P0300 misfires, primarily on driver's side bank
 

Just recently I had two brief CELs about a month apart (over christmas so naturally couldn't get anyone to take a look at them) and another CEL yesterday of the flashing p0300 variety - bought an ODB2 reader to try to figure out what was going on. As soon as the flashing CEL showed up I pulled the car over and let it sit - the CEL cleared by itself after about 5 or 6 seconds and then I limped it home.

I checked the mode6 data for the CEL after pulling over and apparently cylinders 5 and 7 were both misfiring.

Today I tried changing 5/7s spark plugs, coil packs and spark leads over to cylinders 1/3 and then tried to drive the car again from a cold start. As an aside, this is what my plugs look like:

https://cimg8.ibsrv.net/gimg/www.cor...61c9cc238b.jpg

I'm not certain how to gauge the wear on these particular plugs so if somebody can chime in as to wether these need replacement or not that'd be good.

Anyway, Tried to drive the car from a cold start - seems like it didn't change much. I can't really 'feel' the misfires but I presume they are there - the car isn't cammed or anything so I don't see them being false positives.

Also learned to burp the spark leads and what a 'real' misfire on this car sounds like pretty quickly when the Cylinder 3 spark lead came off on the way back from testing. Not a positive experience and not fun trying to thread a spark lead back on between heat shields and naturally I'm already pretty nervous about the car throwing a CEL in the first place.

I also now get a P0300 code set when the car is set to ignition (engine not running); the CEL light itself isn't being lit. Would this be due to the Cylinder 3 misfires being stored in DTC memory?

Since it's beyond me to fix this I'm going to get the car trailered to the local independent garage I use to get the O2 sensors and cats checked out next - this seems like the only thing I can think of that would cause one bank to have considerable misfires over the other. Anyone else have any ideas about this?

I have also attached a pair of the mode6 logs for the car - one for the drive home after the CEL cleared (did not have the ODB reader running at the time so I couldn't get the actual freeze data) and one for today when I took the car out to drive to see what misfires it was getting at that point.

Also ohmed the spark leads - multimeter set to 20k ohm. As they were all reading within a close range I'm presuming that these are all functional and not all broken.

https://cimg1.ibsrv.net/gimg/www.cor...6a01f21040.jpg
https://cimg3.ibsrv.net/gimg/www.cor...8e1cc9e335.jpg
https://cimg4.ibsrv.net/gimg/www.cor...1fdac31d52.jpg
https://cimg5.ibsrv.net/gimg/www.cor...359e3c40d4.jpg

What about the fuel you are running?
Have you tried to throw gas from another source (or a can of Torco) in and see if it stops?
You might also try a fuel system cleaner treatment and see if anything improves.

There are people way smarter than me who might think of something else but it looks like you have eliminated the above as the source.
Those plugs don't look all that bad to me but you might just change them all just to be sure since its not that expensive.

I haven't thrown any octane booster in just yet - the previous temporary CELs were on other tanks of gas but I usually refill before getting too far below half way - which would mean the left side still has old gas in it.

Just an idea.......I had similar problem on a different vehicle and
It turned out to be a intake manifold leak on the particular
cylinders having the misfire.

Never hurts to replace plugs if they are out IMO

Good Luck

I got a CEL due to a cylinder 6 misfire in the paddock at VIR before ever getting on track. After a week of troubleshooting it turned out to be a faulty fuel injector. I guess is was failing over time and chose the morning of a 2-day track event to to degrade to the point of causing the CEL (terrible timing!). 2017 Z06 with 10k miles.

Something to consider if it's a misfire on one cylinder in particular.

Good luck!

Thanks for the suggestions - I will be passing them along as a starting point and see what we can figure

Misfire
 

wow . Same thing happened to my 2017 with 10K miles . A misfire code and it was a faulty injector .
towed it to dealer and got it taken care of

Yes; The misfire algorithm detects the acceleration or speed of the crankshaft every 45 degrees of crank rotation.
If it doesn't accelerate or remain constant (the crank slows down) then the PCM/ECM knows that cylinder didn't ignite any fuel.
Point is lack of fuel or fire can lead to this code.

