Hi all,

I don't post here much, but been a member for some time.
I need some help with figuring out what is causing a random misfire at idle and slow driving (between 2 to 20 misfire count) and huge miss sometimes (200 misfire count)!.

Mods
2016 CTSV, headers with cats, roto fab big gulp, 103 TB, ported blower, upper and lower pulleys (~15 PSi), meth, chiller, 160 stat, catch can.

I took it to the dyno, and all is fine with an excellent number ~735 rwhp and a similar TQ. It ran at the track; 1 and 2 runs were ok (getting better every run), 3rd run was horrible with ~200 misfires and a Christmas tree of light on the dashboard. drove it back to the parking lot and it came back to normal.

I changed the sparkplugs three times with different gapping (40,32,28), although they were fine. I checked the wires, and they were ok. I tested coils, and they were fine. I Did a leak down test, and all was fine; I took it to the dyno and made 750 rwhp and about the same TQ.. !?!?!?!?

I took it to the track; 1st and 2nd runs were ok. The same thing happened after a cooldown on the 3rd run, misfiring like crazy (in ~200 range).

Yesterday, the same type of misfire (~200) happed at 30kph cursing. It gradually faded when I leaned it to the paddle, slowed down and leaned on it smoothly, and slowly returned to the usual misfire of (~2 to 20 while driving).

I am getting confused. I don't know, but can the VVT or DOD cause this? or maybe the O2s? I am unsure of what the problem is.

Can anyone help me out?
attached are 2 logs cruising and at the track.

 

I had a car with similar symptoms as yours. Turned out a DOD lifter had failed. We chased the misfire for a while and rules out plugs, wires, coil and injector. Once we took the valvetrain apart, the DOD lifter was broken. Replaced the lifter and that fixed the misfire.

 

Appreciate your reply! I am leaning towards that too. I just needed someone to point me to it. well, I guess I have to open it up and start fixing

cheers

 

I know this may sound strange but watch your volts. If it goes over 15.0 weird things can start happening. I've had 2 alternators to fail, they were over-charging, and it sent the car into fits. Like your description of the dash looking like a Christmas tree.

 

Thank for the input, I did check that, and it looked fine. I was checking everything I could think of without opening it up. But I guess it has to come to surgery

It might be worth it to change the cam at that point. But, have to choose wisely :P

 

I analysed your data on HP tuners.
You do see misfires on several cylinders but they are at reduced power while decelerating and cruising and they are not to bad.
So lets not worry about those for now. Some misfires are normal under those conditions.

On your third run your issue is the #8 cylinder. Follow me here closely its difficult to convey this in writing.
Look at cylinder #8.

At 10.50.43.5 seconds it looks like you begin a burn out? It doesn't take long and the engine isn't under much load even though you go to 6100 rpm.
However there are 6 misfires on #8 in that short time.

Now lets look at the run down the track.
At about 10.51.13 you launch and as soon as the car starts moving you see 2 misfires.
As RPM builds the #8 misfires increase to as many as 166 misfires at 10.51.17 and 5712 rpm. So we know #8 is your issue.

But lets compare at the same 6100 rpm we saw in your burn out were we only saw 6 misfires.
Lets see if there are any differences and what we can learn from those differences.
At 10.51.18 on the run you hit 6105 rpm and #8 is still misfiring a good bit.

So at 6100 rpm under 2 different conditions the amount of misfires are noticeably more (exponentially more) when the car is accelerating down the track vs during the burnout.
Whats the difference? Why is it misfiring more now?

The answer is boost. Your #8 spark plug is blowing out under boost greater than 6 psi and up to about 14.7 psi.
Now the question is why?

I suspect the plug is bad. I know you changed it. But you should change it again. What heat range plugs are you running? Don't go more than 1 heat range from stock. Verify the gaps don't go less than 0.28. Keeping in mind many folks run the stock gap and stock plugs at your boost levels without issues.
If anything go to the stock ZR1 plug and gap because ZR1 boost levels are about where your boost levels are.

I don't think its a coil or wire because your issue is showing up under load/boost.
But they cant be completely eliminated as suspects as an increase in load increases resistance which could make a weak coil or wire show up as well.
So make another log under boost after you change the #8 plug and lets see what you get.
If you still suspect a wire or coil swap the #8s with the #1s and see if the misfire follows the hardware or stays at #8.

Additionally look for heat damage to the #8 wire as it is often burned by the headers.

Please let us know what you find.
Good luck.

 

If you plan to cam the car at any point in the future, now would be a good time to save money on labor. If you do go for a cam, be sure to get a blower cam and a 32% or 38% fuel lobe. Personally, I like very mild cams in the LT4 to keep the car very streetable but mildly increase airflow and then the major benefit is the fuel lobe so that you can run higher blends of ethanol. Add a pulley with the cam and you're off to the races.

However, you can also save $ if you like the way the car is now and fix the issue.