Injuneer

The PCM watches the crank position sensor and looks for known pattern of irregularities in the rotational speed, caused by the individual cylinders firing. The PCM calculates the exact rotational speed of the crank and which cylinder is firing from the CKP pulses and the high and low resolution pulses from the cam position signal in the Opti. That's why an aggressive cam can throw the misfire detection off, as can a bumpy road. A problem with the Opti cam position signal can cause a code to set for a faulty CKP sensor.

Misfire detection is required to be monitored as part of OBD-2. That's why only the 96 and 97 LT1's have the CKP sensor. And misfire detection is the only purpose of the CKP sensor.

Did you consider the possibility of a faulty injector? Swap #5 injector with one of the others with a "0" misfire count, and see if the misfire follows the injector.

tanmanski

Fred, thank you, that is very helpful info. I was imagining systems looking at resistance/impedance/etc. Knowing that it is actually looking at irregularities in rotational speed is amazing (to me). Logically it also means the system doesn't actually know whether the plug actually fires or not? Only that the cylinder doesn't "combust" (produce rotational acceleration) right? Which could be lack of fuel, air or lack of compression? Since an injector swap is easy, compared to compression check for example, I will do that tonight or tomorrow (if I run out of daylight today) and post up my results.
If misfire remains at #5 cylinder with swapped injector, you agree Opti should be next? Another thought I had is go get a cheap Harbor Freight dial indicator, take off the rocker cover and measure cam lift at the rockers. Just seems like I'd hear noise if there was a serious valve train issue?
Really appreciate the help, it is stressful to be working away from home, living in a hotel and having a car that's sick, with no where to work on it.

shoebox

Could just be a bad cap on the opti. I wouldn't buy a whole opti for that.

tanmanski

Good point Rob, but if you were me, paying shop labor rates, would you do just the cap, gambling (again) that you wouldn't have to have the opti accessed again?
Are Duralast Gold Optis any good? It's "only" $140 plus tax....not sure what just a cap would cost but Im guessing labor is almost exactly the same ($300+) and I really don't want to see the water pump come out twice...

tanmanski

Dang! Auto zone only stocks MSD Opti Spark cap & rotors....for $152....more than the entire Duralast Gold Opti assy! I can check other parts stores, but at this point saving a little at the risk of "double labor" doesn't seem worth it.
So, anyone got an opinion on whether Duralast Gold Opti is or is not worth installing.

thanks guys

tanmanski

and if I buy the Duralast assy and the find it is just a bad cap and or rotor, I'll have a spare opti....

GaryDoug

Have you tried unplugging the #5 wire from the spark plug, hooking it to a spark tester (about $5-10) and checking for spark? Might have to get someone to crank the engine. Compare the spacing max with what you get with #1 wire. If it's the Opti cap, that should show it.

Great Neck/Adjustable ignition spark tester 25069 at AutoZone.com

tanmanski

Hi Gary, that's good, I like it. Cheap, waay easy. How about actually firing it up and seeing what I get? Any reason not too? I'll try to get one with adjustable gap to more definitively judge spark quality. I'm guessing at nominal spark plug gap i'm going to see nothing (since its missing 100% at idle speed) No sense messing with injectors if this confirms weak/no spark. more tomorrow thanks gentlemen!

tanmanski

and since no one has offered a contrary opinion, Im going to assume the opti brand I've been asking about is at least worth installing. I'll take it apart and torque/Loctite the screws as everyone points out. which will probably void the "warranty"! but it's all about that labor getting to it. Never want to go here again!

GaryDoug

Yes, let it run a minute. I was in a hurry to post last time and didn't think it through. No need for another person. The adjustment should be way wide under no pressure, because it takes more voltage to make a spark under the pressure in the combustion chamber.

Injuneer

Opti failure rate varies with the inverse of the cost....... Low cost = high failure rate. Proven over and over again. If you like to gamble, get the Duralast... you might get lucky. Much better odds with an AC Delco. But even those (and the more expensive MSD) fail from time to time.

I think the reason no one else has spoken up is we assume you have taken the time to read through related threads and make your own decision. The subject has been beat to death.

My solution, assuming your Opti optical cam position module is still healthy, would be the Bailey Engineering LTCC with 8 LS1 coils. I've had a similar system since the engine was built 15 years ago, with no problems of any kind. This engine has a 7,400 RPM redline, and I have a couple data logs showing it reached 8,000 RPM when I missed a shift and hit 1st instead of 3rd (back when it was still running the T56).

Getting the high voltage out of the Opti eliminates most of the problems.

Give Gary's MSD cap & rotor idea consideration.

shoebox

If you have no codes related to the opti, MHO is to keep the opti and do a cap and rotor (MSD). The aftermarket optis are very hit and miss with regard to quality and you would be more likely to have to replace it again as opposed to the one you already have. Don't take silence as an affirmation. Not everyone gets around to the board quickly.

tanmanski

Thanks Rob, I do value your experience and opinion.
Car is at the shop now, with direction to test #5 spark, and if no/weak spark is found, replace distributor cap & rotor. If good spark is found, I'll direct them to swap #5 & 3 injectors, but I strongly suspect #5 terminal in cap has a problem getting hi tension out to the plug.
Results, with pictures, to be posted up later today.

thanks everyone

tanmanski

SUCCESS! Opti cap was the culprit! Note I am NOT saying it was the OPTI CRAP! My "gamble" on replacing the whole opti actually would have paid off, but hey, I got new plugs and wires out of the deal, lol. And keep what is probably a perfectly good opti unit.

I'm already in celebration mode (no more driving tonight), so I'll post the pix tomorrow. Ima happy guy right now!

One follow up ?, then quick comments;

I'm guessing this cap was original. Is there a way to confirm if this was original to the car or at least OEM?

I learned a lot about my LT-1 in this painful process, thanks to the knowledge and wisdom of the key players that keep this board running. My thanks to all of you, but especially Fred, Gary & Rob.

Fred showed me how one misfiring cylinder can feel a lot worse because the ECM is now trying to "fix" too much O2 and/or raw fuel in that bank's exhaust stream, which in turn can make the other 3 cylinders on that bank run less than optimally under some circumstances, which is why, at times I would have swore it was missing on multiple cylinders (also supported by misfire counts)

Gary provided the free software and understanding to help me get the data that kept me from going into "parts replacement mode", even though I was desperate to get it resolved.

Rob, pointed out that a working OEM opti with no codes is worth keeping when its likely only the hi tension cap is faulty. I'm a big fan of don't disturb what's not broke. Plus, I think I've done most of your "how to's" over the years and always found your info accurate and easy to follow. I finally ponied up a donation. Thanks Rob!

Finally, the shop pointed out my water pump shaft seal was leaking coolant (just starting by the look of it), so since it was already out, I got a "free labor" water pump installation out of the deal! haha

I'm feeling really good about long term reliability of this mostly stock car, buying me time to do the 389 transplant "right"

thanks again everyone, this board is awesome.

Injuneer

Glad it worked out? Did you examine the old cap? I would suspect the problem could be carbon tracking. That may leave an open question.

Carbon tracking occurs when the cap dielectric strength breaks down, or when there is something in one or more of the cylinders' ignition path that is requiring an excessive voltage to fire. The spark finds it easier to travel across the inside surface of the cap, than to fire the plug.