The OP has 2 cylinders misfiring. The likelihood of 2 injectors going bad on a fairly new car at the same time is low probability.
The issue may be due to fuel or increased boost? I didn't see if the Op said he had any modifications.

First place to start (if you dont want to spring for a new set of plugs just yet) is swap the 5 and 7 plugs with other plugs on the opposite bank that are firing well.
If the misfire follows the plug(s) you know its the plug(s). If it doesn't then you know its electrical (injector or coil), or the result of a modification.
Again you can do the same swap trick with the injectors, coils and wires as well, just keep notes because it can get confusing as to what was where and what the result of moving it was.

No performance modifications yet - seems like the other side bank has 1 or 2 misfires listed over the course of a drive as well but not a huge number. I'm assuming it's most likely to be a manifold issue at this stage because I'd presume that fuel shortages would be more consistent - we'll see though. Not looking forward to the bill for swapping a fuel filter on these cars but that's life.

Two questions. First, is the car still under warranty? Second, how do you burp a spark plug lead?

I assume that the warentee is still in tact and with no mods, why not take it to the dealership for diagnosis and repair. These cars are not simple and not oriented to the home diy types. Leave it to the pro would be my recommendation.

The failure code history report should show you at what rpm the misfires are occurring, what the throttle conditions were and how many misfires occurred for each setting of the code. P0300 misfires aren't specifically limited to one cylinder but the history report will show which cylinder misfired. For instance the history report for my car showed P0300 with cylinder number 8 misfiring 64 times at 5200 rpm at wide open throttle through one 20 minute track session. However, P0308 the misfire code for cylinder 8 didn't get set until the cylinder misfired 521 times during a different 20 minute track session. I never felt the any of the misfires related to the P0300 codes, the engine just felt sluggish above a certain rpm, while I definitely felt the misfires when the P0308 code was set.

Bill

Not under warranty due to it being an international export - I'd surmise that if you loaded it back onto a container ship and sent it back to Florida they'd probably service it there but that might cost a bit much for whatever's wrong with the car. The importer that I used is a fairly large importer of Chevrolet vehicles and has all the diagonistic stuff to work on them - only problem being they're over an hour away, so I'm getting the local independent that works on my race cars to take a look first.

As for the plug leads, apparently the rubber boots will trap oxygen that can expand and blow them off when hot enough so the 'burping' procedure is to use an angled pick tool to bend back the rubber and let the air out - or just fill them with enough dialectric grease that they don't need that in the first place.

I surmise this is for GM specific ODB information - I don't have that level of detail available on my cheapo bluetooth ODB device. Do you know how many misfires constitute a 'standard' driving session (from a cold startup, etc) or should there not be any in standard driving scenarios?

I don't know the answer to those questions. As for getting the Freeze Frame Failure Records I imagine if your mechanic has a GM MDI 2 tool and access to the GM TIS2WEB service site the data would be available. DIYers in the US can get access to the TIS2WEB service and diagnostic information for 3 days for $20.
https://cimg6.ibsrv.net/gimg/www.cor...f897108f81.png

My history reports had 24 pages of data.

Bill

Did you say you are oversees?
What type fuel are you running and how do they measure octane in your area?
Is it RON ? And if so what RON are you using?

Back in the day when I was in Germany I ran my plug gap about .005 less than I did in the States.

We use an Octane rating system over here - our 95 and 98 is analogous to US RON 91 and 93. I primarily drive the car on 98 as the AFR runs fairly rich (12 to 10 depending on if fuel enrichment is active) on 95.

Note that I've ran the vehicle on both grades of fuel (but never our 91 / RON 89) and haven't had any misfire issues in the previous 40000km I've had the car - only noticed it being an issue very recently.

Cool good info. Thanks
I guess unless you got a bad batch of fuel we can eliminate fuel as the cause.

Would love to see a data log from HP tuners or EFi live.
Not sure what else we can tell you without more information.