It sounds like most of the usual items have been changed, I would first of all check for fault codes to see if any codes have been stored which may indicate a problem with a component.
Usually this kind of fault is down to low pressure in the fuel rail which will prevent the engine from firing up - this can be confirmed with a diagnostic too reading the pressure sensor while the engine is cranked over - if it is not reaching the required pressure set by the ECU then the engine will not fire up.
All the low pressure lines should be inspected - you have said the O rings have been replaced but have all the seals been replaced on all hoses? If not then I would ensure all are replaced.
The fuel filter can cause problems & we have many problems with non genuine filters installed, so would ensure a brand new Mercedes Benz fuel filter is installed.
Injectors are another problem - were new injectors installed or used ones?
There are also seals on the fuel regulator valve at the end of the fuel rail - if this has not been inspected then I would replace the seals on this.
If all of the above checks out ok then the next step would be to have the pressure from the high pressure fuel pump checked - Mercedes has a special tool to connect up to check this.
There are of course many other issues that can cause the problem but would have all of the above checked first.
Let me know how you get on.
Best regards, Paul
You would think that it would not be like that intermittently but in my experience most problems where the engine cranks over but does not fire up it is down to low pressure in the fuel rail! It almost always only causes non start issues & unless severe the engine will run & maintain rail pressure just fine once it is running.
The only way to be sure is to actually monitor the rail pressure when the fault is present, which can obviously be difficult if the fault does not occur often.
Usually if there is a voltage issue to the CDI ECU then the engine will not actually crank over as the ECU sends a signal for the starter to turn.
Has the vehicle been checked for codes? Another issue for cranking over but not starting can be a faulty camshaft sensor on these engines - but this will usually store a code.
Have any codes at all been recorded do you know?
Has the garage been able to reproduce the fault?
Have all seals on the fuel lines & on the regulator in the rail been replaced?
Are any air bubbles present in the fuel lines or are there any signs of fuel leaks?
No need to apologize & if I can help in any way I will be glad to! It can be frustrating for both the owner & techicians when a fault is intermittant!