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Discussion Starter #1 (Edited)
94 Caprice 9c1 lt1
Background:
Car was given to me with an idle issue.
Previous owner:
Replaced optispark three times chasing the issue, most recent less than 3k miles ago (has vent now)
Replaced plugs
Replaced wires
Hollowed out cats
When I first started it up to get it off the trailer it ran like a top, only ran it for a minute or two to get it into driveway. Shut the car off and charged the battery (I had to jump it to start it, it was sitting for a couple months).
Next day I start the car runs fine, take it down the road and it stumbles hard trying to maintain a speed(Accelerates fine, coasts fine). Get it back to driveway and it's idling super rough to the point of stall unless I rev the motor. I checked all the vacuum hoses going to the TB, EGR and Opti and everything was fine pulled air filter and it was clean, MAF seemed clean. Cleaned the front of the TB and IAC valley with throttle body cleaner (don't have gasket couldn't take TB off and clean) and pulled the IAC and cleaned it. Did dvm test on TPS and it's working fine. Put everything back together and it started up, seems to idle better for a second (I think I was just being hopeful) and started low idling and running rough.
Next day I start it cold and it runs like a top, took it for a drive and it started stumbling about 1/4 mile down the road (closed loop I would assume... O2 sensor?) when maintaining speed, turn around get it back to driveway and now it's idling rough again. Check fuel filter, couldn't blow air through it, think holy shit it can't be that easy, go get new fuel filter, put that on and... Runs worse, starts, runs for about 4 seconds and stalls, check engine light comes on at this point but I don't have my obd1 code reader handy so I call it a day.
Next day try to cold start it for shits and giggles, runs for 4-5 seconds and stalls, starts cranking more to start and kills the battery. So I throw battery on charger and call it a day.
Next day I go out and start it and... It runs like a top but it's super cold outside, check egr diaphragm and it seems fine but I snap an egr vacuum line in the process (fixed with hose). Decide to call it a day because it's cold.
Next day take car for a ride to end of the road and back after I let it idle for a minute or two and it runs great the entire time. Get my obd1 scanner and come across a laundry list
DTCs:
16
42
41
36
46
78 (fans are on a toggle)
18
51
Which to someone who doesn't own a car that the last owner gave away because he was convinced optispark was the devil after replacing the damn thing three times may seem to point to a bad optispark but I'm not convinced. Now keep in mind those are also codes stored from when I was unplugging various things while it was running. Alldata mentions 16 could be cause by a short between the high res and low res signal between the Opti and pcm and the issue is intermittent so I'm gonna do their DVM test on the connector and the Opti then clear the codes and run it until something happens again then check and see what codes are actually related to it. If anyone has any other ideas or theories in the meantime I'm all ears, if you need me to run tests or anything I have the equipment to do most of them
 

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Discussion Starter #2
Update, started car today idles high then stalls, left O2 sensor reading is all wonky, code 16 came back after stall but not 36.
I have a video of the O2 sensor readings on my scanner that I'll upload when I get some wifi
 

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Update, IAC steps are jumping all over the place when it had the idle issue, ordered iac and an O2 sensor for tomorrow. Only code showing now is 78(fans) and 16 will show if it stalls
 

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Discussion Starter #7
MAF readings were fine the last time I had it hooked up if I remember it correctly, I'll check the logs when I get home I have it torn apart right now waiting for the right Opti since MSD sent me the wrong one the first time.
 

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While a problem with the high voltage part of opti may occur all the time, a problem with the optical section can be intermittent. You need to buy a $60 PC based oscilloscope from Ebay, Amazon, etc.

With the scope you can monitor the low & high resolution pulses of opti to see if they are the proper square wave. Also see if waveform distorts or drops out. The scope is only 100% way of testing optical section.

So your job would be to see if, when engine stalls low res signal drops out before the stall. Or if it drops out because of stall.

Also, remove opti wiring pigtail that runs from passenger side of intake manifold to opti. Use your multimeter set to ohms to ohm out each of the 4 wires. Expected resistance is 0 ohms. Any more resistance and you should get a new opti wire pigtail.

And scope is also good to see if your 50 hertz vats security fuel enable signal is the proper frequency and square wave.
 

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Years ago I did an LT1 swap into a 1976 Corvette starting with a 1995 LT1 from a Caprice. I ran across an issue with the wiring when I was prepping the harness, on the IAC connnector 2 of the wires were switched. I don't remember which ones, but I always wondered if it would have caused a problem if I had not switched them. I put the wires (tracing from ECM to IAC) where the wiring diagram showed they should be. Always thought it was a bit strange, and I have never had an issue with idle speeds.
 

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The 2 iac wires are iac coil high & iac coil low. I think that switching the wires would result in faulty iac operation. This is because 1 coil is for low counts of operation and the other is for high counts of operation.

This would probably cause the iac to have low counts when it should have high counts. And visa versa.
 

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And since computer has no way of knowing what actual iac counts would be, I imagine engine would not be happy.
 

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Discussion Starter #12
Replaced the Opti and the iac at the same time and the problem went away, I'm not 100% sure which one fixed the problem but she's running good now.
 

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Discussion Starter #14
I demolished it taking it apart, previous owner siliconed the cap on and it didn't want to come apart, there was definitely moisture in there, the rotor shaft had rust on it.
 
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