part throttle higher load miss

[blkchevyz]

missjuly.hpl
2008g8mineo2closed loop idle startup.hpt


i've been battling this hesitation/missfire for a while now. cars a 08 g8 gt. bolt ons, cam, dod delete....


seems to only do it under a higher load, and you can feel it missing and you get some pops from the exhaust.
so i figured it was a spark issue. tried a few different plugs and wires.
it doesnt seem to do it at very low throttle or wide open. but not sure if i just dont notice it as much then.


the logs i'm not seeing much, a few missfires but seems to be mostly all the cylinder so i'm not sure how accurate that is. also the short term fuel seems to go lean then rich for a short time then back to normal. doing it on both sides not sure if thats what it looks like when a cylinder drops.

i've done a compression test and leak down and everything seems fine.

[blkchevyz]

miss8-11.hpl

here is a longer drive. i made some timing changes and turned long term fuel trims back on.

timing change seemed to jack up my fueling a little but really dont want to retune till i get it figured out.


i'm starting to get more backfires and occasionally out the intake.

also i dont know why but sometime my missfire history actually shows history. others times it just shows the current then clears out.

[GHuggins]

You need to check your valve train. Make sure the springs are good and valves still have good pressure on them without the springs moving around or anything like that. Also make sure they aren't being held open by incorrect pushrod length. If it's popping out the exhaust then raw fuel going straight into the exhaust. If popping out the intake then the valve is open when the plug is firing. I assume it's starting good? Can also be slipped timing chain or spun reluctor on the crank, but would check valves closely first. I also assume the lifters were replaced and the DOD ones removed?

[blkchevyz]

lifter are new along with the dod stuff is all gone.
it starts good, sometimes feels a little lazy but still tweaking on that. it use to flare up way to much.
i've popped the valve covers off a few time but really just was looking for loose rocker or something visually wrong with them since i did the trunnion upgrade too. i'll go through it again and look closer at the springs. also thought about the timing being off but was thinking it would run like crap all the time if that was the case.

[edcmat-l1]

Cracked plug or plug wire not snapped on completely on either the plug or coil. I see it almost daily.

[GHuggins]

Cracked plug or plug wire not snapped on completely on either the plug or coil. I see it almost daily.

Would normally say that first too, but he said he "tried a few different plugs and wires". I assumed several sets were tried. Guess it's possible to do several and do the same not full snap each time... There should be a double click when seating the wires.

[edcmat-l1]

missjuly.hpl
the logs i'm not seeing much, a few missfires but seems to be mostly all the cylinder so i'm not sure how accurate that is. also the short term fuel seems to go lean then rich for a short time then back to normal. doing it on both sides not sure if thats what it looks like when a cylinder drops.

i've done a compression test and leak down and everything seems fine.

Ya gotta know what you're looking for. Otherwise it's there staring you in the face.

Tuner tip. You're looking for sharp downward spikes in the O2 signals. This one is pretty evident it's on bank 2. See attached screen shot. In the line graph, second from bottom, light blue line. The bank 1 sensor has a few but not near as many as bank 2. That's the telltale sign of an ignition misfire.

[GHuggins]

That just means a lot of oxygen due to a non firing cylinder went into the exhaust. Doesn't mean ignition specific...

[edcmat-l1]

That just means a lot of oxygen due to a non firing cylinder went into the exhaust. Doesn't mean ignition specific...

Well duh. But 9 times out of 10 it's an ignition misfire. That exact pattern is very indicative of an ignition misfire. Other causes (broken valve train parts, etc.) won't cause that same pattern and sometimes you'll have a miss and there is no sign at all in the O2 pattern.

[TheMechanic]

I'm looking at 95kpa at only 50% throttle and 1,800 rpm. I'm with GHuggins on timing chain/valve timing issue

[edcmat-l1]

I'm looking at 95kpa at only 50% throttle and 1,800 rpm. I'm with GHuggins on timing chain/valve timing issue

Valve timing will not cause a random single cylinder misfire. It can cause all kinds of other issues, but not that.

[TheMechanic]

Valve timing will not cause a random single cylinder misfire. It can cause all kinds of other issues, but not that.

That's odd. I have had several worn out loose timing chains cause all kinds of drive ability problems including P0300's. New chains and gears fixed it. Guess I got lucky.

[edcmat-l1]

That's odd. I have had several worn out loose timing chains cause all kinds of drive ability problems including P0300's. New chains and gears fixed it. Guess I got lucky.

