Misfire when the engine is warm
Thread Starter
Intermediate
Joined: May 2024
Posts: 38
Likes: 8
Misfire when the engine is warm
I started my c5 base yesterday after a clutch swap and differential rebuild. It all seemed to be going well, but then the engine started to misfire after it warmed up (runs smooth when cold). On 2nd startup, the CEL came on (presumably for misfires). Note that before doing the drivetrain work, the engine ran great in all conditions
Here is what I plan to do to troubleshoot:
1. Check what the specific CEL code(s) are
2. If it is a random cylinder misfire: Check O2 sensor resistance and check intake manifold for crack (seems to be a common issue for those that lower the drivetrain?)
3. If it is a specific cylinder misfire: Check plug wires, spark plug, and injector for the specific cylinder that is misfiring
Anything I'm missing or should check in addition to this? I haven't really diagnosed misfires before, so this is all I could think of
Here is what I plan to do to troubleshoot:
1. Check what the specific CEL code(s) are
2. If it is a random cylinder misfire: Check O2 sensor resistance and check intake manifold for crack (seems to be a common issue for those that lower the drivetrain?)
3. If it is a specific cylinder misfire: Check plug wires, spark plug, and injector for the specific cylinder that is misfiring
Anything I'm missing or should check in addition to this? I haven't really diagnosed misfires before, so this is all I could think of
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,630
Likes: 4,076
From: Cape Coral, Florida
That is a start and to be honest with you diagnosing misfires are over the heads of most DIY mechanics...if you want to change plugs, wires, coil packs, O2 sensors, injectors and check all the wiring associated with this you can go this route...I normally don't check O2 sensor "resistance"....find yourself a diagnostic shop that has the advanced equipment and knowledge to fix it.
Thread Starter
Intermediate
Joined: May 2024
Posts: 38
Likes: 8
That is a start and to be honest with you diagnosing misfires are over the heads of most DIY mechanics...if you want to change plugs, wires, coil packs, O2 sensors, injectors and check all the wiring associated with this you can go this route...I normally don't check O2 sensor "resistance"....find yourself a diagnostic shop that has the advanced equipment and knowledge to fix it.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,630
Likes: 4,076
From: Cape Coral, Florida
Yeah I honestly saw that o2 sensor check on a youtube video that is probably just bad advice. I wonder if I can check the O2 sensor/ AF readings on a OBD2 scanner? that would be the easiest way for me to check the sensor. If it's pegged high or low, that would be a clear sign that it's bad (I would think)
Thread Starter
Intermediate
Joined: May 2024
Posts: 38
Likes: 8
A good O2 sensor will oscillate between 200-800 Mv's when engine warmed up...yes, if it's pegged high or low that's bad...I had that issue a few years back...pegged low (100 Mv's) and car ran rich and misfires on that bank !!...if you see it pegged high or low let the car cool off after shutting down...now disconnect that O2 sensor and pull the O2 heater fuse...turn key ON and look at O2 sensor voltage with scan tool...it should say 450 Mv's if wiring is good...if still pegged high or low you have a wiring issue.
great info. I will give it a shot this week and report back
Thread Starter
Intermediate
Joined: May 2024
Posts: 38
Likes: 8
Code is coming back
p0171: system too lean bank 1
p0640: MIL control circuit/ open
im not sure if number two is relevant to my misfire?
All my O2 sensors are within 200-800 me except one oscillates between 60-70
p0171: system too lean bank 1
p0640: MIL control circuit/ open
im not sure if number two is relevant to my misfire?
All my O2 sensors are within 200-800 me except one oscillates between 60-70
Last edited by C5jax; Aug 13, 2024 at 06:40 PM.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,630
Likes: 4,076
From: Cape Coral, Florida
You sure it's not a P0650 ??...what year is the car ??...that is the first thing you should mention...that P code has nothing to do with your misfire and we won't look at it right now...ONLY the upstream O2's are supposed to oscillate between 200-800...the downstreams (S2) are fairly steady around 600-700 Mv's...which one is 60-70 ??...look at bank 1 and 2 fuel trims...is only Bank 1 elevated or both ??...the MIL illuminates when the long term fuel trims exceed 23%...if Bank 2 is close to that we need to know...if either Bank 1 or 2 are elevated run the engine at 3000 RPM and see if the fuel trims DECREASE....if so that is a vacuum leak...if it is only Bank 1 vacuum leak that may be an intake runner leaking...if fuel trim does not decrease that may be a clogged or faulty injector or an ignition problem....and NO a no spark issue will not cause a rich condition !!…my friend Matt who has a great YouTube channel did a great video on engine diagnosis....does your scan tool have a misfire counter ??..the P0640 is a circuit code for the MIL (check engine light).
Last edited by C5 Diag; Aug 13, 2024 at 07:22 PM.
Thread Starter
Intermediate
Joined: May 2024
Posts: 38
Likes: 8
You sure it's not a P0650 ??...what year is the car ??...that is the first thing you should mention...that P code has nothing to do with your misfire and we won't look at it right now...ONLY the upstream O2's are supposed to oscillate between 200-800...the downstreams (S2) are fairly steady around 600-700 Mv's...which one is 60-70 ??...look at bank 1 and 2 fuel trims...is only Bank 1 elevated or both ??...the MIL illuminates when the long term fuel trims exceed 23%...if Bank 2 is close to that we need to know...if either Bank 1 or 2 are elevated run the engine at 3000 RPM and see if the fuel trims DECREASE....if so that is a vacuum leak...if it is only Bank 1 vacuum leak that may be an intake runner leaking...if fuel trim does not decrease that may be a clogged or faulty injector or an ignition problem....and NO a no spark issue will not cause a rich condition !!…my friend Matt who has a great YouTube channel did a great video on engine diagnosis....does your scan tool have a misfire counter ??..the P0640 is a circuit code for the MIL (check engine light).
https://youtu.be/5WnM_NsOtd8?si=mvmImsD9NP_ZEgxd
https://youtu.be/5WnM_NsOtd8?si=mvmImsD9NP_ZEgxd
thanks for all your help. I figured it out
Just to close this out: the previous owner had installed headers. When you install the o2 sensors on headers, it looks like the primary AND secondary o2 sensor plugs criss cross (driver side sensor plugs into passenger wire annd vice versa). However, the primary are supposed to plug into same side and the secondary are criss cross
so I mixed up the o2 sensors which caused each bank to be randomly lean or rich.
All fixed!
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski