Newbie here, i have a 1996 chevy blazer 4.3l and have just replaced the engine with a motor i aquired from a junkyard around town. Since start up i have only had one code a P0300 random/multiple misfire, i have replaced the plugs wires, cap, rotor, fuel filter, and ignition coil, have checked the egr valve for being stuck open/closed, all checked out good, any help would be appreciated, thanks

 

Did you play with the cap or rotor? That usually throughs it out of sync.

 

no i put the rotor and cap on the waythey came off. Some more info the truck ran great no check engine lights, the only reason i replaced the motor was because the bottom end had a knocking noise

 

Did you transfer any sensors from your engine to the new engine??? If not I would recomend that you might check the crankshaft position sensor and the camshaft position sensor (located in the distributor), these two sensors have to synchronize with each other to get the proper timing for the engine computer to fire the injectors off at the correct time...this might be the cause of the P0300 DTC

 

How does it run? If it runs good I would reset the light and try it again. I have in the past have the P0300 code from doing major engine work. If you did not tune it up before you started it, and the truck ran rough for evena short time during the initial run, it can set a P0300 code. So just reset the code and see if it returns. If it does then it can be any number of things, but usually the cam/crank sensors will set their own code as well. Let us know what happens.