3 years of random misfire
 

Hi all,
I have a gmc sierra which has a misfire without trouble code .ive done many things nothing seems to work .things I changed :
plugs two times
wires two times
injectors two times
complete intake two times
swapped the engine (my old engine was installed in another truck and runs smoothly .
o2 sensors changed
installed kooks headers with off-road y pipe cause I thought maybe the cats melted .
throttle swapped .
pcm changed
all things didn't make a change in misfire engine running rough with random misfire and no codes .




please help if some one have like this condition.

Bad knock sensor?

But in scans i cant see a knock retard

How about coils, and what about a harness issue... How are all the grounds? There has to be something going on electronically at this point.... If you havent changed coils, thats where i would start.... Is there a specific cylinder that misses more than the others or is it just all over the place? Have you checked fuel pressure?

Yeah all coils swapped from a good running truck . Fuel pressure is 300 kpa at idle which in spec. All grounds is good as alot of shops tested it . Mostly all cyl but which a little more than others is number 1 and 6

300 kpa is actually low at 43 psi should have closer to 55-58 (roughly 400 kpa)at the fuel rail.... I would test it at the pump and if its not better, look into whether you are getting proper signal to the pump, if it is where it is supposed to be start looking at fuel filter, lines etc!!!

My truck have fpcm
Which will increase the speed of pump to 400 kpa at loads and wot as per specs from dealer , also i will try to change the pump if it will fix it , cause i lost alot of money on this truck .

My 08 has a fpcm as well and at idle it is still 58psi... 43psi(300kpa) is the low side of spec.... Basically the bare minimum required for the engine to run!!!

I will be so sad if the fuel pump was bad . I will order a new fuel pump ill give it a try . Thanks a lot man .
If any have same problem and have another fix just tell me

key on engine off command pump on with a scan tool... Pressure should be between 345-690kpa (50-100psi) after pump is turned off it should stabilize between 500-599kpa(72-87psi) (if more than specified, replace fpcm, if less check for restricted fuel feed pipes or poor harness connections, if the test normal replace fuel pump)