Tip for a no-start M103 engine.......
 

This is kind of a simple thing, but it happened to me a few years back and it might be useful info to someone else.

My mother-in-law called and said her 300E wouldn't start. I was out of town, so she had it towed back to her house. When I got back, I went and looked around on it for a few minutes before she said it had backfired when she had last shut it off. Checked the vacuum hoses and sure enough the hose running from the intake to the idle control valve had popped off. Stuck it on and fired right up.

That's it! I only mention it because it is kind of hard to notice in its position under the fuel distributor. If your car backfired, and won't restart, this is a good first check!!