Lee96Z28

My 1996 Z28 has just shy of 39k miles on it. I've owned since new. it is mostly stock, just has GMPP 1.6 rockers & LT4 springs and LT4 knock module, an Ed Wright mail order PCM tune, catback exhaust, SLP cold air kit. I changed the plugs to NGK iridiums at 30k miles, which was about 15 years ago. I also did new front O2 sensors.

Anyway, it has always run fine until last December when I picked it up from getting new tires. On the drive home, it just felt a little "off" like it had a slight miss and just wasn't running crisp. I started looking into in January this year. The miss is still there, but seems to be worse than it was originally. It's there when cold, but it's subtle. As the car warms up, it gets way worse. Below is the what I have done in the order I have done it.

1. I scanned it with AutoTap and found it was missing on #1, #3 and #4. There were no codes.
2. Removed valve covers to check for any broken parts or loose rockers. All seemed good. Put covers back on.
3. Pulled #1, #3 and #4 plugs all looked pretty good. No fouling, none looked super lean or anything. Put them back in.
4. At the advice of a couple of buddies that are pretty well versed in LT1s, I replaced the fuel injectors with rebuilt Bosch 3's from FIC, also changed the fuel filter
5. Took it to a mechanic that has done some work on my wife's Honda and my Toyota truck. Reasonably knowledgeable guy, but didn't know LT1s. He replaced the EGR valve. No change.
6. Took it to another mechanic who works on Vettes (including LT1s), he did a compression check on #1, it had 160 PSI. He replaced the #1 plug wire and the car ran way better. Not perfect, but the worst miss was gone. It just a had a little flutter.
7. Based on the fact that the #1 wire replacement fixed the issue with that cylinder, I decided to replace all of the wires. That project was no good for my blood pressure and since I could not get the y pipe off, I decided to only replace the front 4 wires (1,3,2 & 4). I also found #5 plug was loose, so thinking that may have been the smoking gun, I thought I was golden. Started the car up, the bad miss was back. Confirmed by AutoTap that the worst was #1.
8. Put the wire back on that the Vette mechanic put on #1. Same miss.
9. Replaced # 1 plug in case it had a non-visible crack. No change.
10. Double-checked the wires were tight in the Opti, found #2 was loose. Got it to click in correctly. Also verified they were on the proper terminals. No change.
11. Found another mechanic that works on LT1s and came highly recommended by someone on the Corvette Forum. Took it there yesterday. They confirmed that #1 is the worst, but #3 and #4 also miss occasionally. It is intermittent. They told me it's the Opti and they want $1900 to swap out the old and install an MSD. I don't want an MSD Opti and I can't justify spending nearly $2k on something that might fix it. I had the car towed home.

So my car is sitting back in the garage and I am not sure what to do next. It was suggested that it might just need a cap and rotor, but even that is a giant PITA.

I'm looking for insights/suggestions as to what it might be. It's a really clean car, probably one of the cleanest 4th gens out there, but I can't keep throwing money at this.

Thoughts?

Injuneer

Perhaps you are overestimating the difficulty of the Opti replacement. Shoebox has a pretty good write up as part of his cam removal/install procedures. You should be able to avoid pulling the radiator and the crank hub for doing just the Opti. Probably some other guides out there too. ~$1,300 -$1,400 for labor sounds excessive.

Your position on the MSD Opti is correct - avoid at all costs. Even their caps seem to be having problems. Appears you have already found Petris.

The decision to replace the cap/rotor, or the complete unit can be made by pulling the cap first, without removing the base. Cylinder specific misfires can be due to carbon tracking, cracks, burned contacts, etc. Have you examined the plug wire contacts in the Opti for corrosion? You can also see if a worn bearing is allowing wobble of the rotor shaft, or whether there is oil buildup in the Opti due to problems with the seal in the timing cover.

WhiteWS6

Definitely make sure all the plug wires are "clicked" in properly, I had to double check them when I replaced my opti just before starting and found a few that actually "clicked" in a bit further. I understand if you can't replace the optispark due to health reasons, but if you'd like to save money and know the quality of the work performed I would definitely recommend it. A year ago I was practically too scared to even touch most of my vehicles, thinking I would somehow screw something up. I finally got tired of paying other people for questionable work and started to do it myself, best decision of my life. Removing and replacing the optispark is nowhere near as hard as most perceive it to be. I'll link a video just below here of a guy replacing his opti and water pump seals on an Impala (removes opti and water pump to do so), you can see all the work involved and it's quite easy. I spread out replacing mine over about 3 days to take it slow and double check things such as cam dowel pin length etc..
The only thing I had to do different from the video was remove the fans which is literally only 2 electrical connectors then you push it up and pull it out the bottom, and then turn my motor over to slide my opti in and out a bit easier past the crank hub.
I didn't even have to use a crank pulley removal tool, I just grabbed and pulled with my hands and it came right off. I've seen others just pry it a bit but I'm not sure I'd recommend that. That is a very excessive amount of labor to replace it, good local shop by me charged roughly $450 for a friends. I went with a Petris Enterprises opti and am very pleased so far. My car even idles a little bit smoother after the process. If you want any pictures of a set of steps and pictures from me just shoot me a message, I'll be more than glad to try and help.

