My mercedes will start but will not run I have replaced the fuel pump same thing

are you loosing spark or fuel pressure

5.6L V8? Gas engine?? It has a distributor with wires coming out going to each cylinder.....Then there are 'spark' plugs in each cylinder...are they getting spark from the distributor? Spark from coil? IF you are unfamiliar with how to check this , it might be a good time for a tow truck, to be safe!

Check the Engine knock sensor