I forgot to mention my O2 wires were burned on one side, but since fixing that the O2 code that popped up after fixing the P0336 is now gone too. I have unplugged them and drove around before anyway, don't think it's the misfire source since it still misses when they're unplugged right now.

 

Well I just came back from getting it scanned. Right now it misses at idle and a little above that maybe about 1200 rpms really bad, then when you go above that it doesn't miss. When it revs down from any rpm it backfires a lot out the exhaust. It also does this when starting
I've driven it 50+ miles and the computer had 0 recorded misfires. Even as we ran it for 10 minutes and could see and hear is missing in front of our eyes the whole time the tech2 picked up in total 3 misfires in cylinder 5 and that's it. My fuel trims were -3-5% so taking fuel away, even though every other time I've driven it they've been 5-15% adding fuel. The only thing that was even slightly out of the ordinary was the vacuum is at 16 inches which is a tad low since I'm at 3000ft and the IAC counts are 16 which I read here should be 30-40 (32 ideal). It idles at ~820 rpm. I couldn't find a vacuum leak with engine start fluid or water. Tested for an internal vacuum leak, no problem there. I can't just drive it like this because it gets like 10mpg and it runs so rich it's gonna destroy my new cat. 2 mechanics said it's gonna be practically impossible to find the problem so they don't try to and I don't blame them. The IAC counts are the only thing really our of the ordinary but I can't find any vacuum leaks and today the fuel trim was trying to remove fuel not add it like what happens when there's a vacuum leak. Anyone got any idea of what I should do now?

I can get the data saved with a tech2 tool since obviously letting you guys look at the data is best but before I do that, would the data from only the tech2 be enough to you guys to be useful? I'm reading the manuals and tutorials about the tech2 but I can't actually find it's sample rate and a full picture of what it gathers and what it doesn't.

 

How about the MAF? Did you unplug it and drive around?

 

Yeah hesitates for a second when I unplug it but misfires all the same. Should I leave it unplugged and drive around and log it? Would that make a difference?

I logged it today but only got basic info from HPTuners. I'll have better logs with GaryDougs software on Monday when I get an adapter to plug into the OBDII.

Here's the CSV of the data of me driving then revving within the badly missing rpms and just idling at the end:
https://drive.google.com/file/d/1iBB...ew?usp=sharing

and here's the HPTuners file
https://drive.google.com/open?id=1zR...JxgAVJ5GoAuxrg

 

When i have time i’m going to have to read through this again. I too think it’s quite possible it could be a vacuum leak. You can do a ‘smoke test’ and see...

 

I put new injectors in because the ones I had were for some 150hp V6. I put proper injectors in and the car doesn't run rich at idle anymore, seems normal, fuel trims read about -7- to -10% at idle. I wiggled my IAC wires and the car started surging 600-900 rpms slowly. After I turned it off then back on it stopped surging but still misfires a little bit. The misfires are way less than before and it will still backfire occasionally when I rev it up but nothing like before. I'm going to drive it and try to get more codes, I got P1371 which indicates a bad opti considering it's not the wires because I have a brand new opti harness I ran. This is my 3rd autozone opti. If I have to buy another opti that has a decent change of being good like a reman or from someone who tests them with scopes, I might be in this car more than it's market value.

To see how far I've come, when I bought this car and it had a bad head gasket, egr valve, vacuum lines, icm, weak ignition coil, harness to the CPS, both window motors, cat, exhaust leak because of no gasket, opti, defroster / hvac control unit, tranny mount, high pressure power steering line, radiator cap, A/C, peeling clear coat all around, long gone worn shocks, and more I'm probably forgetting. Everything that isn't the powertrain still needs to be fixed. Obviously if I knew all of this would be broken I would have scrapped the car but you can't really tell for sure some parts are bad until you eliminate the others you know are bad. Or at least that's what the mechanics helping me back then told me.