Part 1 of 4 (others can be found on right side):

Lee96Z28

The Opti replacement looks like a moderately difficult job. I’m mechanically inclined, but I’m kinda sick of wrenching on things. Just trying to do plug wires I cursed the engineers who designed these cars incessantly.

I looked at the terminals when I changed the 4 wires. They looked ok. I haven’t been inside the Opti, so I don’t know what is going on in there.

Lee96Z28

I don’t really have health issues per se, other than the usual 50-something aches and pains. Although my blood pressure is a concern. What I meant was that I get really f-ing pissed off working on cars, especially ones that are difficult to work on. Like I want to smash things because I get so mad.

Thanks for the video link. The guy obviously has done it many times before. An Impala is a different animal, though. I wish I had that much room.

I may put the cover on and let it sit for a while. Just not enthusiastic about it right now.

WhiteWS6

Ok gotcha, yea these cars can especially lead to some wrench throwing sometimes, my floor would know lol
Yes he's done it a few times for sure, just a bit more patience with the Camaros and Firebirds but virtually identical, sometimes it's just best to let it sit while you think on it

-Grant

Lee96Z28

The biggest thing that concerns me is going through all that work and then not have it fix the miss. I really want to be sure that it will fix it. I know, no guarantees in life...

WhiteWS6

From what you've described it's a likely candidate, diagnosing issue's on these cars can be a real hassle.

Chimera96

OP

you have a ignition problem specific to the same cyl. The most likely candidate, given you have replaced a few plug wires for those cyl, is the Opti.

2 things. Either cap is bad and or you have oil and or coolant in the opti corrupting the optical sensor reading the reluctor wheel.

You would need to pull the opti though to find out. I am older than you and yes can get very frustrated (to say the least) working on the car. But it is all doable with hand tools

Unfortunately MSD anything Opti is now a total crap shoot. Their C&R used to be good and I have those on my original AC Delco Opti as my spare now. It works fine but there is some slop in the bearing which does allow oil to get inside that will eventually cause problems like you have

If I had to buy a C&R today I would get this Standard Motor Products one but have no direct experience with it. It has brass terminals
https://www.rockauto.com/en/moreinfo...049701&jsn=446

If you have contamination in the opti, oil or coolant, you can carefully clean it. I have. I do it on the table with the opti sitting on a roll of tape. Q tips and tissue and carefully clean the inside.

If you have a AC Delco Opti do not throw it away. The optical sensor can be used in those cheap crap off shore optis if you need to "rebuild" one of those.

If some of your plug wires are still original they are likely toast by now or moving/unplugging them has now corrupted them. I find on the LT1 its easier to change the plug wires from under the car. The DS I remove the metal brackets and 4 wires as a set and reassemble the set with new wires on the bench then install as a unit. The PS I drop the starter for more room and also remove front tire to access through wheel well. Yeah a PIA but doable. IMHO plug wires should be run through the stock looms and brackets

Also if you have those metal heat shields over old plug boots you can get plug arc (misfire) between that metal shield and boot that is very hard to see. You can usually hear it though. Spray mist of water on plug wires in the dark can reveal bad plug wire arc...but you really need to have the car securely up on jackstands to be able to get under to see plug boots on some of the plugs

If you have a leaking WP, from weep hole, and or a leaking WP drive seal in timing cover those are the two main sources of crap that gets inside the opti so you would want to replace those if pulling the opti and you clearly see signs either is leaking. It takes some form of "tool" to properly install the WP drive seal. Can buy one for about $20 on ebay, use a Sharpie pen body, a long socket the right size or even the coupler that connects the drive to WP but IMHO use the $20 tool as the leading taper really prevents the inner lip of the seal folding under which is 100% guaranteed leak if it does

Unfortunately qualified mechanics to work on these 25 year old platforms is far and few between so it is always better to do this work yourself if you can deal with it