Dtc P0300 Random Misfire

6-1500HD · March 23, 2009

2003 1500HD 6.0L vortec. Have been getting this code only when the engine is running atleast 2000 rpm's or more. Truck can be in park, on the highway or even slowly moving as long as the engine is turning atleast 2000 rpm's the code will set. Strange thing is truck runs great at all speeds. It idles smoothly, it always starts and it runs great on the road! I've had the injectors ultrasonically cleaned and flow checked. Checked for vacuum leaks around manifold. Found none. Checked vacuum level, 18" at idle and a steady increase to 21" at 2000 rpm's. This also shows that the exhaust and cat's are not plugged. Removed sparkplugs, looked good. Checked compression on each cylinder, had low of 171lbs and high of 180lbs. Changed fuel filter. Checked fuel pressure only had 50 psi @ idle with 18" vacuum pulled vacuum hose off of FPR pressure increased to 57psi. Used a manual vacuum pump to cycle FPR, pressure would change. Live data from scanner STFT bank 1 swing from -0.78 to 7.81%. Bank 2 swing from -3.13 to 9.38%. This is all at about 2100 rpm's. I think fuel pump is in question. Shouldn't the fuel pressure increase with the engine speed? The pressure on the gauge actually goes down with increased engine speed. I don't think this is right. Any ideas where to go from here. Maybe crankshaft position sensor? Again, I can't feel any stumble or miss or loss of power while the light is flashing.

silveradobobby · March 23, 2009

When the check valve or the little transfer hose on the fuel pump starts to leak alittle it will cause a misfire at higher rpms and cause that light if only everything else is in check which u said it was. Check to see if it holds fuel pressure when you turn the key on and then turn it off and see if it holds fuel pressure it should have no more that about 5-8 psi drop in 3-5 mins. Then we can go from there to see if the issue is back at the fuel tank or up front in the engine area.

6-1500HD · March 26, 2009

Cycled the key 3 times fuel pressure goes to 57 psi. Then in 2 secs with the key still on the pressure drops to 53 psi. Turn the key off the pressure goes up to 55 psi and stays there for 10 minutes. It has 48 psi with engine running @ 2000 rpms. Shut the engine off then the pressure goes back up to 55 psi and stays there. With a quick stab of the throttle the pressure goes up to 54 psi then back down to 48 psi. Fuel pump bad? There is no signs of any fuel leaking in the fuel pressure regulator on the vacuum side. 54 psi to 48 psi is with the engine running.

silveradobobby · March 26, 2009

Looks like your fuel pressure is in check, it should run good with those pressures.

silveradobobby · March 26, 2009

Sorry i cant help much with this one as it would be alot easier with my gm scan tool and could see really what happens above 2000 RPM's. It could possibly be the crank shaft position sensor as it only happens about 2K rpms. unless you have a very small intake leak that would be hard to pick up. Might have to bite the bullet and pay the chevy house to tell u whats up. Should be an easy find with the scan tool.

6-1500HD · April 10, 2009

Went to dealer and had them use Tech2. Mostly misfires on cylinder #1 and #6. Some on #5. Swapped coils on #1 and #3 did not change the misfire. Now I'm thinking there might be a small vacuum leak someplace on the underside of the intake manifold. The tech said it could also be the valves are sticking. Have heard about a bad ground at the PCM causing the injectors to not fire every time. Where would I look for this ground? The Tech2 did show misfires on these cylinders while idling and up to the "magic" 2000 rpms. Any suggestions

6-1500HD · April 22, 2009

Went to dealer and had them use Tech2. Mostly misfires on cylinder #1 and #6. Some on #5. Swapped coils on #1 and #3 did not change the misfire. Now I'm thinking there might be a small vacuum leak someplace on the underside of the intake manifold. The tech said it could also be the valves are sticking. Have heard about a bad ground at the PCM causing the injectors to not fire every time. Where would I look for this ground? The Tech2 did show misfires on these cylinders while idling and up to the "magic" 2000 rpms. Any suggestions

Truck sometimes now seems to fire a little slow. Will crank good but seems to not fire right away. This only happens after driving for a while then restarting truck. I think something is failing maybe just run it till it quits. Then we'll know what was wrong