 

You see, this is why it's so difficult to diagnose these things over the internet. You had V6 injectors before? Why would you do that? Do those injectors have the same impedance, flow rating and pressure as a stock LT1 injector? Your backfire could most likely be related to a timing issue of some sort, or too much unburnt fuel in the exhaust. Are you without a doubt, absolutely, positively, one-hundred-percent certain your plug wires are on the correct plugs? I have re-edited this post from last night due to thinking more about the injector issue. I am not at all familiar with fuel injector specs. What I do know is if incorrect injectors are used they can dump fuel into the engine at a very high rate.

 

I may be missing things here but I have one simple question. Is the miss only at idle? If so and if I didn’t miss you already having done it, do a compression/ leak down test. I have seen in some engines at times when they’ll run pretty well normal out on the road yet consistently miss at idle. In these cases it’s usually a burnt valve or a carboned valve/seat. It leaks off enough compression to miss under idle or light load conditions yet with a bit of rpm it’s able to build sufficient compression and straighten itself out. This is one more thing you can do to check off your list. I know It’s a bit of an *** pain but at least you’ll be able to rule it out.
From there I’d make certain that I have no vacuum leaks, verify that every plug wire is where it should be etc. Also, the good thing about the test I mentioned is it gives you the opportunity to inspect every plug. If you find one or two that are very black or are damp, you may just have found your misfire anyway. Keep in mind that a plug that’s on the verge of fouling will also cause a pop through the exhaust. Usually you’ll notice the pop when you bring rpm’s up on the engine. The pop is actually the plug attempting to clear itself. Sometimes they will and sometimes they won’t. Just ideas, friend.

 

Vacuum looks low for a stock LT1. I'm pulling 16in with a CC503. I think a stock LT1 should pull at least 18in. Idle sounds high as well....was it fully warmed up? Where did you pull the reading from? I took mine off the FPR hose using the various attachments of the mityvac.

I shot this years ago. I'll see if I can get a better video up later....but here is what my needle looks like....last half of video.
http://www.ace1252.com/images/CC503_...ant_tester.WMV

 

The car just came with the v6 injectors, I put proper injectors on and now it appears to run lean at idle not rich. It misses a lot less now but still misses. It misses at higher rpms but only a tiny bit, and I can't even tell above like 2500 rpms so I think the higher you go the smoother it gets. I did a compression test but I think I did it on only every other cylinder. I should test the others I might soon but the plugs are so hard to pull on this car and I accidentally started stripping one last time. It will be good to be able to see the plugs, but I think they will all be fouled a little from it running rich with the wrong injectors. It idles mostly ok now at about 800 rpms but sometimes it surges from like 600-900 slowly, I think that's my IAC cause it started doing it when I wiggled the wires. I also am getting P1371 code which usually means an opti going bad, and I have a whole new wiring harness for the opti so I think it's the opti itself. I have a lot of stuff to fix on this car so it might be a few days before I update again but I'm gonna scan the next with GDScan.

 

This is what it sounds like to me:

-If its running rich it could very well be a leaky injector. Ive had issues with cars in the past that will dump fuel in straight from ignition and never stop dumping. Causes a massive, temperamental misfire.
-Pull your plugs and see which plugs are covered in fuel, then focus on that cylinder soley.
-Your plug wires could also have a temperamental current. That could cause that misfire.
-FOR THE LOVE OF GOD, CHECK YOUR OPTISPARK TIMING!!! If your distributor is off by even a LITTLE BIT, it will misfire
-Also, check your coil. A bad coil can cause random misfire when it warms up, thats what happened with my LT1 T/A.

 

The optispark can't be adjusted in any way that will affect timing. Pin drive or spline drive. Timing can only be adjusted in the PCM. Also, OP, use an IR thermometer on the exhaust ports. If one cylinder is noticeably different from the others, investigate that cylinder.

You got the opti code and said you have a new harness. Swap the harness out to the old one and see if the code returns. If it does, then its definitely the opti

 

Not what I had meant. Im talking about the teeth markings on the male and female splines. If they are not lined up, it will misfire or run horribly. I had that issue the first time I did optispark on my T/A

 

Not saying this could be an issue with it missing. Now that you have different injectors you need to confirm the data/tuning in the PCM. I have no idea what V6 injectors are rated at, but if it's different than the injectors you have now you will have fuel delivery problems.
There have been those who've forced an opti on the wrong way regardless if it was splined or dowel.