Randomn misfire P0300
Thread Starter
Intermediate
Joined: Jun 2018
Posts: 34
Likes: 3
Randomn misfire P0300
i bought a 2008 auto C6 with 140,000
the car is in great shape and the motor is very strong.
It has a randomn misfire while idle and it clears up under load
so far I have
replaced
all 8 fuel injectors ( AC Delco )
new plugs and wires ( AC Delco )
all 4 new O2 sensors ( AC Delco )
had valve cleaning done
new battery
so had fuel pressure test and it came back at 61 lbs which by the book was great
what else could it be. Please help
dealer has the car and 1 dealer done turned me away saying they couldn’t find the fix. This dealer is trying they are now leaning to it needs a valve job.
The motor doesn’t smoke or knock at all.
the car is in great shape and the motor is very strong.
It has a randomn misfire while idle and it clears up under load
so far I have
replaced
all 8 fuel injectors ( AC Delco )
new plugs and wires ( AC Delco )
all 4 new O2 sensors ( AC Delco )
had valve cleaning done
new battery
so had fuel pressure test and it came back at 61 lbs which by the book was great
what else could it be. Please help
dealer has the car and 1 dealer done turned me away saying they couldn’t find the fix. This dealer is trying they are now leaning to it needs a valve job.
The motor doesn’t smoke or knock at all.
Drifting
Joined: Nov 2011
Posts: 1,843
Likes: 330
From: western NY
Are they seeing certain cylinders have more misfires than others? With a scan tool you should be able to look at the individual cylinder misfire counters. Also, how did they 'clean valves'? Did they use a Motorvac type of machine or just pour a can of something in the gas tank? Did they do a 'top engine' clean? GM makes a real good product you spray in the intact tract to clean the valves & combustion chamber of carbon that could be causing lean misfires. If it's really carboned up, it may take 2 tries to clean it up. Compression or leakdown test?
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,591
Likes: 4,055
From: Cape Coral, Florida
I’m a C5 guy but heavy into diagnostics !!...like the previous post did the dealership look at misfire counters ??...well if not since you changed all the injectors did you check to make sure each injector has power and control ??...you ALWAYS have to check circuit integrity before you change a part...you changed plugs and wires...could be a bad coil..was that addressed ??...I’m lucky enough to have an oscillosope and can check all primary and secondary ignition waveforms fairly quickly...easy to see a bad one. I also like to look at O2 sensors and fuel trims to see if the O2 sensors are reporting a lean condition but you changed all 4...you have to check circuit integrity for those sensors too...do you have a scan tool that can graph those O2 sensors ??...if you have a compression gauge I’d do a compression test...maybe bad valve or spring...I’m pretty sure you can do a “clear flood crank” on the C6’s...accelerator pedal to the floor and crank...engine won’t start...listen to how the car cranks...should sound even and steady when cranked...also with 140K if that timing chain is worn that can throw off the cam/crank signal which will set a random misfire...to be honest without some diagnostic equipment there is only so much you can do...if it’s misfiring at idle hold a piece of paper or a dollar bill up to the exhaust...if it IS misfiring that paper will get sucked into your exhaust..I still use that quick test
Le Mans Master
Joined: Oct 2014
Posts: 8,673
Likes: 2,727
From: Atlanta GA
My 2011 had a random miss at idle that I chased for months , looked at the misfire count on a Tech ll and it was all cylinders that would randomly pop a misfire not just one or two cylinders
read a thread about the alternator cable being to close to the number 1 spark plug wire , this would induce RF in the ECM and it would cause havoc , just like different plugs do. I said what the hell let me try it .
loosened the nut on the alternator, rotated the cable 40 degrees , just enough to put a big air gap between the cable and the spark plug wire
started it up , fired up the TECH ll , miss was gone ....
now LS motors don't idle as smooth as your fathers Buick so don't confuse a miss with a lumpy idle
good luck finding your problem
Dave
read a thread about the alternator cable being to close to the number 1 spark plug wire , this would induce RF in the ECM and it would cause havoc , just like different plugs do. I said what the hell let me try it .
loosened the nut on the alternator, rotated the cable 40 degrees , just enough to put a big air gap between the cable and the spark plug wire
started it up , fired up the TECH ll , miss was gone ....
now LS motors don't idle as smooth as your fathers Buick so don't confuse a miss with a lumpy idle
good luck finding your problem
Dave
