JD_AMG

Just started getting random missfires this week and I want some opinions here. Car only has 73K miles, and typical bolt ons (lid, LT headers, ORY, catback, HPtuners tune) also have aftermarket valve springs and chromemoly pushrods - all of which was done a while back (like 10,000+ miles ago). Car has ran great. Replaced plugs and wires when I did the LT install (have NGK 55 plugs and MSD wires). The most recent thing Ive done to the engine was paint the coil packs, so that had me thinking that was the problem. I swapped coils with another cylinder, and swapped plug wires as well and still got cylinder 5 missfire. I just replaced the plug in cylinder 5 today, disconnected the battery and then reconnected and drove around and the code came back. The car doesn't feel too bad, the engine isn't shaking too bad (hardly noticeable, but its there) but it does seem to lack some low end right now.
So its not the coil, not the wire, and not the plug. Now, what should I go for next? The injector? If so how hard is it to replace?
02 sensor maybe? But If its an 02 why would only cyl 5 be going off?

Also it comes and goes and seems to do it most often when cruising on the highway in 6th. The engine light will flash a couple times and then stay on, and sometimes the light doesn't stay on.

Here are some pics of the plug I pulled out:

JaYZeig

iTrader: (11)

maybe try swapping injectors too?

if nothing after that, id pull the valve cover and make sure everything is kosher in there..like a loose rocker, bent pushrod, possibly broken valvespring..and hopefully its not somthing deeper like a collapsed lifter, or anything tough to get at..

in my experience, ive also seen a cam go bad...the "peak" (for lack of a better term) of the cam flattened out, causing misfires

JD_AMG

Thanks for the help.
Got different injectors and fuel rail from a friend. Car is now giving the p300 (random missfire code). Friend has HPTuners and was scanning my car while driving and isnt seeing anything unusual.
Then changed all the spark plugs, swapped some wires around(got a couple different wires from friend), different wiring harness on drivers side, cleaned the MAF, and had the valve cover off (drivers side) to inspect, and no change.
The car drives and accelerates good, but in low RPMs under load (like >1000RPMs in a higher gear like 5th or 6th) its missing pretty bad, car is bucking hard - but not flashing the SES light...
Highly doubt pushrods are bent/broken, im on a stock cam with chromemoly pushrods with only like 10K miles on them - no strange noises either.

RPM WS6

iTrader: (3)

A valve spring could be cracked without it being easily visable. What springs are you using?

Rocker arms were mentioned above....I'm assuming you checked those for excessive movement when the valvecover was off. Guess you could also drain the oil and check for needle bearings.

It would be unlikey that a cam lobe would go flat on a stock hydro lifter roller cam. This is more of a flat tappet issue, though it's still possibile.

Did anything happen shortly before the issue began, something along the lines of a mechanical overrev or a hard smack against the rev limiter? Also, what is your limiter set at, and what rpm do you normally shift at (WOT)?

That plug looks pretty good. You've already covered the basics with moving wires/coils/injectors around, and the plug indicates that the cylinder is certainly firing enough to prevent serious fouling, so there isn't much else to look at besides the valvetrain.

Have you recently had the engine at high rpms (say, 5k+)? If so, how did it behave? Normal revving? Normal power?

JD_AMG

Im using patriot gold (650 lift), got these and the pushrods because I had a valve seal go bad and was buring oil, so instead of just buying the seals I just decided to buy the springs and pushrods together because I do plan to do a cam in the future anyway. As I said they have maybe like 10,000 miles on them. But I will likely pull the valve covers off again to check again - on this same note if it were a spring why would I only get a miss under load and otherwise drive good?
Yep, rocker arms were checked by my friend, no movement. And the oil was just recently changed as well (like 2 weeks ago), nothing unusual there - but good suggestion and thanks - I may end up doing it anyway.

I can't think of anything that happened recently to cause this, it would be a hell of a lot easier if there was something. Haven't hit the limiter in a long time, and have only hit it hard one time I can think of and that was over a year ago.

Limiter is at stock setting (6200Rpms) and I shift right around that.

I ran the engine in the high RPMs today and it feels strong up top, no unusual/new noises, and above 1500RPMs the engine seems normal, but below it seems lacking and under 1000RPMs under load (like 5th or 6th gear) you can REALLY feel the missing, causes the car to buck like crazy- BUT the SES light doesn't come on, the only time I've had it come on or flash is when Im cruising on the highway 60+mph in 5th or 6th, and you really can't feel it then... weird...
My friend and I are going to try more on sunday (he found a misfire detector on HPtuners) and we may replace the crank position sensor and the 02 sensors, maybe a passenger side harness too for the hell of it...

Any other suggestions are welcome, thanks.

RPM WS6

iTrader: (3)

Hmmmm.... have you checked plugs from other cylinders?

You mentioned that your friend did some real time scanning while the issue was occuring. What did the fuel trims look like (ST & LT), and how big was the spread bank-to-bank during the condition and otherwise? And since you mentioned that he didn't see anything "unusual", I'm guessing that cylinder specific misfire counts were uneventful?

01ssreda4

iTrader: (96)

I say its a good possibility its the 02s since you seemed to have checked everything else. They are very important in fueling, and lugging the engine at under 1500 rpms in an overdrive gear would require precise control of the fueling to prevent bucking. If they were off a tad, it would show here.

JD_AMG

Wanted to update this thread, heres my findings:
https://ls1tech.com/forums/generatio...k-pistons.html