1998 Mercedes Benz when i excelerat number two and three cylinder stop sparking and the injectors also stop

Three things. A bad spark plug, a bad spark plug wire, or a bad coil pack

when you say not firing have you removed the plug and checked for spark across the plug points. If you haven't done that then do so as misfiring can also be from faulty injector( blocked tip or faulty electronics)

bleed the air out of the fuel system . crack the injector nuts and turn over until fuel drips out .re-tension the injector nuts . Check that the stop solenoid is operational. Ensure that the battery is fully charged as the diesel has to spin fast during cranking . Ensure that the glo-plugs are working especially if the ambient temp is low.

Generally speaking it can be a spark, fuel, or compression-vac leak problem.
So the spark plug, coil, fuel injector, vac leak, or internal engine problem would do it.

Check wiring harness.. could have a break in it... causing it to stop firing..

Sounds like its off a cylinder. Most likely an injector or a vacuum leak. You will need a mechanic to check it.

1- Check if you see spark at spark plugs when start. If if is not then look for problem at ignition system.

2- Check the cold start valve, it is supposed to open only less than 8 sec during the start to assist the combustion. If it is faulty then it becomes always open causing too much gas in the cylinder chamber. Remember the golden ratio of Air: Gas is 14.6 to have a good combustion.
3-Check injectors. These injectors are normal close and only open when there is a signal from injection module control.

SOUNDS LIKE NO FUEL TO 2,3,5,8 CYLINDERS

Check to ensure that there is fuel to the injectors using a pressure gauge on the schrader valve on the injector rail or fuel filter (which ever has it).

Check engine compression with a proper compression tester.

Buy an inline spark checking tool and install it between each spark plug and wire to ensure that you're getting spark to each cylinder while cranking.

These three things cover the basics. Once you've tested all these things, you can further diagnose the problem.

I took it to the mechanic