I have a 94 Blazer, 4.3 Vortec 4wd. It is a work truck. The engine started knocking and loosing power so I put another used engine in it. It ran fine for about 6 months then started to knock. It doesn't do it all the time. Sometimes when I crank it up it runs fine with plenty of power. At other times it will knock terribly and have a hard time getting it to change into overdrive. This may happen several times a day going between knocking and running fine. Does anyone know what might cause this?

 

Does it sound like a bottom end problem or more a top end ping problem? The fact that it comes and goes makes me think that it is related to either fuel supply or ignition timing.

 

Been researching this and it seems to be related to the counter-balance shaft that comes in this particular engine. It is also a possibility that the timing chain has too much slack in it allowing it to jump timing at times which would account for the difference in idle speed. I am taking it off the road this week and pulling down the front of the engine to see if this is the problem.