Even a loose timing chain won't cause that "dut dut dut" miss that a secondary ignition system miss will. It won't cause that telltale O2 wave form either. In 30+ years of being a driveability tech and/or tuner, I can't remember the last time that same O2 pattern was caused by anything but an ignition miss.

[edcmat-l1]

seems to only do it under a higher load

it doesnt seem to do it at very low throttle or wide open.

i've done a compression test and leak down and everything seems fine.

You guys are completely overlooking this combination of symptoms. It is telltale secondary ignition misfire. There's no logical argument for it being anything else.

As a matter of fact, the "only under higher load" part is the ideal way to check the integrity of the secondary side of the ignition. Get whatever vehicle in high gear at a low rpm/mph and roll into the throttle. If you have an issue on the secondary side that's when it'll show up.

[Alvin]

Could you do a log where you floor it.. Its easier to track down misses that way.

Several times in my life I've put brand new plugs in a car.. didn't drop them or break them.. and the car has a miss. I've had brand new plugs go in and come back out looking brand new never once fired. Its possible you just got a dud.

[blkchevyz]

i'll have to get wot log. i thought i had one but guess i didnt save it.
i've tried 2 and a half sets of plugs so far. swapped some here and their when it seem to point to specific cylinder, then just swapped them all a few weeks back. a coil pack as well. and 3 different wire sets.
i'm with you thinking its ignition related since higher load should mean higher cylinder pressure and harder to ignite. only thing i havent done is swapped all the coils out. you did lose me at the secondary ignition part though. no clue on that one lol

going to go through the valvetrain today. but besides re doing the rockers and visual inspection i'm not sure i'll find anything.


here are a bunch of other logs i pulled previously.
missjuly.hpl
miss1.hpl
missing.hpl
miss.hpl
1-12.hpl

[blkchevyz]

also timing chain was new.

[GHuggins]

I'm not going to say I've seen this dozens of times, but I can think of 3 right off where valves were bent on decently cammed motors. Idled somewhat fine - unless you knew the difference between a strong cammed idle and a weaker one you might never tell it kind of thing. MAP read high due to lack of vacuum and pressure finding it's way into the intake a little bit. They would even drive with light throttle OK, which was surprising without a single miss, but the second you went wot they would miss every other second or so just like a secondary miss (a secondary miss is just a complete misfire too by the way). The only way your going to tell is to do a compression check. Compression should be nearly identical cylinder to cylinder. If it's 12 or more different you need to start looking at mechanical items.

A secondary miss, as it keeps getting referred to, is basically the output of the coil and everything associated with it there of, so plug and wire on the ignition circuit plus output side of the coil. From what you described in the very first post had me thinking improper valve spring preload or valve train especially with the fact that you've already done plugs and wires, BUT it can even still be something like a restricted injector, however you usually see that in trims and with mid throttle misses as well. There for a while the fuel lines rubber sections were known for coming apart due to ethanol and rubber pieces moving around in the rails causing random misfires most noticeable pulling hills. At one time there was actually a tsb about this...

If a compression check doesn't show anything then yank the injectors and rails together then disassemble and peck them upside down on a clean white towel. If junk comes out then that might be your problem, but the high map reading won't be caused by this.

[Alvin]

i'll have to get wot log. i thought i had one but guess i didnt save it.
i've tried 2 and a half sets of plugs so far. swapped some here and their when it seem to point to specific cylinder, then just swapped them all a few weeks back. a coil pack as well. and 3 different wire sets.
i'm with you thinking its ignition related since higher load should mean higher cylinder pressure and harder to ignite. only thing i havent done is swapped all the coils out. you did lose me at the secondary ignition part though. no clue on that one lol

going to go through the valvetrain today. but besides re doing the rockers and visual inspection i'm not sure i'll find anything.


here are a bunch of other logs i pulled previously.
missjuly.hpl
miss1.hpl
missing.hpl
miss.hpl
1-12.hpl

If you get a log with WOT or if you tell me which one of these that do have that I can likely help you track it down to at least which side of the motor the miss is coming from.

[blkchevyz]

i did do a compression and leak down test and everything seemed good. leakdown was less the 20% compression i think my lowest was 189 highest was 220.
did them all cold with cheap amazon tools, so figure the numbers aren't the most accurate but nothing really stuck out.

haven't gotten a wot run yet. tore the valve train apart making sure rockers and springs where good visually. everything seemed good. rockers all operated smooth, pushrods looked fine, springs i couldn't move any by hand and nothing looked broken.
probably wont get it back together till the weekend... its going to be hot as shit all week lol

thought about pulling the intake and inspecting/replacing the seals since i did reuse the old ones.