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C4 Tech/Performance L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine

1996 LT4 ICM Issues and Schematic

 
Old 04-29-2019, 01:28 AM
  #21  
cardo0
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Congratulations! A happy ending finally.

Thx for posting and sharing as it helps us all.

The PCM has compensation for changes in batt voltage and temperature but a weak battery may keep voltage to low under different loads and and engine RPM. Probably more science to it than that but I know I learned my lesson and and will test my batt to ensure it's healthy. Generally I can tell by a slow start the batt is over the hill. Low voltage makes for a slow start and a crappy startup to idle RPM.

Good night.
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Old 04-29-2019, 05:27 AM
  #22  
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Thanks to all of you for your help!
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Old 04-29-2019, 09:58 AM
  #23  
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Originally Posted by grandspt View Post
I installed the new Borg Warner ICM (the box showed made in the USA but I don't know which one was; the box or the ICM). Anyway it works no codes!!!!
I meant to add that my box said "made in USA" too. That may actually matter. We're seeing a wave of optisparks from Autozone (Duralast brand) that have bad sensors right out of the box. It makes me think there's a serious quality-control issue when sourcing solid-state electronics from some factory in China. You noted that the electrical measurements on the two ICMs you tried weren't close to spec. That's a red flag to me! What a pain - all the time we are spending to diagnose problems that turn out to be new parts that are bad!
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Old 04-29-2019, 10:11 AM
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Thanks for choosing OPTIMA! Don't forget to submit your $50 rebate form!

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Old 04-29-2019, 03:57 PM
  #25  
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Originally Posted by OptimaJim View Post
Thanks for choosing OPTIMA! Don't forget to submit your $50 rebate form!

Jim McIlvaine
eCare Manager, OPTIMA Batteries
Hi Jim, thanks for the rebate information. I just filled it out online.
Hopefully this Optima battery will also last over 13 years😁
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Old 05-09-2019, 09:12 PM
  #26  
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Originally Posted by MatthewMiller View Post
I meant to add that my box said "made in USA" too. That may actually matter. We're seeing a wave of optisparks from Autozone (Duralast brand) that have bad sensors right out of the box. It makes me think there's a serious quality-control issue when sourcing solid-state electronics from some factory in China. You noted that the electrical measurements on the two ICMs you tried weren't close to spec. That's a red flag to me! What a pain - all the time we are spending to diagnose problems that turn out to be new parts that are bad!
I agree with the poor quality parts. I actually ordered and received a second BW CBE122 ICM just to have as a backup since all the other brands throw codes.
**But my misfires returned after driving the car in warmer weather
Also I am noticing that the cranking time is taking longer when the car sits for several days.
I am wondering if my Optispark is dying. I had rebuilt the Optispark after the water pump puked but maybe the optical sensor got affected. I am getting no codes and no knock counts even though I see random cylinder misfires. The car is running well but my idle seems maybe a tad rougher. At idle the short term fuel trim varies around 0 to -3% and long term fuel trim is around -5%. A bit rich but not too far off.
The O2 sensors left and right bank are varying at idle. I am not seeing on the Tech 2 scanner that is out of the ordinary.
This really has me stumped.

Last edited by grandspt; 05-09-2019 at 09:24 PM.
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Old 05-09-2019, 11:21 PM
  #27  
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Originally Posted by grandspt View Post
I agree with the poor quality parts. I actually ordered and received a second BW CBE122 ICM just to have as a backup since all the other brands throw codes.
**But my misfires returned after driving the car in warmer weather
Also I am noticing that the cranking time is taking longer when the car sits for several days.
I am wondering if my Optispark is dying. I had rebuilt the Optispark after the water pump puked but maybe the optical sensor got affected. I am getting no codes and no knock counts even though I see random cylinder misfires. The car is running well but my idle seems maybe a tad rougher. At idle the short term fuel trim varies around 0 to -3% and long term fuel trim is around -5%. A bit rich but not too far off.
The O2 sensors left and right bank are varying at idle. I am not seeing on the Tech 2 scanner that is out of the ordinary.
This really has me stumped.
Ugh! That sucks - sorry to read that. I can't believe you're feeling random misfires but not getting a code for that in a 96. They have extra crank position sensor and seem pretty sensitive to that. I tend to doubt your opti sensor is failing that intermittently. I'd think if that's happening then your tach would probably be bouncing all over the place and/or the car would flat-out die, quickly. And if you had just lost the high-res from the opti (and still had the low-res signal from it), or vice versa, you should get a code for that too. I'm not sure where to look next. Hopefully someone else has a good idea.
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Old 05-10-2019, 05:13 AM
  #28  
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Originally Posted by MatthewMiller View Post
Ugh! That sucks - sorry to read that. I can't believe you're feeling random misfires but not getting a code for that in a 96. They have extra crank position sensor and seem pretty sensitive to that. I tend to doubt your opti sensor is failing that intermittently. I'd think if that's happening then your tach would probably be bouncing all over the place and/or the car would flat-out die, quickly. And if you had just lost the high-res from the opti (and still had the low-res signal from it), or vice versa, you should get a code for that too. I'm not sure where to look next. Hopefully someone else has a good idea.
Yeah I am baffled that the misfire is not throwing codes. Last night I hooked up the tech 2 and set it to misfire graph under special functions. I drove 18 minutes in the city so I had a bit of stop and go traffic. The graph lit up cylinder one as the gross misfire but all the other cylinders we're misfiring as well!
Really strange that no code is being thrown especially for being an OBD2 car.
I hear the mis slightly at idle through the exhaust when the engine is hot. But mainly around the 1500-2000 RPM is the roughest where you can see and feel the engine shake.
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Old 05-10-2019, 11:52 PM
  #29  
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Reads like you are using to cold a spark plug. That or bad fuel. Sorry to read problems continue but I think you have a simpler problem than before. OBD II diagnosis-es a change in crank speed as a misfire (or maybe better to say diagnosis-es a misfire as a change in crank speed). Pretty technical stuff but if your PCM is sensing misfire then that verifies what you hear and feel. And with no codes I don't think its an electronic control problem. You may think the warmer weather has something to do with it but a spark plug with too cold range will load up unless cylinder temps are a lot more than atmospheric temperatures. Bad fuel wouldn't matter much for atmospheric temperature either.

