Q: p0300 misfire on 2003 GMC Sierra 2500 HD

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The truck has 88kmi.At 60-70 mph I get a p0300 code.The misfires are listed as cyl 1 and cyl 6.Here is a list of things I have tried with little or no change:new plugs/new wires/8 new injectors/new fuel rails with new pressure regulator/8 new coils with brackets/intake manifold gaskets/cam sensor/fuel filter/fuel lines/fuel pump/cleaned MAF sensor...The truck runs great,smooth,plenty of power etc.Other than the light flashing on the dash you would never know there was a problem (if there is one?)What's next?
(3) Answers
(14) Comments
Check for vacuum leaks, and check to make sure all the ground wires on the engine and chassis are clean and tight.
Refer to the GM technical service bulletin #06-06-04-046: Information on Engine Misfires - MIL/SES Light Illuminated or Flashing DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0420 or P0430 - (Sep 12, 2006)
This bulletin describes checking a specific ground wire on the right cylinder head.

Also, read this for a ton of info about misfire diagnosis.
Hey, this data helps, but unfortunately it's incomplete. We need to know the engine coolant temp and MAF sensor readings (voltage and flow rate).
We also need to know what other diagnostic codes are stored, the readings indicate a very rich mixture and you should have other codes stored.
What is the fuel pressure?
Have you checked the MAP sensor, do you have the specifications for testing he sensor? Your MAP sensor seems to be telling the ECU that the vacuum is around 10"hg, and that is either a mechanical problem with the engine or a bad MAP sensor.
Here is some info on the MAP sensor, which is completely different the specs you have. Where are you getting your data? Have you checked the engine vacuum at idle, what is it?
What needs to happen is the engine sensor data needs to be monitored as the fault is occurring, or recorded and checked to see where the problems are occurring. This takes special equipment and skill to handle this, otherwise it is a guess.
Were you able to perform a check of the ground wire I referred to in the bulletin? If so, how did you check it?
*More info:Fuel trim STB1-28 avg -29.56.LTB1 -31.90 avg -26.43. STB2 -36.43 avg -27.21 LTB2 -29.56 avg -25.65.
open loop
H20S B1S1 121.52mv B1S2 716.14mv H2OS B2S1 768.22mv B2S2 273.43mv
Injector PWM B1 3.6469ms PWM B2 3.7232ms
Do these #'s help?
ECT 91c, MAP36kpa,RPM 563,Spark 16.5*MAF g/s 6.85,MAF 2836.4HZ,No codes...right now...Fuel pump is new 55 psi/holds 45psi engine off for hours with no drop
The only thing I found on the MAP is it should be around 40kpa with a stock cam @sea level I live in the hills and it's 36kpa.but I plan to go over the grounds over the weekend so I will try a voltage test on it.One other thing I did notice before on the scanner (but not always) is it will say tac/pcm fault. So it may still be a grounding or bad PCM?? Thanks.
I have seen that chart,and it would explain the over correction on the fuel trims and most likely the highway misfire problems.The other info I found is probably not correct I will try a new MAP and let you know. Thanks for hanging in there with me.
I changed the MAP sensor-no change (the old one had some clear goo leaking out of it). The vacuum at idle is 19 and steady.Runs great no misfires at idle,it will burn the tires off until it hits the rev limiter etc...take it on the highway,hit 70mph and flash,flash,flash,CEL p0300 code stored.I'm out of ideas,bad pcm?
I got a copy of the TSB and checked the ground.It looked good,but I changed the wire end scuffed the block,scuffed the mounting bolt/washer with a wire wheel and for grins and giggles added an additional 14ga. wire from the ground wire/bolt to the neg. side of the batt.I have 2 scanners that will freeze frame.neither one really catches anything out of the ordinary. I ordered a pcm and will try that next at this point I really don't have anything to loose.Thanks.
I have a gmc 2006 2500hd v8 check engine light flashing 60mph and over po300 comes up replace crank sensor and still flashing any suggestions someone said something about ground wire I just don't want to screw up my converter
I had the same identical problem, completely identical. The crank sensor has to be realigned with the ecm, it is a computer alignment that needs to be done. Good luck.
I have the same truck,and had the same problem.The crankshaft position sensor caused the light to be on.(Think about it,the crank is in the same position to fire the #1 and the #6 cylinders.)I replaced the sensor, and the light is gone.You may not want to tackle this yourself.The starter has to be removed,because the sensor is behind it.Also, make sure you can twist the sensor before you try to pull it out of the block.Even then,my sensor broke off flush with the block making it real fun to remove.Good luck!
I have a 2003 gmc 2500hd 6.0 with the same problem. po300 misfire. engine light starts to flash then comes on when traveling over 70mph. can't really feal anything. drive at lower speeds around town and the light will go back out.
The problem is ethanol gas "wooo write congress and tell the to keep corn out of gas" They all run like crap and Big Momma GM could care less They are toe to toe in bed with congress. The best fix,that your GM dealer will refuse to do is reflash the pcm. they will tell you it's not the problem. Then when you tell them "just flash it,I'll pay the cost!!. It will be done and the problem will be solved. There is a reason that '04 and newer computers and fuel systems were completely changed...with NO explanation. GM Hiding The Truth?...Never...LOL F-ing JUNK just buy a Powerstroke Ford and forget it.....