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Discussion Starter #1 (Edited)
Huge misfire issue under boost, tried lots of stuff, still no success, not sure what to do next. Sorry for the long post . . .

Story:
Car had been sitting for a couple weeks or so, when this problem just came on out of the wild blue sky. Prior to that, I had no apparent spark energy issues - she would hold strong at 14psi with water meth as well (stock lower end motor also!).

So, went to drive the car, find out that it misses badly etc at anything over 5psi. Pulled over, and car actually had trouble restarting at the time. Hmm odd. Car runs OK , maybe a slight miss detected, but mostly OK until misfires and carries on badly at about 5 psi, consistently. (also turned off water meth to see if that was operating improperly, and no effect was seen)

So decided not to go on the next day's roadmeet (bummer) and fix the system before it leaves me stranded somewhere.

Car already has upgraded MSD cap/rotor, stock OEM opti crazily enough, and upgraded MSD plug in coil, installed in Oct. 2018.
ALso new Taylor wires a few months ago.

Plugs are now gapped at like .021" I believe it is. Had to go there when I turned the boost up.

Got to really working on the car finally this weekend to see what's really wrong. Was thinking the cap/rotor was likely the issue, but decided to do more checking before blindly pulling the opti.

So I find that using my adjustable Lisle spark checker on the coil wire, that I only get 1/4" or so of spark, and it's not real consistent and looks kinda weak. Last year when I did the work originally, it had blue thunderbolts and an audible snap at over 1/2". Hmm. (opti isn't even in the loop at this point of course).

So, I seem to have weak spark from the coil. Go through the factory manual - checked the terminals at the ICM, and getting good ground, and also showing 11.5 volts at both the wires that come back from the coil. So all that checks out - and the manual basically says replace the ICM.

So I replace the ICM, and no real change, except that somewhere along the way of testing the MSD coil has failed to produce spark at even 1/4". Now that's odd, - so I try the old original LT1 coil (knew I saved that for a reason), and how bout that, I have spark again. I start to widen the gap on the spark tester, and was able to get to about .350" or so, not stellar by any means, but the spark looks better anyway. Still not a real great reading it seems. So I can't believe it, and swap back to the MSD coil, no spark again. Swap back to original - spark comes back. Ok, guess the MSD coil is dead, although the DVM says otherwise - no apparent open circuits in the coil - but there must be more to it to properly test I guess.

Also took the ground post nut loose, cleaned the nut and the ground lugs, and put that back. No change to spark energy.

So kinda decided that maybe the MSD coil had gotten weak - so test drove the car with the old original coil (knowing that the spark still doesn't look very good, especially when testing at the plug wires) and of course it has the same result. Will not run good at all above 5 psi.

I'm fairly sure it's a spark issue, kinda acts like when your plug gaps are too wide and starts blowing the spark out - I know what that feels like, and this seems to be close to that.

Checked voltage at ICM plugs, replace ICM, swap coil, clean grounds, problems still exist???

So - any thoughts out there?
 

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I'd look at the cap & rotor as a suspect. Rotor has a tendency to back out it's screws. This will cause all kinds of problems. Cap could be in failure mode also.

You also might want to invest $70 in a PC based oscilloscope. Use it to make sure opti low & high resolution signals are the proper square wave signal. And that signal doesn't change shape or drop out.
 

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Discussion Starter #3 (Edited)
Not to be dismissive of advice, but I'm seeing that I have weak spark with the opti not even in the equation - straight from the coil wire. Once I have good spark there, I'll be more suspect of the opti or wires, etc..

I did red loctite on rotor screws as well, but who knows. I will probably have to rule out the cap and rotor as you said, but I just can't believe that's going to save me when the spark straight from the coil is so weak.

I do have a MSD box laying around - so maybe I'll throw that on there (and try the MSD coil again, or get another one) and see what that does. That puts the power load on the MSD box I think, which should be far more capable than the stock ICM and coil I'd say. Probably a better setup for a boosted engine anyhow.

Kinda browsed for a PC based scope - do you have any in particular that you know work well? I'd like to save myself the trial and error portion of the purchase.

