More issues on 2015 Z06

Hi all,

Made a post on here a few days ago about some issues with how my new to me 2015 Z06 has been running
https://forum.hptuners.com/showthrea...5-Corvette-Z06
I've taken some more data logs and unfortunately seem to have discovered even worse issues than anticipated. Misfires on all cylinders but noticeably more on low rpm/cruising than WOT which I find strange.

Prior to this discovery we (friend and I) believed it to be a single cylinder giving issues and may be tied to a bad injector, now with it being all cylinders while also seemingly running OKish (no catastrophic failure or inability to start) it led us to check out the spark plugs again. We found that the previous owner installed "NGK LTR7IX-11" plugs which I believe are aftermarket plugs that are 2 steps colder and is likely the reason all 8 cylinders are reporting misfires as well as why the car has a very long crank before turning over. What do you guys think? Can someone verify that the plugs currently installed are incorrect at least for how the car is currently setup and let me know the part number for the correct OEM plugs. I plan to make the change this weekend to hopefully clear up the issues I'm seeing if this is the case. Thank you all for the help!

Timing is right at peak for optimal conditions at that boost. I would probably pull 2 or 3 degrees as I'm sure fueling wasn't dialed in to stay the same all the time - in fact I know it wasn't. If it had been and had more safeties still for temps and whatnot then that timing wouldn't be a huge no no. I don't like to run them that lean on gas. I know a lot out there do. Just not me... I run them there on alcohol, but still .8 on gas.

Fueling changes a lot bank to bank. That could easily be the torque model being wrong as the torque model will completely change the fueling or cylinder 1's misfire. Cyl 1 appears to miss the most. While you have the plugs out just do a quick compression check. I think you'll be OK, but who knows for sure until it's checked. Could easily just need plugs and wires. Common enough on these.

Previous owner changed the plugs and wires 2k miles ago (stating oem plugs and wires) I think what I?m suspecting is he put in the wrong plugs. Are you able to confirm this? I just can?t see true oem plugs needing to be replaced a mere 6 months after installation with minimal mileage.

Also I?m pretty new to tuning, only found this page as a friend ran the data logs for me and got me setup on this page to troubleshoot. With the issues I?m seeing, would putting correct plugs in (assuming these are wrong ones) correct it? Thanks for the input! Let me know what you think.

The LTR7IX plugs are only 1 step colder and is the right plug if you have added a pulley and headers to the car. Otherwise the 6's are the stock heat range for the LT4. The 5's are for the LT1 only, if you put 5's in the LT4 you will have issues.

If the previous owner installed plugs and wires, I would start to doubt the wires themselves. They are easily damaged if removed incorrectly. You could get some LTR6's and MSD wires for the car and start there. There is a chance the plugs are fouled as the LTR7's need WOT runs every so often to stay clean.


TLDR, install new plugs and MSD (or OEM) wires and ensure the plugs are gapped properly and the wires are clicked on the plugs and the coils. It's a single click onto the plug side and a double mini click on the coil side. If you don't hear/feel this, then the wires are not installed properly.

Another thing to check is pop off the valve covers and inspect the valvetrain. And lastly, make sure your O2's are switching back and forth when warmed up. I've seen bad O2 sensors cause fuel trims to go rich on one bank and foul all of the plugs and start misfiring.

Thanks for the info! The car has a 15% lower but stock manifold, it?s also street driven so not many opportunities to go WOT to clean them. I ordered a set of OEM plugs and wires (ACDelco 41-128 Iridium Spark Plug) and will check the gap (stated they are pre gapped to 0.030 but will verify) I?m assuming these are equivalent to the 6?s you mentioned. Either way I think I?m starting to narrow down the issue, I appreciate the help!

Yes, the stock heat range is equivalent to the LTR6IX.

I have been running the same set of LTR7ix in my Whipple SS since 2019 with no issues. I doubt the plug heat range is the issue.

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Originally Posted by 1scls1z

I have been running the same set of LTR7ix in my Whipple SS since 2019 with no issues. I doubt the plug heat range is the issue.

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It?s not. :)

But if the plugs aren’t gapped right, got dropped during install, etc then they could still be a problem. But heat range is spot on.

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Originally Posted by Joshinator99

But if the plugs aren?t gapped right, got dropped during install, etc then they could still be a problem. But heat range is spot on.

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Definitely...but I thought that was a given.

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Originally Posted by 1scls1z

Definitely...but I thought that was a given.

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Figured I’d mention it since the OP is not the guy who installed the plugs. Anytime you have to make assumptions about someone else’s work lol…

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Originally Posted by Joshinator99

Figured I’d mention it since the OP is not the guy who installed the plugs. Anytime you have to make assumptions about someone else’s work lol…

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Yeah, mistake on my part lol.

