C7 random missfire
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1st Gear
Joined: Jun 2026
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C7 random missfire
hi i have a 2014 C7 Z51. lately the car has been missfiring code p0300. i have replaced wires with MSG replaced plugs Ac Delco replace 2 ignition coils cyld 1 and 3. after driving the car heavy acceleration 50/60 mph 3 4k rpm heard rumbling in the exahaust eng light flikered. nack on the the reader now its says pendingbrandom missfire. what is the next step injectors not avialiable at any dealer and there 140 a piece. any ideas berfore i sart tonthrow money to chase this issue. anybway of testing injectors.has anyone experience this isssue
Last edited by bacas; Jun 18, 2026 at 10:23 PM.
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Moved to C7 Tech for assistance.
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
Your NEXT step is to take your car to a diagnostician who can quickly and accurately diagnose your misfire and not a dealership or independent shop !!...you don't have the knowledge or the equipment to accomplish this yourself so don't waste your money and time...so cylinders 1 and 3 were misfiring per your scan tool ??...the injectors have 2 ways to be tested...AFIT (Active Fuel Injector Tester) which if you can find one are around $3000 or more or using a factory level scan tool...much cheaper to pay for 1-2 hours of diagnostic time than fire the parts cannon...since you changed the coil packs how do you know that the wiring for them is good ??...you never "assume" that...what do your O2 sensor look like on bank 1 since you have a cylinder 1 and 3 misfire ??...and you're not the only one on the C7 Forum with a misfire so there is no silver bullet for every case...there are MANY things that can cause a misfire.
Last edited by C5 Diag; Jun 19, 2026 at 07:33 AM.
Melting Slicks
Joined: Feb 2018
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From: North Texas
If you don't have a good scan tool, just for starters, and the knowledge to use it, follow C5's advice.
Seriously, you will probably need alot more 'tools', diagnostic and hand than most people have.
Unfortunately, cars now days aren't like the ones 30 years ago, shade tree and DIY with minimal equipment just won't give you the results you're looking for.
Best of luck repairing your car.
Seriously, you will probably need alot more 'tools', diagnostic and hand than most people have.
Unfortunately, cars now days aren't like the ones 30 years ago, shade tree and DIY with minimal equipment just won't give you the results you're looking for.
Best of luck repairing your car.
Pro
Joined: May 2022
Posts: 576
Likes: 245
From: Peoria, Arizona
Your NEXT step is to take your car to a diagnostician who can quickly and accurately diagnose your misfire and not a dealership or independent shop !!...you don't have the knowledge or the equipment to accomplish this yourself so don't waste your money and time...so cylinders 1 and 3 were misfiring per your scan tool ??...the injectors have 2 ways to be tested...AFIT (Active Fuel Injector Tester) which if you can find one are around $3000 or more or using a factory level scan tool...much cheaper to pay for 1-2 hours of diagnostic time than fire the parts cannon...since you changed the coil packs how do you know that the wiring for them is good ??...you never "assume" that...what do your O2 sensor look like on bank 1 since you have a cylinder 1 and 3 misfire ??...and you're not the only one on the C7 Forum with a misfire so there is no silver bullet for every case...there are MANY things that can cause a misfire.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,707
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From: Cape Coral, Florida
What's a "phone book" ??...LOL !!...any repair shop with "auto diagnostics" in its name...found this one in Phoenix that may be good..."mechanics" just change parts...a diagnostician is a highly trained and knowledgeable technician who uses advanced test equipment like oscilloscopes, pressure transducers and other such tooling...video below of one of the best automotive instructors and diagnosticians in the country...if a shop isn't using this type of equipment they're guessing !!
Last edited by C5 Diag; Yesterday at 08:33 PM.
