Need help with diagnosis
Thread Starter
Advanced
Joined: Dec 2018
Posts: 68
Likes: 30
From: Chattanooga, TN
Need help with diagnosis
2017 Grand Sport. Supercharged, methanol, cam, internals beefed up, no cats, tuned by James Short. Stock fuel system.
It's been running great for a couple of years since James tuned it. No issues at all until now. It's happened about 3 times now. It suddenly starts running very rough. Feels like it's hitting on 6 or 7 cylinders. It's backfiring HARD. Constantly. Even at idle. Restarting does nothing. After 5 - 10 mins it's like flipping a switch and it immediately smooths out. Sometimes it comes back within 30 seconds and sometimes I can drive 30 miles without an issue.
No check engine light. I've used the scanner on HP Tuners and it has not recorded any DTCs (I fully expected to see some misfires at least). I just got it home from a drive and once I hooked everything up to scan, it started running smooth. There was nothing obvious while it was running smooth.
Any ideas?
It's been running great for a couple of years since James tuned it. No issues at all until now. It's happened about 3 times now. It suddenly starts running very rough. Feels like it's hitting on 6 or 7 cylinders. It's backfiring HARD. Constantly. Even at idle. Restarting does nothing. After 5 - 10 mins it's like flipping a switch and it immediately smooths out. Sometimes it comes back within 30 seconds and sometimes I can drive 30 miles without an issue.
No check engine light. I've used the scanner on HP Tuners and it has not recorded any DTCs (I fully expected to see some misfires at least). I just got it home from a drive and once I hooked everything up to scan, it started running smooth. There was nothing obvious while it was running smooth.
Any ideas?
Intermediate
Joined: Aug 2023
Posts: 44
Likes: 29
Keep your scanner in the car, misfire code will not show on an intermittent issue
You need to run a non-continuous scan and see what cylinder(s) show for misfire, if its even that.
Check your o2 sensor readings for both banks when the car runs fine, they should flip flop between each other near evenly.
Then check again when it runs poorly, if one sensor is reading out of wack, you can at least pinpoint to which half of the engine is operating poorly and diagnose from there.
Same goes for all other basic checks, fuel pressure, etc
Get some basic info, otherwise everyone on here is guessing the same as myself
Or just pay the 400$ to get it diagnosed.
You need to run a non-continuous scan and see what cylinder(s) show for misfire, if its even that.
Check your o2 sensor readings for both banks when the car runs fine, they should flip flop between each other near evenly.
Then check again when it runs poorly, if one sensor is reading out of wack, you can at least pinpoint to which half of the engine is operating poorly and diagnose from there.
Same goes for all other basic checks, fuel pressure, etc
Get some basic info, otherwise everyone on here is guessing the same as myself
Or just pay the 400$ to get it diagnosed.
Thread Starter
Advanced
Joined: Dec 2018
Posts: 68
Likes: 30
From: Chattanooga, TN
Thanks for your reply. I'm keeping the scanner in the car but so far it didn't repeat yesterday.
The O2 sensors are flipping back and forth like they're supposed to and the values look right as well. I'm not as familiar with the fuel flow numbers, but nothing is obviously off.
In looking around yesterday, I found the wires coming out of the top of the injectors are cracked and I can see bare wire in places. That can't be good and might be related. It would also explain why the problem comes and goes. I've got those cracks covered now with liquid tape to see if this makes the difference. Might be replacing those wires.
As far as getting it diagnosed for $400 - not sure where that would be around me. I haven't found anyone that knows what they're doing with a modified LT until I got to James Short. And he's a 5 hour drive from me. I can get some remote assistance from James but I've got to get something on the scanner first.
The O2 sensors are flipping back and forth like they're supposed to and the values look right as well. I'm not as familiar with the fuel flow numbers, but nothing is obviously off.