But also, why don?t we think the heat range is the issue. All signs point towards that being the problem? Long crank at cold start, misfires on idle and low rpm cruising but less at WOT, excess carbon build up with minimal mileage, misfires on all 8 cylinders and not any specific cylinder. Colder plugs as far as I?m aware would cause all of those issues alone, unless I?m missing something here. Let me know, I?m newer to this type of stuff so any educating is appreciated.

All heat range plugs are the same temperature on a cold start.

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Originally Posted by ShawnC7

But also, why don?t we think the heat range is the issue. All signs point towards that being the problem? Long crank at cold start, misfires on idle and low rpm cruising but less at WOT, excess carbon build up with minimal mileage, misfires on all 8 cylinders and not any specific cylinder. Colder plugs as far as I?m aware would cause all of those issues alone, unless I?m missing something here. Let me know, I?m newer to this type of stuff so any educating is appreciated.

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None of that stuff would apply to a 1 step colder projected tip plug. Even a non-projected tip plug even 2 steps colder won't be a drastic difference in any of those things. They do end up looking darker over a short time but I wouldn't say excess carbon.

I run non-projected tip plugs in pretty much anything FI especially if it makes any boost. A real cold non-projected tip plug on cold mornings will start rougher and run a little rougher than a projected tip plug will for maybe a minute. As soon as it gets the smallest bit of heat in it they come around.

Sounds to me by your description the motor might have some damage.

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Originally Posted by edcmat-l1

None of that stuff would apply to a 1 step colder projected tip plug. Even a non-projected tip plug even 2 steps colder won't be a drastic difference in any of those things. They do end up looking darker over a short time but I wouldn't say excess carbon.

I run non-projected tip plugs in pretty much anything FI especially if it makes any boost. A real cold non-projected tip plug on cold mornings will start rougher and run a little rougher than a projected tip plug will for maybe a minute. As soon as it gets the smallest bit of heat in it they come around.

Sounds to me by your description the motor might have some damage.

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Maaaaaaannnn, well I guess I?ll see what happens this weekend when I install the oem plugs. I was really hoping this was gonna fix the problem.

What confuses me is how the misfires go away after going WOT. Wouldn?t the misfires get worse if there were motor damage? I guess I?m just being optimistic here, I?m sure there?s other issues I?ve yet to diagnose.

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Originally Posted by ShawnC7

Maaaaaaannnn, well I guess I?ll see what happens this weekend when I install the oem plugs. I was really hoping this was gonna fix the problem.

What confuses me is how the misfires go away after going WOT. Wouldn?t the misfires get worse if there were motor damage? I guess I?m just being optimistic here, I?m sure there?s other issues I?ve yet to diagnose.

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I agree that is a strange scenario. I haven't looked at any of your data logs, but are you able to log the misfires? Can you feel them or are you looking at misfire counters on your scanner?

By far (95+%), the most common causes of misfires in an LS or LT engine are cracked plugs and plug wires not snapped on all the way on either the coil or plug. If there's any doubt, replace all the plugs, and when you snap the wires on, you should feel and even hear a solid click on the plug and most of the time a double "click click" on the coil.

Use an insulated spark plug socket. Take care not to over tighten them. A tapered seat plug only requires 15-18 ft lbs. It just needs to be snug. Spark plugs with compression gaskets are different. If you hear an audible "snap" when tightening your plugs, you just cracked the porcelain.

OK, going back and reading your original post, my best advice would be to do an accurate compression and leakdown test. I have a very specific way I want both done. I do a lot of email tuning for other shops and I have them follow my exact procedures for compression and leakdown testing.

Then, in addition to that, I have them pull the plugs, take a piece of cardboard and poke 8 holes, 2 sets of 4, stick the plugs in the holes and with a sharpy number them on the cardboard. Then take close up pics for me.

That's step 1 and 2. That's going to tell you a lot. I also highly recommend everyone who does this even as a hobby, own a borescope. They're cheap and readily available now.

Step 3 is do a bore scope inspection. Using a cheap USB bore scope, readily available on Amazon, you take pics of the top of the pistons and even the cylinder walls.

I've had a bore scope for probably 15 years now. They were expensive back then. And kinda crappy even for good ones.

This is the one I have now. It's an articulating bore scope that uses your phone for the screen. It's an amazing tool. You can stick in in a spark plug hole and look all around. If you twist it just right you can look back up at the valves.

https://www.amazon.com/Inspection-Ar...hlbWF0aWM&th=1

Following all of these steps will tell you exactly what the mechanical health of your engine is.

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Originally Posted by Joshinator99

Figured I?d mention it since the OP is not the guy who installed the plugs. Anytime you have to make assumptions about someone else?s work lol?

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Valid point! :cheers:

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Originally Posted by edcmat-l1

OK, going back and reading your original post, my best advice would be to do an accurate compression and leakdown test. I have a very specific way I want both done. I do a lot of email tuning for other shops and I have them follow my exact procedures for compression and leakdown testing.

