My 86' 300SE W126 starts normally when the engine is hot but it is very hard to start when cold. I tried the best to pointout the main cause, it give me the following summary:
1. To start the cold engine, it need about 8-10 seconds.
2. To start the HOT engine, it starts instantly.
3. When the cold engine was started, I instantly switch it off and restart, it was started instantly.
4. I measured the battery by the voltmeter, the reading is about 11.5V.
5. I tried to press the acceleration pedal deeply when cold start, no improvement! (still need about 10 seconds)
6. I lived in Asia and the temperature of my area is about 20-30 *C.
I really want to make my car perfect and keep it forever (at least as long as I can!!)
Please help!! Thanks.

Ivan, ...for proper operation, ..your battery should be able to store 12 V or more in order for the relays and controls to function properly. Also, ..check your charging system, ..it should be able to deliver 13.8 V or more. Your battery is about ready to give in. change it and let me know what happen..

Also check, or have checked the fuel system pressure during operation and after shutoff.

If the fuel system pressure bleeds down, like overnight, it may take a bit of time (cranking) to repressurize.

After engine has been running, if you restart before enough time has passed the pressure will still remain.