So I looked back over your posts and the original problem was a high RPM misfire that became complicated by bad replacement ICM's. This was after replacing the plug wires. I would look over those plug wires for heat damage and test resistance. Multiple misfire sounds like more than one wire could be bad or bad termination crimps. The biggest symptom I see here is the misfire w/o DTC's.

Hope this can help.
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Old 05-11-2019, 07:23 AM
  #30  
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Well I am still working on this issue and I may have stumbled across something that leads back to the Optispark. I watched this video like 4 times:
Very interesting information about the forced ventilation system and how it works. The video also talked about how a problem with the venting system can cause random misfires. Unfortunately they didn't go too deep in how to troubleshoot it.They show the two check valves but call them out incorrectly however they are shown in the correct position.
So I pulled the elbow vacuum line from the bellows that gets fresh air from behind the MAF while the engine was running at idle and I felt very little vacuum almost zero. I shut the engine off and got out my Mightyvac. I tried pulling vacuum through that same elbow for the fresh air and nothing. The vacuum lines are new, they were replaced when I changed the water pump and rebuilt the Optispark. Next I went to the Intake manifold side and checked vacuum with a gauge about 20 inches of vacuum. I checked the vacuum at each check valve and I still had good vacuum. So I removed the check valve that has the filter and restrictor (it is basically a screen on one side of the vacuum barb. And a metal sleeve that is pressed on the opposite barb that decreases the diameter see the attached picture with the light shining through it). I cleaned this check valve with some intake cleaner. It did not make much of a difference.
Bypassing this check valve is the only way I can get vacuum to the fresh air elbow.
This means my Optispark is not sealed properly and is leaking somewhere even though I used the OEM GM AC Delco kit which includes the Cap, Rotor, Midplate,Seals, and screws!
I guess it could be leaking air from the bearing in the Opti or I also read about the 4 wire connector not having the weatherproof seal installed (mine is installed and looks good visually).
I am going to bypass the check valve that has the screen and restriction because if I do this it does pull vacuum through the Optispark and I can feel the vacuum from the fresh air elbow.
I will update everyone on my findings. I sure don't want to pull the Opti out again!!!
I also took a screen shot of my misfires with the Tech2 after a 15 minute drive in the city with the engine warmed up (no faster than 35 MPH with stop and go traffic so RPMs were in that off idle to 2500 RPM range). The picture was taken after I parked the car.

Misfires during a 15 minute city drive, cool night but engine was fully warmed up.

Check valve with restrictor and screened filter inside.
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Old 05-11-2019, 08:56 AM
  #31  
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I had to replace the check valve and filter in the Optispark vacuum line. My filter was clogged and the check valve was not checking. This vacuum line assembly is still available from GM part #12555323 https://www.ebay.com/i/390214844747?chn=ps. This assembly is slightly different than what is used on the Vette because the rubber hoses are longer. But they can be cut to fit.

My Optispark also does not pull air 100% from the input port that connects to the engine air intake. I suspect that it is pulling air through the 4 pin connector. However, what is important is that there is air flow through the distributor.

Warren

Last edited by Warren Seale; 05-11-2019 at 08:57 AM.
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Old 05-11-2019, 09:41 AM
  #32  
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Originally Posted by Warren Seale View Post
I had to replace the check valve and filter in the Optispark vacuum line. My filter was clogged and the check valve was not checking. This vacuum line assembly is still available from GM part #12555323 https://www.ebay.com/i/390214844747?chn=ps. This assembly is slightly different than what is used on the Vette because the rubber hoses are longer. But they can be cut to fit.

