2002 EF Volvo Penta 7.4 gsi problems continued

[basscat348]

It has not run for 5 years. Originally the engine would hardly run and you could not accelerate the engine in or out of the water.I have pulled out the injectors and sent them out to be cleaned(did not help).I have replaced the map sensor (that somewhat helped but not alot) I have replaced the high pressure fuel pump(did not help).I have diacom software to look at codes and everything else and the only thing it said was bad was the map sensor.The problem continues to be the same....the engine will start perfect and will accelerate for about a mile or so then slowly it will loose power until it comes off plane and if I continue to leave the throttle control forward it will then start backfiring through theair intake NOT out the exhaust until I pull back on the throttle.Now after this occurs I cannot get the boat back up on plane unless I shut the boat off and let it set for a minute or so. Originally I thought that the fuel pump was bad and not holding enough pressure but I have replaced that and does not help.When the engine begins its backfiring the engine sounds to only be running on a few cylinders , but if I put the control in neutral and rev the engine it sounds normal.I am throughly confused here and need some help.I had another post about this problem but I was not getting any replys.I probably wont here either because this is beyond the average guy to figure out.I have performed all tests that can be done except for fuel pressure leak down,maybe thats it I dont know.