Then, in addition to that, I have them pull the plugs, take a piece of cardboard and poke 8 holes, 2 sets of 4, stick the plugs in the holes and with a sharpy number them on the cardboard. Then take close up pics for me.

That's step 1 and 2. That's going to tell you a lot. I also highly recommend everyone who does this even as a hobby, own a borescope. They're cheap and readily available now.

Step 3 is do a bore scope inspection. Using a cheap USB bore scope, readily available on Amazon, you take pics of the top of the pistons and even the cylinder walls.

I've had a bore scope for probably 15 years now. They were expensive back then. And kinda crappy even for good ones.

This is the one I have now. It's an articulating bore scope that uses your phone for the screen. It's an amazing tool. You can stick in in a spark plug hole and look all around. If you twist it just right you can look back up at the valves.

https://www.amazon.com/Inspection-Ar...hlbWF0aWM&th=1

Following all of these steps will tell you exactly what the mechanical health of your engine is.

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Unfortunately read this after doing all of the work.

Just finished swapping out the plugs and wires and data logging. The original plugs were gapped properly, all had carbon build up, at least 2 were fouled with oil.

Ran a compression and leak down test on all 8 cylinders. All cylinders held pressure at 225-230psi, no outliers no leaks.

Data logging showed the bad news. Still misfires on all 8 cylinders. Couldn?t determine if it was less than before, still had a dozen or more misfires in a 10 minute drive. Still seeing less to no misfires on WOT than idle/cruising speed.

The mechanic/knowledgable fellow who was helping me out looked over the tune and said with it being misfires on all 8 cylinders it?s likely the tune, and the timing is way too high. He recommended bringing it to a new tuner and having it retuned from the ground up and knock the timing back a few notches.

This needs someone competent who can go hands-on with this car. Further tuning won't fix it if it's been hurt from a previous bad tune.

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Originally Posted by ShawnC7

Unfortunately read this after doing all of the work.

Just finished swapping out the plugs and wires and data logging. The original plugs were gapped properly, all had carbon build up, at least 2 were fouled with oil.

Ran a compression and leak down test on all 8 cylinders. All cylinders held pressure at 225-230psi, no outliers no leaks.

Data logging showed the bad news. Still misfires on all 8 cylinders. Couldn?t determine if it was less than before, still had a dozen or more misfires in a 10 minute drive. Still seeing less to no misfires on WOT than idle/cruising speed.

The mechanic/knowledgable fellow who was helping me out looked over the tune and said with it being misfires on all 8 cylinders it?s likely the tune, and the timing is way too high. He recommended bringing it to a new tuner and having it retuned from the ground up and knock the timing back a few notches.

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Can you data log it again? This time pay attention to your timers at the bottom. Point out a time stamp or multiple time stamps where you feel it or where you think you're having misfires? That way maybe someone here can "see" literally what you're talking about.

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Originally Posted by edcmat-l1

Then, in addition to that, I have them pull the plugs, take a piece of cardboard and poke 8 holes, 2 sets of 4, stick the plugs in the holes and with a sharpy number them on the cardboard. Then take close up pics for me.

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Is there any possibly you can do this ^^^? Even if you didn't keep track of which plug goes with which cylinder, can you take some close up pics and post them here? I'd like to see all of them together if possible.

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Originally Posted by blindsquirrel

This needs someone competent who can go hands-on with this car. Further tuning won't fix it if it's been hurt from a previous bad tune.

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Would there be a reason a new tune wont fix it if it seems like the engine is in good health?

There is a shop a few hours away that has tuned several 1000whp+ street/drag cars that have lasted well into the 80k+ mile mark with little to no issues with nearly all hard driven track use. I got several recommendations to see them for tuning and was told they would make it run right.

But obviously if there is a reason I should avoid paying to retune I would like to know. But from what Ive heard that?s my best option. Let me know

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Originally Posted by edcmat-l1

Can you data log it again? This time pay attention to your timers at the bottom. Point out a time stamp or multiple time stamps where you feel it or where you think you're having misfires? That way maybe someone here can "see" literally what you're talking about.



Is there any possibly you can do this ^^^? Even if you didn't keep track of which plug goes with which cylinder, can you take some close up pics and post them here? I'd like to see all of them together if possible.

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Ive got the spark plugs, but we did smear them on a towel to separate carbon from oil that?s moreorless how we determine some were indeed fouled. The fellow helping me said it?s likely a non-issue and is a result of the lack of spark and running rich and something about crank case (I?m 20 and know little about mechanical problems, so sorry if my explanation is a little poorly worded). I can still do this, but not sure if the smearing/wiping negates what you are looking for specifically.

