I guess I should get into more detail, Over the winter the high pressure fuel rail burst, I replaced that and had no problems with it. then in the spring it was hesitating a lot and was getting an EVAP error small leak detected, I decided to replace the fuel filter, spark plugs, spark plug boots, fuel pressure regulator, there was a hole in the fuel filler pipe that goes to the tank that I replaced, it had good pressure in the lines and one thing that I overlooked was the hose that goes to the MAP sensor, it was rotton, replaced that and everything worked great up until 2 weeks ago, I checked everything over I replaced the IAC valve because it was broken, thought that would fix it, the throttle body was all full of carbon, I even had it at a GMC dealer and he said it showed misfires on all 4 cylinders and the fuel trim rich, fuel line pressure at 44psi, backpressure is less than 1/2 LB, cylinder compression at 200lbs, I ohm tested the injectors and they were normal. engine vacuum is low at idle 9-11in. 18 in @1400RPM checked and verified that the pcm had the latest calibrations. I been doing some testing and found that when I remove the hose to the MAP sensor it idles fine for about a minute and then rough again, been doing some research and the only things I could think of now is to replace the knock sensor or the mass airflow sensor, EGR valve, or IAT sensor because they produce data to the pcm which makes the adjustment to the timing and everything else. Does anyone have any other ideas that I can try? the GM Mechanic could not even figure this one out, he is leaning twards the timing chain. I would not pay him to check it but I think it is something else. but I do plan on checking it sooner or later.
Have you checked to see if you coil pack is bad? Does it get worse when you give it gas?
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