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I have an 08 ls3 a6 with 60k on it. Getting p300 random misfire code. I changed plugs and wires with change. I logged with HP tuners and saw random misfires on many cylinders. I did see more on number 7 and 8. I swappes the coil pack from 1 to 7 with no change. Tested vacuum and have 14 inches of vacuum at idle. Have miss primarily only at idle. Also get service active handling while idling for a few minutes without moving. Once I start to drive it goes back to normal and traction control light turns back off. Other than that car runs and drives good. Open to any ideas
I have an 08 ls3 a6 with 60k on it. Getting p300 random misfire code. I changed plugs and wires with change. I logged with HP tuners and saw random misfires on many cylinders. I did see more on number 7 and 8. I swappes the coil pack from 1 to 7 with no change. Tested vacuum and have 14 inches of vacuum at idle. Have miss primarily only at idle. Also get service active handling while idling for a few minutes without moving. Once I start to drive it goes back to normal and traction control light turns back off. Other than that car runs and drives good. Open to any ideas
About 75% of misfires are ignition related and the rest divided between fuel (injectors/pump) and mechanical. What do your Long Term Fuel trims look like ??...if injector related your Long Terms will be quite elevated...you have all that "excess" oxygen in the cylinder not being burned.Can you hear the injectors "click" when running...using a long screwdriver is best instead of a stethoscope to listen...stethoscope is too sensitive and you may be listening to an adjacent injector fire !!
I can hear injectors clicking. I will check my fuel trims and see what they look like. I also turned off each injector one at a time and it makes an impact to idle. It's just a slight miss if any. Just during idle the misfire counts get high enough to pop a code.
Are these misfires that you can actually feel or you are just getting a P0300 ??...sometimes a “crankshaft variation relearn” with a Tech 2 or bi directional scan tool will solve those “perceived” misfires !!
If you CAN feel the misfire don’t bother with a crank relearn...I handle misfires like this by doing a relative compression test to rule out anything mechanical...also look at injector voltage and current and secondary ignition but specialized equipment like a labscope is required to nail it down..,if it looks like it may be injector related a injector balance test would be in order...you basically check how much fuel is flowing through each injector using fuel pressure for a certain amount of time !!...you can try swapping injectors and see if the misfire moves !!
Just noticed your misfire is on 7 and 8, the rearmost cylinders. Maybe a vacuum leak near the back of the intake? Did you try to spray carb cleaner around the area to see if the idle changes? Not sure myself of the best vacuum leak test...
Do the plugs looks different on 7/8 vs the rest?
Do you get any significant misfire counts on other cylinders?
You might try swapping plugs/coils/wires again between cylinders.
Don't think you stated your long term fuel trims. Crank relearn?
Should probably do a compression test if all else fails.
Last edited by steel_3d; Nov 12, 2019 at 12:52 AM.
Just noticed that you said you have 14 inches of vacuum at idle...is this a stock engine ??...14 seems a low...my LS1 hovers between 20-21 inches at idle !!...also since you swapped out the injectors did you check them "electrically" ??...if you have good spark at 7 and 8 I'd start focusing on mechanical...if you can find a shop that knows how to do an in cylinder pressure analysis you can rule out any mechanical or breathing problems with the engine...I just started doing this myself and just want to get more experience with the waveform analysis...here is what you would be looking at...my 2001 engine at idle...no fuel or spark so you're just looking at the engine as an air pump !!!...looking at TDC, exhaust valve opening and intake valve closing.
P300 misfire help