In terms of data logs, I only have what one of my friends set up for what it?s looking for. I do have some data logs already from after the spark plugs replacement, from what I?ve been shown by friend, he?s having me look at Long Term and Short Term fuel trims and misfires which he setup to report on the data log. To be honest I?m not sure what I?m looking at half the time.

Lastly for when I ?feel misfires? I can?t honestly say I even know what to feel for. 6.2L Corvettes already feel choppy to begin with so identifying little misfires as in idling or cruising are next to impossible for me since I?m not even sure what I?m supposed to feel for. Only reason I knew there was an issue to begin with was because I had an issue where I was initially running rich to the point it was shuttering on idle kicking out black smoke from gas so my friend set up the data logging with misfire checks and we noticed the insane amount of misfires.

Let me know if the pictures of the spark plugs are still wanted, and if there?s any tips to identifying misfires. I can post the data log here too.

What have you seen so far that would give you good confidence that the engine is in good health? Your report of the leakdown & compression tests up above are complete gibberish, leakdown results are a number in percent, and whether a compression test 'holds' or not is because there's a little Schrader valve in the gauge so that it'll record the peak. Never seen anything but a diesel that would do 230 PSI. You think it's possible the chamber and piston tops are caked up with a 1/4" thick layer of oil and carbon, if that's a real number from a real compression test?

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Originally Posted by blindsquirrel

What have you seen so far that would give you good confidence that the engine is in good health? Your report of the leakdown & compression tests up above are complete gibberish, leakdown results are a number in percent, and whether a compression test 'holds' or not is because there's a little Schrader valve in the gauge so that it'll record the peak. Never seen anything but a diesel that would do 230 PSI. You think it's possible the chamber and piston tops are caked up with a 1/4" thick layer of oil and carbon, if that's a real number from a real compression test?

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When it comes to the exact PSI I was told that the number doesn?t matter unless it?s super low. As long as all 8 cylinders are reading within 5-10% of each other then we know that a cylinder isnt damaged. All 8 cylinders read at around 230 psi, even if it isn?t truly 230psi we know that they are all giving similar readings of compression. As for leak down, We did hold the compression tool for a minute on each cylinder to see if it went down, and the needle only moved 1-3psi. If you?re saying that it automatically holds it at its peak value then that was a miss on my part.

Honestly though, I can?t imagine any damage being so consistent between all 8 cylinders that they would simultaneously read the same compression while having damage AND running somewhat decently.

There are misfires being reported on all cylinders so obviously something is wrong, but the car can still idle, cruise, and pull just fine. We aren?t seeing any catastrophic events or anything warranting immediate mechanical attention (yet) so I sincerely think there is not something as serious as engine damage.

I also want to take a second to say, appreciate the helpful feedback. I dont want to come off as someone who sounds like he wants to be right and ignore the people trying to help. I can genuinely say probably everyone in this forum is more knowledgeable than me. I?m just trying to connect what I see in person to the feedback I?m getting to see where we can troubleshoot next, and what im seeing now doesn?t sound like an unhealthy engine.

On that note, just to confirm suspicions, what is the most logical/time and cost effective method to check for more signs or symptoms of engine damage? Previous commenter suggested a bore scope inspection so I am inclined to start there.

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Originally Posted by ShawnC7

Ive got the spark plugs, but we did smear them on a towel to separate carbon from oil that?s moreorless how we determine some were indeed fouled. The fellow helping me said it?s likely a non-issue and is a result of the lack of spark and running rich and something about crank case (I?m 20 and know little about mechanical problems, so sorry if my explanation is a little poorly worded). I can still do this, but not sure if the smearing/wiping negates what you are looking for specifically.

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You have a late model fuel injected car that runs clean as a whistle for the most part. If you have shit on your spark plugs that you have to smear off to tell if it's carbon or oil your shits got issues. How many more times does someone have to say it?

Oil diluting the air/fuel charge drastically reduces the knock tolerance. If the plugs are crapped up then everything in the chamber and likely the backsides of the valves is crapped up just as bad. 230 PSI is too high for a pump gas street engine - most engines with high static compression have enough camshaft duration that cranking compression will still be in the normal range of 150-180 PSI. A leakdown tester has two gauges, and the test result is a percentage (2% leakdown, 7% leakdown...), it is not a 'put some air in and see how long it will hold pressure' test. This ain't like big truck air brakes.

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Originally Posted by blindsquirrel

This needs someone competent who can go hands-on with this car. Further tuning won't fix it if it's been hurt from a previous bad tune.

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I agree with this. From the sound of it and the logs, it looks to me like it has or had a terrible tune in the car and now the damage is done. Take it to someone that can do a true compression and leakdown test. Hell even here at the dealer I only charge like 250 to do that. If your plugs are getting fouled out that fast, then theres some sort of damage done. Even heavily carbon deposited DI engines wont foul the plugs that fast......just run like crap on a cold start.

:banghead::banghead::banghead:

Lots of this going on in here.