i bought an 87 honda civic si a few months ago and it ran fine, but then all of a sudden randomly it won't start, cold or warmed. somedays it starts up perfect. it rotates at 2000 rpms until its warm. everything seems functional when it doesn't start: all electronics, starter rotates. it fires then shuts off and won't start again until many hours later, sometimes days. I replaced the main relay, I tried disconnecting the water temp sensor under the distributor, nothing. the ecu has the 2 center lights on. the pgm-fi light is not on, but it does turn on when the key is turned to the second position. I don't know where to go from here.

 

need pics

 

The ECU is giving you a clue. Go to www.redpepperracing.com where the factory service manual is online and you can see the code the ECU is giving you.