Hey guys, just did a heads and cam swap on my car a few weeks ago (97 LT1 T/A M6) and it has been running great. It has a basic tune from when i put headers on it way back when so the EGR, EVAP, AIR and post cat O2s are gone. Here's my problem...

The other day the check engine light popped on and i was a block from home so i got home and just parked it. Started it up to go to class a few hours later and realized it was misfiring. Since then it has had the same misfire and the light has stayed on. It's a constant misfire, throughout the entire RPM range. It's extremely noticeable under a load (like 1,600 RPM in 5th gear say) but it never comes or goes, i can feel it all the time no matter what RPM or how much throttle i'm giving it.

I'm almost positive the car has original plug wires which i planned to replace, also replaced the 140k mile opti with a used but good one with an MSD cap. here's what i've gathered so far...

All the valvesprings are good, i haven't done a compression check yet but i pulled the valve covers and the springs and everything look good (I ran it and watched them all, as well as felt them all). I was reading that the Pre cat O2s can throw this code sometimes, and thinking maybe since the tune is so far off with the new ported heads and big cam that it may have fugged up the O2s and causing this but that seems a bit less unlikely than a plug wire. The only reason i'm leaning toward it NOT being a plug wire however is that i was driving the car for a few weeks with ZERO problems and it suddenly threw the code and started misfiring, which doesn't seem like a plug wire would do (it's not like i drove it 4 minutes, a wire burnt up and it misfired), but again i could be wrong... Anybody got any experience with this code or a similar issue? I'm gonna swap plug wires this weekend i think but wanted to see what you guys thought or had to offer. Thanks!

 

So are you saying you have 140k on oringinal plug wires? Did you put colder plugs in there?

 

I don't know what cam you have, but my CC503 will throw false misfire codes. The code is disabled on my setup, but I see you mentioned that you can actually feel it.

If you can get hold of a scanner with GM enhanced parameters, it should tell you what cylinders are having the misfires(misfire counts, IIRC). If it is across the board, then the ICM or Opti could be to blame. I'd start running through the ICM and opti checks on Shoebox's site.

Have a look at these too.....from my 1996 factory manual.

http://www.ace1252.com/1996_Z28_Fact...TC_P0300_1.JPG
http://www.ace1252.com/1996_Z28_Fact...TC_P0300_2.JPG
http://www.ace1252.com/1996_Z28_Fact...TC_P0300_3.JPG
http://www.ace1252.com/1996_Z28_Fact...TC_P0300_4.JPG
http://www.ace1252.com/1996_Z28_Fact...TC_P0300_5.JPG

 

well the plug wires are numbered and definitely look stock or close to it (my understanding is factory plug wires were the only ones that came numbered). I know i should have changed them when i did the work but it literally came down to barely being able to afford coolant to put back in it so i did the opti and figured i could always do the plug wires in the near future lol.

ACE, thanks for those links ill be sure to have a good look at them once i get a scanner on it. I'm gonna do the plug wires and see if anything changes. The car ran strong as hell and then BAM misfire, so i'm thinking and hoping a new set of plugs and wires will do the trick (plugs were changed but i know it wouldnt hurt to change them since it's still a stock tune and they probably arent burning all that great right now) Thanks guys

 

Interesting...I have a CC230/236 with LE Heads and have a constant P0300...sometimes it goes away but rarely. Isn't it bad to disable that code? Doesn't the engine pull timing if it is misfiring?

I didn't know the factory manual had that information. I use Hayne's manual but didn't see that info in it. Can we still order that particular factory manual? Sorry for the slight hijack...hopefully it's pertinent to your question.

 

You can find the factory manuals used on ebay for a lot less than a new one, if they even print them any more. Check the publisher - Helm

http://www.helminc.com/helm/homepage.asp?r=

 

I see, after seeing the original price I remember why I didn't purchase them...$139! My Haynes was like $15. I did not know it had that much more information though. Not too bad on ebay ranging from $30-$89 but not in great condition either.

 

I'm not sure on the timing, but I don't think I've had any ill effects from it. I initially re-enabled the misfire code after getting the PCM reprogrammed(I purchased Jet DST software so I could tweak the programming...but I also got a mail order tune). It would throw the misfire every now and again, but my misfire counters would never increment....so I disabled it, as it originally was on the re-program. So far, no trouble and the car runs strong.

 

Have a look at this video too.

https://www.camaroz28.com/forums/sho...d.php?t=795663

Just after the halfway mark, there is mention on how a plugged vent system can generate enough vacuum, in the opti, to pull the case to the rotor and actually cause misfires. They also discuss how to diagnose the opti and vent system. It's an excellent training video.

Just something else to look at.

 

changed plugs and wires and all good now! thanks guys