So it was doing it before the LS6 valvesprings on the same cylinder? And when you changed the springs, you didn't notice if any of the pushrods were bent? Correct? If so, I doubt it's a pushrod, because it should have been real obvious. And if it's not a pushrod, and it was doing the exact same thing before the ls6 springs, I hate to say it, but you need to do a leak down test.

 

It was doing this on Cylinder 5 before the LS6 springs, we did not remove the pushrods when doing the valve springs so we're not sure if one's bent or not.

Tomorrow at lunch we're going to check the pushrods and do a leak down test as well so hopefully we'll have some answers, if not we'll eliminate a couple things....although there won't be much left to check after that.

 

Wow...didn't know you could change valvesprings without removing the pushrods. You fellas got some talent!

 

I would not run LS6 springs on any boosted setup whatsoever.

 

I am 100% for sure ordering springs next week and will put them in. Don't think this is the cause of this problem for Cyl 5 though but at least it will be safer in the long run.

 

Hi rob,

I dont' understand this post? you need to remove rockers to change springs, but why do you have to remove the pushrods? We just left them sitting in the motor. I really don't understand? Maybe we have a magic single cam pushrod motor?

In any case, we just tested compression on the offending cylinder and it looks great. Builds pressure extremely fast (120psi on first stroke) and compares equally to other cylinders. We also just checked the pushrods in that cylinder, and they are as straight as can be. Hopefully lowering the gap on that one plug (and changing to a new one) to 0.021 for ***** and giggles will tell us something. Otherwise I guess it could be crank sensor/wiring... I just don't understand how it could always offend the exact same cylinder, and only above a certain rpm.

 

Oops. I guess I'm just used to using the double spring tool (allows you to do 2 valvesprings at one time). If you use a single spring compressor, I suppose you wouldn't necessarily need to remove the pushrods. My bad. You aren't magic.

 

Update, lowering the gap on this one cylinder to 0.021 did nothing. A wot scan showed 125 misfires on that single cylinder still. We're now going to swap coil pack modules from left to right to see if it's the wiring that triggers the coil which has an issue.

Any other ideas people?

 

Coil bad? Wiring to that coil bad?

 

We had swapped coil 5 for coil 1 and we still had the problem on cyl 5.

What I am going to do right now and swap the whole coil assembly from 1-3-5-7 to 2-4-6-8 if the problem is in the wiring to the coil it "should" move to the even cylinder......I hope we see something, this is getting annoying.

 

we've swapped coils with another cylinder already, no change. So this afternoon the entire coil pack module will be swapped to see if it's the wiring within the module. Defiantely running out of ideas.

 

Leak down test.

 

You still havent said if you checked the pushrods yet ?

And I cant see swapping coil for a different unit, making any difference than swapping from side-side of the engine did.

Although I see you only swapped cylinders on the same bank. Id swap side-side of the engine. There is only 1 wire linking all 4 coils on each bank for fault diagnosis.

 

If you read a few posts up, you'll see that I noted that we checked the cyl #5 pushrods and they are "straight as an arrow".

The first coil test was to see if the actual coil was the problem. This new test is verifying if the wiring to the 4 pin connector to the coil is the problem. Each test is eliminating a different component.

 

Ok, swapped the whole coil assembly from one bank to the other, still had the Misfire on #5.

Did a Case Learn....didn't help

This blows!!

 

Ok, So now I have a new problem, When I get into boost and hit roughly 5psi, it sounds like the car is backfiring out the cutout like CRAZY, like a machine gun goes off in my exhaust.

I re-gapped plug #5 to about .035 because at .021 I was getting small misfiring all the time so I thought this might be it. This helped the overall misfire at cruise but I am still getting HUGE backfiring, this blows.

 

Bent valve? Cylinder loading up? Got a vid?

 

I am going to unplug my Alky at lunch and see if this is causing an issue since it usually starts firing at 5psi.

If it's still doing it I will get a video of what's happening when I get home.

 

verify that actual timing on the crank = commanded timing doing this has become very important to me

i would also purposely misfire another cylinder to see if the computer can tell. then at least you would know if it is correctly picking #5 vs a general misfire

also looking at your afr graph, sure it is solid, but it could be solidly below 9.5:1 out the tailpipe and will absolutely kill hp production all together. if you don't have wideband data it will be good to refer to.

 

if you really think no5 is the problem. Unplug that injector and see what happens.....or even, remove the pushrods/rockers from that cylinder.

Has the missfire just appeared from nowhere on a healthy engine, or gradually over time ?