1998 e320 wagon-explosion under hood
 

Car was running great until....one day I started it and BOOM from under hood followed by lots of smoke coming from right in front of the intake manifold. When I tried to start again it ran very rough with an assortment of noises and the brakes are working but no power assist.
I have no ideas

Need a picture.

Does this have a plastic intake manifold? If an engine backfires through the intake it could cause the issue you describe.

On non MB specific engines I've seen exhaust valve rockers come off and cause severe intake backfires. There is also the possibility of a ignition problem that fired the spark plug as the intake valve was open causing an intake backfire as well.

With such a intake manifold vacuum leak, the engine can run away since air is being admitted through the broken manifold and not through the throttle body.

If the air is going through a broken manifold, the MAS is not seeing it and adding fuel, so the engine cannot run away.

For some engine computers in general, if MAF readings are too far out of spec, the computer can go into limp and run fuel based on RPM. And if there is a MAP sensor, it can run from manifold pressure ( until that gets to far out of spec and goes into limp. )

Maybe I'm crazy but once my car goes BOOM followed by lots of smoke, I'm not going to try to start it again until I figure out wtf is broken.

It's probably the ugliest Mercedes engine, has a plastic cover over the manifold to make it look prettier. In front of the intake manifold you have an enormous amount of stuff...alternator, A/C compressor, hydraulic pump, water pump, radiator, hoses, oil filter housing, cam position sensor, various other sensors. You're going to have to begin with your own eyes and ears. Remove the plastic cover so that you can actually see the manifold. If nothing is obvious, remove the belt and turn each ancillary by hand.

You may have an now cracked intake manifold from a back fire ive seen 2-or 3 in last few years , inspect manifold carefully especially at ends , may be hard to see, or if it starts hissing , probably not a good idea to rev engine as broken intake pieces could get sucked into engine

That's was it. Backfire caused intake manifold to crack. Thank you, all fixed now.

still what causing it,Timing?