I am not a happy camper right now. I've had a misfire for a while now, and I'm at the point where I need some additional help. The misfire occurs at WOT from ~4500 RPM to redline. It's at least one cylinder misfiring, but could be two (it's pretty bad). AutoTap does not record any misfires or DTCs, which is immensely frustrating - I don't know which cylinder(s) is/are the culprit(s).

Two weeks ago I replaced my TR-6's (with all of ~9k miles on them) with TR-55s, and replaced my Jacobs wires with Taylor Thundervolts. The only effect this had was to lighten my wallet by $70. <img border="0" title="" alt="[Mad]" src="gr_images/icons/mad.gif" /> So much for an easy fix. So now where do I look, the coils? Injectors? Other?? With AutoTap not detecting the misfires ( <img border="0" alt="[bullshit]" title="" src="graemlins/gr_bs.gif" /> ), I'm shooting in the dark.

FWIW, the last time it was on the dyno (and misfiring at >4500), the wideband O2 put the AF smack dab at 13:1, so I doubt my misfire is an injector problem, but ya never know. Also, the car is strong as hell below 4500 RPM, running absolutely flawlessly until that magic 4.5k barrier.

Any and all suggestions are welcome at this point. Thanks! <img border="0" alt="[cheers]" title="" src="graemlins/gr_cheers.gif" />

<small>[ November 02, 2002, 11:28 PM: Message edited by: Nightclaw ]</small>

 

Yes, and it does show the odd occasional random misfire at low RPM. On throttle, AutoTap seems to think they're clean as a preacher's sheets - even while the engine's chugging on 7 cylinders. Argh.

I don't know why ATap is not catching the misfire data. It's there at the track, and you can see it on the dyno sheet. But either the PCM or ATap is turning a blind eye on it, and I can't tell you why... <img border="0" alt="[Banging Head]" title="" src="graemlins/gr_banghead.gif" />

<small>[ November 03, 2002, 02:41 PM: Message edited by: Nightclaw ]</small>

 

it the plugs were misfiring then atap should show them.. are you logging current misfire history for each cyl??

 

Anyone have any suggestions at all? I hate to start blindly throwing parts at a problem, but it's coming to that unless someone has a better idea. Anyone know offhand what coil packs cost?

 

Wow, coil packs are a lot cheaper than I though! $30/ea. through GMPartsDirect.com. Screw it - I'll just buy four packs and keep rearranging them until the misfire goes away. If it STILL doesn't, then I'll really need some help... <img border="0" alt="[guns]" title="" src="graemlins/gr_guns.gif" />

 

Let me know if swapping coil packs out works or not. I've had what I believe is the same problem for a few months now. I purchased my 02 Z28 in April and broke it in normally though I went to 1000 miles before running the engine above 4 grand. However, it seems like I drop a cylinder right around 4500 RPM, like you mentioned. At about 3K miles on the car, I had new exhaust put on (headers, et. al). While they were at it, I requested new plugs and plug wires, thinking (like you) that this would remedy the situation. Alas, it didn't. I later had the car dyno'd however, a peculiar thing occurred. Though I was suppose to have three pulls done, they had problems with the lead which was hooked to what ai believe was the number one cylinder. The tach would climb steadily to about 3000 RPM, then it would jump back and forth between 3000 and 5000 but it leveled out around 3500. Then, the same basic thing occurred at 4500 RPM but leveled out again at 5000. The tach swept right up to 6000 (200 short of red line) but never went above that. To me, that alone is weird. But, when looking at the dyno chart, I have a gap in my horsepower curve between 4200 RPM and 4600 RPM. But, there is no gap in the torque curve. Also, my AF was right at 13:1 if I remember correctly. The place that did the dyno also informed me that they saw arching between the front two coil packs on the driver side (I believe that these are cylinders one and three though I could be wrong). I took the car to the dealership but alas, they wouldn't look at it due to the header install (that's another story all together). I haven't hooked the car up to any diagnostics computer. I'm looking to buy software but I'm not really sure what I should get. I have however, attempted to find this misfiring again. I've noticed that sometimes, when I'm in third and punch it, it feels like I dropped a cylinder when I hit 4500 RPM. Other times, it doesn't. Also, if I launch and row through the gears, this phenomenon doesn't seem to occur. I've come up with two ways to fix this. Either, buy two coil packs and alternate replacing them until the issue is resolved. OR, take it to a shop and let them figure it out. I'd prefer to do this myself; I just haven't had the time. I apologize as I haven't really helped out your cause, but I am very interested to see what happens when you change out the coil packs. Please keep us informed.

 

Damn, that was a sh*t ton longer than I thought it would be. Sorry about that.

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Norrie:
<strong>Damn, that was a sh*t ton longer than I thought it would be. Sorry about that.</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Heh! No worries - this is frustrating as hell, and I can understand needing to vent. <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" /> I haven't received a tracking number from GMPD yet, but they're coming UPS ground, so I probably won't see 'em this week. As soon as I get them done I'll post the results. Definitely keeping my fingers crossed...

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">The place that did the dyno also informed me that they saw arching between the front two coil packs...</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Whoa! That's pretty wild, if true. Why it wouldn't arc at all RPMs I have no idea, but I imagine that the coil field strength of each is more prominent at higher RPMS, so I guess it's sorta plausible. Hrmm...

 

Read this and see if it helps you :

Misfires

 

Great info! If the new coils don't help, then I think my next step is to check the compression, injectors and pushrods. I do get some oilsmoke on startup (valve guides), so maybe there's a correlation if the new coils don't banish this ghost. No errors on the injectors, but then again, ATap thinks there's no misfire, too... <img border="0" title="" alt="[Roll Eyes]" src="images/icons/rolleyes.gif" />

*IF* none of the above are the culprit(s), it's time to take off the heads and look at the valves - maybe a cracked/burned valve, but good lord I hope that's not the case. No time/money for THAT project in the forseeable future... <img border="0" title="" alt="[Sad]" src="gr_sad.gif" /> I mean, if I end up having to go to all that effort, I'd be a moron not to upgrade the bump-stick and heads, ay? <img border="0" title="" alt="[Wink]" src="gr_images/icons/wink.gif" /> <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />

See, that's the problem with me - I'm what you'd call a pess-optimist. <img border="0" alt="[devil]" title="" src="graemlins/gr_devil.gif" /> <img border="0" alt="[judgement]" title="" src="graemlins/gr_judge.gif" />

 

Thanks Team ZR-1; that was some very informative information. Unfortunately, I have to wait another month or two before I can go tearing into the issue(s) with my car. However, I agree with you Nightclaw. If I'm going to tear into the engine because GM won't even look at it due to headers being installed, the outcome is going to be well worth the effort. I haven't rally started looking into H/C packages much. But, what I've seen local in Dallas tells me to go with AMS (who is running a special, wouldn't you know it) or LG with the G5 cam. Oh, who knows? I'll cross that bridge when I get to it.

 

Holy crap - it's been 15 months since I started this topic?! Well, here's an update, and I think I finally found my misfire (though I won't know for sure until I get it on the track/dyno). I pulled the valve covers three weeks ago, and found 5 bent intake pushrods! Badly bent too, I might add (ie: I'm amazed they bent that far without breaking).

So this weekend, what started out as "I'm just going to replace the pushrods" became "Hey look: Thunder Racing is having a sale on camshaft packages!!" I still have to do the valve springs, but the car's back on the road. The springs are going in next weekend, and the car's going to the track the weekend after that. Is the misfire finally gone? Stay tuned!