

hi,

I tried to pinch the fuel return line when the pressure drops. It doesn't help restoring fuel pressure. So the problem is not related to fuel rail or fuel pressure regulator.

Next step is to check the fuel lines and the fuel filter.

[marcjero]

I got the used pump from a 450 assembly. The assembly is a bit different so we had to move the 450 pump to the roadster assembly. The 450 assembly doesn't have any hole at the bottom. Knowing that the plastic bit that should block this hole is broken...

[tolsen]

Air lock being the cause is as just wild as blaming pedal or valve rubber. Better trying to fathom out why rail pressure drops to 1 bar when engine misfires.

[Thrumbleux]

How airlock van happen ?

When the fuel filter was changed, its volume of air would have been pumped through the system. You'd expect it be purged right out through the injectors but as the path to the injectors is downward possibly there is a pocket of air in the fuel rail obstructing fuel flow.
If so (and I admit it is an outside chance that this has occurred) releasing a pipe on the fuel rail (given it is higher than the injectors) should allow escape of any air. I'd do that if only to eliminate it as a possibility.

[marcjero]

How airlock van happen ? What do you think about the hole at the cup ?

[tolsen]

Yes an airlock it must be!

[Thrumbleux]

A bit leftfield...... could there be an airlock? A trapped bubble that under certain conditions (heat, increased fuel flow) moves to a narrower section of pipe and obstructs flow and hence available pressure?

[tolsen]

Why does pressure drop to 1 bar when it should be around 4 bar?

[marcjero]

Well weak power supply should show low voltage reading

[tolsen]

Fuel pressure drops to about 1 bar when misfire occurs. Fuel pump noise is different too when this happens.

Fuel rail regulator is purely mechanical. Must be power to pump. I would try external supply.

[marcjero]

Already checked : steady 13.7v at the fuel pump connector during the test. Will try to block the return line when the pressure drop occurs. Can a fuel rail fail ?

[tolsen]

Perhaps there is a problem with power to pump? Not uncommon on diesel 450 Smarts with SAM. The cure is to fit external relay to power fuel pump.

[marcjero]

We got a used pump for testing today. Guess what : problem is still there. Massive fuel pressure drop at wot and misfires when warm. FPR was changed, filter has few mileage. No visible leak. So what ???

[marcjero]

The pump is a walbro gss. It's a well known design. What happened during the test is the pump wasn't able to push fuel all the time. I saw walbro failures video on youtube where the walbro pump is running and nothing is flowing at the exit. In my cause the flow is sluttering : it still flows but there a a lot of interrupts. These interrupts could create severe fuel pressure drops in the rail.