Something like this?? https://www.amazon.com/Hantek-Portable-Oscilloscope-Analyzer-Diagnostic/dp/B01N74XEAN/ref=asc_df_B01N74XEAN/?tag=hyprod-20&linkCode=df0&hvadid=312278522835&hvpos=1o11&hvnetw=g&hvrand=3259569453131524165&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9016679&hvtargid=aud-801381245258:pla-569046058279&psc=1

Is there a video tutorial at all on reading the scope for the Opti / where to hook up etc?
 

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Yes the Hantek scope will be fine. I have one and it works well.

The positive probes of scope hook to the low & high resolution signals found at connectors of opti pigtail. You can search youtube for how to use an oscilloscope.

Don't be so fast to dismiss optical side of opti as problem. The correct low & high res signal is a 5 volt square wave. If voltage is low, or signal skewed from proper square wave, fire might be insufficient. Signal from opti, causes pcm to trigger icm, which in turn fires coil.

Even though you are firing just the coil, opti is still in the loop. icm won't fire coil without proper signal from pcm. And pcm won't send that signal to icm unless at least low resolution pulse is coming from opti. If high res pulse is missing, coil will still fire, but with reduced performance.

Here's something I just thought of. Remove the opti pigtail. It runs from opti to passenger side of intake manifold. Use your multimeter set to ohms. Test each wire in opti pigtail. Make sure it has 0 ohms resistance. Any resistance in this pigtail will lower performance.
 

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Discussion Starter #5
I did pull the opti pigtail. All wires checked out to 0.1 ohms. Not zero, but close. And all the same.

My 8876 MSD wiring harness came in. I'm gonna try that this weekend I think and see how it goes. Hoping for a big blue nasty spark haha.
If that doesn't solve it, then I guess I'll buy a scope and get on with that.

Can you explain why the performance of the coil would be reduced if the high res signal was lost? I would have thought that the signal to fire the coil was the "same" no matter what ECM logic was behind it. In other words, the signal is digital not analog, and thus it's just full power all the time. But ???

I had a fuel tank / fuel pump problem on this same car that drove me nuts for months. After 2 new pumps, pulling the tank probably 6 times, I finally figured out it was the actual bulkhead connector inside the tan that would have intermittent connectivity issues at the strangest times. Removed the connector and soldered the wires to the bulkhead terminals, and the problem has never come back.

Starting to wonder if I have some sort of weird thing like that going on here. :banghead:

An LS3 5th gen. seems like the right solution. >:)
 

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When high resolution pulse is lost, computer only has low resolution pulse to determine spark advance. The pcm will retard timing and go into limp home mode. I'm not a boost expert, but I believe with retarded spark timing, the plug fires later in the cycle. This makes it more difficult for spark to jump the plug gap as it's now working against the boost pressure. Rather than with the boost pressure, if it had fired earlier in the cycle.


Lower voltage sent back to pcm on low & high resolution signal would still be a square wave, but with lower amplitude. This would confuse pcm, which was expecting a 5 volt square wave, and in turn, it would send a weaker ic control signal to icm, thus a weaker spark. Well that's my story & I'm stickin with it. Running out of reasons for your weak spark.


Lastly, that 0.1 ohm resistance in opti pigtail, doesn't seem like much. If this was a high voltage circuit, the effect would not even be noticed. But on a 5 volt circuit that opti & pcm use to communicate with each other, the effect could be weaker spark.
 

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Discussion Starter #7
FIXED for now

Update:
I installed the old MSD box and did some quick tests - to find out that the old coil fired a pretty good spark.

Also tried the MSD coil (thought to have failed from before tests) and found that it works just fine in fact. Strange. No real explanation to go around here. So far so good.

So took the car out for a drive and she ran great at 9 or so psi. Success!! I'll put the boost controller back on and see what happens next.

But then I notice a little steam from right front tire area. Hmm. Headed home and the steam got worse. The original '91 radiator has started to steam at the tank connection. So now I have a radiator to install. But at least the ignition is solved for now.

Sure wish I knew what started the weak spark deal - and wonder if whatever caused it will maybe get worse and start causing issues again. I'd feel a lot better with a smoking gun fixed :screwy: I hate an "unsolved mystery".
 
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