Misfire- but wait.....
Thread Starter
Racer
Joined: Mar 2021
Posts: 262
Likes: 60
Misfire- but wait.....
So I had a previous thread about some hesitation at wot. But we got some early snow in Wisconsin. So I dropped it at a shop to look at rather than my outside work area.
so these guys did a basic scan, found a misfire in #8 and were convinced it was a bad injector. I had him do a few other things since he had it. Knock sensors. Oil pressure sensor.
I bought the car used. Had some goodies. But who knows who did the work. Has a cam. Heads. Headers. Always screamed until the wot issue.
I had a fapk to put on. So I did a new MAF and AIT while in there. Anyways, he says now it wasn't the injector AND its a 6 liter swapped in as well? News to me.
in either case, he says it's a rocker or lifter. But that's not making a ton of sense to me. In addition, why guess that without pulling the valve cover to look.
I'll pick it up tomorrow when most this snow is melted and take a look at the plugs. But just sounds like a fishy diagnosis.
so these guys did a basic scan, found a misfire in #8 and were convinced it was a bad injector. I had him do a few other things since he had it. Knock sensors. Oil pressure sensor.
I bought the car used. Had some goodies. But who knows who did the work. Has a cam. Heads. Headers. Always screamed until the wot issue.
I had a fapk to put on. So I did a new MAF and AIT while in there. Anyways, he says now it wasn't the injector AND its a 6 liter swapped in as well? News to me.
in either case, he says it's a rocker or lifter. But that's not making a ton of sense to me. In addition, why guess that without pulling the valve cover to look.
I'll pick it up tomorrow when most this snow is melted and take a look at the plugs. But just sounds like a fishy diagnosis.
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Next he will tell you it needs a new oil pump... Did they do any of the basic troubleshooting like plug, wire, coil swaps??
If it were me, Ide get a scanner that can read live data (misfire counts on specific cylinders, LT & ST Fuel Trims, O2 Sensor Data etc..) Figure out what cylinder/s are getting misfire counts and swap the stuff that is easy to move to another cyl. It prove out the low dollar stuff.
Have you read your DTCs using the internal DTC routine? What is the condition of your plug wires?
How to Access C5 Corvette Diagnostic Codes (motortrend.com)
BC
If it were me, Ide get a scanner that can read live data (misfire counts on specific cylinders, LT & ST Fuel Trims, O2 Sensor Data etc..) Figure out what cylinder/s are getting misfire counts and swap the stuff that is easy to move to another cyl. It prove out the low dollar stuff.
Have you read your DTCs using the internal DTC routine? What is the condition of your plug wires?
How to Access C5 Corvette Diagnostic Codes (motortrend.com)
BC
Melting Slicks
Joined: Sep 2021
Posts: 2,555
Likes: 1,213
Those are great concerns to bring up with your next mechanic. This one does not sound very thorough.
Thread Starter
Racer
Joined: Mar 2021
Posts: 262
Likes: 60
So I got the car back. As I stated, they told me I had a misfire in #8. Yesterday I pulled the valve cover and coil packs up. Pulled the #8 plug. No surprises with rocker or springs. No chunks of metal. Plug looked fine. Put it all back together. Had a neighbor come over with a scanner. No misfires across the board. Took it out for a rip. Pulled hard. It does have a different throaty sound. But im chalking that up to the FAPK intake I put on. I'll give it some time, but maybe the bad knock sensor? But then why a misfire on #8 ? Maybe that plug wire wasn't fully seated? I'll give a few days and update
Tech Contributor
Joined: Dec 1999
Posts: 32,910
Likes: 2,402
From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Misfire c5 ls engine information
MISFIRE C5 LS ENGINE INFORMATION
When the PCM detects a misfire, YOU will see a check engine light FLASHING during the misfire event.. If you just pull over and read the DTCs "WITHOUT TURNING THE IGNITION OFF" You will see a P-0300 DTC which is a misfire DTC.
. Unfortunately the C5s DO NOT RETAIN the P-0300 DTC or give you a P-0301, P-0302, P-0303 etc...like some platforms do.
If you turn the ignition off and back on the P-0300 DTC clears from the PCM. You have to have another misfire event for it to set again.
With the right scanner, you can see misfire events for each cylinder. 1 - 8 and go directly to that/those cylinder to look for the cause. It will also give you the P-0301, 2, 3, 4, 5, 6, 7, 8. DTC/s Most any scanner that can give you live data and selectable PIDs can show you misfire data for induvial cylinders.as well as EFI LIVE and HP-Tuners
Another C5 module that clears some of its DTCs upon cycling the ignition is the EBTCM. If you are having EBTCM related issues, it best to read the DTCs for that system when you see the messages in the DIC and without turning the ignition OFF and then back ON.
Bill
When the PCM detects a misfire, YOU will see a check engine light FLASHING during the misfire event.. If you just pull over and read the DTCs "WITHOUT TURNING THE IGNITION OFF" You will see a P-0300 DTC which is a misfire DTC.
. Unfortunately the C5s DO NOT RETAIN the P-0300 DTC or give you a P-0301, P-0302, P-0303 etc...like some platforms do.
If you turn the ignition off and back on the P-0300 DTC clears from the PCM. You have to have another misfire event for it to set again.
With the right scanner, you can see misfire events for each cylinder. 1 - 8 and go directly to that/those cylinder to look for the cause. It will also give you the P-0301, 2, 3, 4, 5, 6, 7, 8. DTC/s Most any scanner that can give you live data and selectable PIDs can show you misfire data for induvial cylinders.as well as EFI LIVE and HP-Tuners
Another C5 module that clears some of its DTCs upon cycling the ignition is the EBTCM. If you are having EBTCM related issues, it best to read the DTCs for that system when you see the messages in the DIC and without turning the ignition OFF and then back ON.
Bill
Thread Starter
Racer
Joined: Mar 2021
Posts: 262
Likes: 60
I cleared the codes the other day prior to taking things apart. i wanted to see if i could get it to replicate. however once everything was back together, we could not see any misfires on the live data. He had a scanner that could show data for each cylinder individually. i drove it twice since, and have plans to later today. After i run it today, i will check codes again. I have read other posts that have had some really oddball causes. so in the event it returns i would like to be able to narrow it down. I did have a CEL illuminated when i got the knock sensor code. but the problem with the hiccup at WOT was long before i got the CEL. i know the bad knock sensor could pull timing, but why it didnt give me the CEL is confusing.
Race Director
Joined: Oct 2004
Posts: 12,864
Likes: 4,668
It is my experience that on modded/cammed/tuned cars, many times the tuner either tunes out the misfire section, or changes its parameters. Even when my car had a wire off, it never threw a code. When all plugs/wires are good, plugging in my old Predator handheld, using it as a scan tool in real time, it read "random multiple cylinder misfire, cause unknown," but no CEL. Tuning/changing parameters are almost a prequisite, because a good performance cam will cause misfires at idle. If the tuner didn’t tune this out, every decent cammed C5 would throw a CEL......
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