JUST BOUGHT MY 95 SILVERADO 6.5 DIESEL. and the engine light is flashing code 18 when i do the test with paperclip..... It says its the pump cam reference pulse error which is code 18... what can that be or mean??

 

Optical sensor error,is the optical sensor electronic noise filter still installed(black rectangular rubber piece with wires out both ends)?

If so remove it and plug the harness directly into the OS-you might have to go to radio shack and get enough ferrite core to put a piece on every OS wire-it does the same job as the filter but doesn't fail over time.