In looking around yesterday, I found the wires coming out of the top of the injectors are cracked and I can see bare wire in places. That can't be good and might be related. It would also explain why the problem comes and goes. I've got those cracks covered now with liquid tape to see if this makes the difference. Might be replacing those wires.
As far as getting it diagnosed for $400 - not sure where that would be around me. I haven't found anyone that knows what they're doing with a modified LT until I got to James Short. And he's a 5 hour drive from me. I can get some remote assistance from James but I've got to get something on the scanner first.
Safety Car
Joined: Jul 2019
Posts: 3,817
Likes: 1,295
From: nashville TN
since you have hp tuners start a data log and just let it run. 20-30- maybe 45 minutes and hope it reoccurs. then you need to sift thru the data.
as for shops I use dbr high performance in Spring Hill tn. David the owner does all my tuning but they have 8+ bays techs fab room mainline dyno visually everything needed to diagnose any problem and they are very familiar with modded lt4 motors. not cheap by any stretch and pretty booked up best way is drop it off and let them see what they find
as for shops I use dbr high performance in Spring Hill tn. David the owner does all my tuning but they have 8+ bays techs fab room mainline dyno visually everything needed to diagnose any problem and they are very familiar with modded lt4 motors. not cheap by any stretch and pretty booked up best way is drop it off and let them see what they find
Melting Slicks
Joined: Feb 2018
Posts: 2,126
Likes: 1,012
From: North Texas
Sounds like rodents are eating at your wiring which will no doubt cause these intermittent issues.
You got to repair the wiring first then move on to any other issues.
Troubleshooting 101.
You got to repair the wiring first then move on to any other issues.
Troubleshooting 101.
Thread Starter
Advanced
Joined: Dec 2018
Posts: 68
Likes: 30
From: Chattanooga, TN
as for shops I use dbr high performance in Spring Hill tn. David the owner does all my tuning but they have 8+ bays techs fab room mainline dyno visually everything needed to diagnose any problem and they are very familiar with modded lt4 motors. not cheap by any stretch and pretty booked up best way is drop it off and let them see what they find
Thread Starter
Advanced
Joined: Dec 2018
Posts: 68
Likes: 30
From: Chattanooga, TN
It wasn't rodents, but yeah, I think the bare wires might be my issue. As I said, I've at least got the wires temporarily fixed for testing purposes and I've got several more hours of driving done over the past couple of days with no issues. I'm hopeful.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
5 MOST and 5 LEAST Popular Corvette Model Years in History!
Joe Kucinski
2027 Corvette Buyer's Guide: Everything You Need to Know!
Joe Kucinski
Melting Slicks
Joined: Oct 2010
Posts: 3,016
Likes: 790
From: Tollesboro Kentucky
I couldn't imagine James would ever turn the misfire counter off.Probanly raised it some to allow for the cam.He definitely knows what he is doing.Sounds like you may have found your issue.
Instructor
Joined: Mar 2018
Posts: 222
Likes: 138
From: Fountain Hills, AZ
I have a 2016 and my injector wire insulation were all severely cracked where the wires feed into the connectors. It was bad enough that I could see bare copper on many of the wires. It got me a little worried even though I wasn't having any problems. I gooped/sealed them at the connector lead-ins with black silicone. That was 2 months ago. No problems so far.
Thread Starter
Advanced
Joined: Dec 2018
Posts: 68
Likes: 30
From: Chattanooga, TN
I agree. I don’t think he turned off the misfire counter. And he definitely knows what he’s doing. More than anyone else I’ve ever seen.
Thread Starter
Advanced
Joined: Dec 2018
Posts: 68
Likes: 30
From: Chattanooga, TN
I have a 2016 and my injector wire insulation were all severely cracked where the wires feed into the connectors. It was bad enough that I could see bare copper on many of the wires. It got me a little worried even though I wasn't having any problems. I gooped/sealed them at the connector lead-ins with black silicone. That was 2 months ago. No problems so far.