My Optispark also does not pull air 100% from the input port that connects to the engine air intake. I suspect that it is pulling air through the 4 pin connector. However, what is important is that there is air flow through the distributor.

Warren
Thank you for the information. Did you have any misfire symptoms?
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Old 05-11-2019, 11:15 AM
  #33  
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It's an interesting question: should we be able to measure vacuum at that elbow on the bellows? I really don't know. The amount of air movement through the vacuum harness with the restrictor in place is intentionally very low - otherwise it would be an open vacuum leak to the engine. And the purpose is to move just enough air through the opti that it keeps ozone from building up (it isn't suppose to blow-dry the insides of a wet opti, IOW). And the opti itself is not hermtically sealed. FWIW, a lot of use use RTV on the "layers" of the opti, and I also put silicone o-ring lube on the seal for the electrical connector. Taht's really to help keep moisture out, rather than to seal in a perfect vacuum. In short, we know that the purpose of the vacuum harness is really to keep a small amount of fresh moving through the opti rather than pull a vacuum on it.

So keeping all that in mind, it may not mean anything that you can't measure vacuum with the restrictor in place. It may just be able to pull enough air through leaks that the pressure doesn't drop measurably. OTOH, rest assured that if the elbow is connected, then it's moving air through there as intended, because that will be the entry point of least resistance. I'd suggest that you try measuring vacuum at the inlet side of the restrictor itself. You should be able to see vacuum build up there, because there would be no leaks that far upstream. As long as you see that, I wouldn't worry about the rest of it. This a longer way of reiterating Warren Seale's post just above.

Aside from all of the above, it seems instructive that only one or two cylinders show most of the misfires. If the opti were crapping out, one would expect the misfires to be evenly distributed among the cylinders. It's also interesting that it occurs mostly around 1900rpm. At the opti itself, the only thing I can think of that would favor some cylinders over others is the cap itself - if it had issues with conductivity on the affected cylinders' terminals. Like cardo said, I think I'd take a hard look at the plug wires and plugs right now.
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Old 05-11-2019, 12:42 PM
  #34  
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Bypassing the check valve made no difference at all. I put the valve back in and measured vacuum at the elbow. It does pull close to 18 inches! But I notice it takes a while to get there.
So the the plugs are brand new AC that replaced my perfectly good NGKs.
The wires were replaced at the time my water pump puked and I installed Taylor wires made for the LTX engines.

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Old 05-11-2019, 02:44 PM
  #35  
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Do you still have your old plug wires? You could try a swap back to the old wires if you do. I wanted to mention that dialectic grease is for helping install and remove the wire boots - don't use it on any conductors/terminals. Boots can become damaged pulling them off and on dry.

It's just that I think you would get a code if the Opti had a bad enough low resolution. I still drove my camaro with high resolution failure though it lit the SES light - but no misfire. And if the Opti was contaminated it would not be corrected with just good vacuum flow. Don't you think the sensor would stay clouded once it is contaminated?

Hmm, the vacuum does build up to 18"Hg means the system is working and the orifice/restrictor is doing its job slowing the flow. You could also plug one of the opti's vents and pump it down with your Mighty Vac to check the Opti for leaks. Something to note is the vacuum signal would be much lower if you tee'd into the line rather than dead end the line to the gauge.

Last edited by cardo0; 05-11-2019 at 02:48 PM.
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Old 05-11-2019, 03:05 PM
  #36  
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Originally Posted by grandspt View Post
Bypassing the check valve made no difference at all. I put the valve back in and measured vacuum at the elbow. It does pull close to 18 inches! But I notice it takes a while to get there.
Okay, so the vacuum harness is fine. With the restrictor in place, it should take some time to pull full vacuum because the flow is so low.
So the the plugs are brand new AC that replaced my perfectly good NGKs.
The wires were replaced at the time my water pump puked and I installed Taylor wires made for the LTX engines.
FWIW, I always use NGK TR55 (or TR6 if I want a colder range) in my LT4. I would ***-ume these cars came with AC Delcos, so one would think you're fine with those.
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Old 05-12-2019, 08:52 AM
  #37  
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Originally Posted by grandspt View Post
Thank you for the information. Did you have any misfire symptoms?
No misfire. My attention was drawn to the vacuum problem when my optical sensor died. I lost both high and low resolution signals.

Warren
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Old 05-12-2019, 10:33 AM
  #38  
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Hey, again I appreciate all of your help!
I pulled out the Opti last night along with the new Taylor wires. I was so. Pixxed off I didn't even take a look at the inside of the Opti. I did break one of the distributor cap towers. So now a new cap and rotor again! Oh yeah I bought another set of Taylor wires universal fit with the LT1 boot kit. I am going to reroute the wires this time so they don't go behind the power steering pump. They seemed to have gotten pinched really good. But I did not see any evidence of Arc through.
What a pain in the **** this job is. The harmonic balancer came off the hub easy thanks to my liberal coating of antisieze. But I had replaced the power steering rack. I forgot about the little stub that sticks out just enough so you can't pull the balancer from between the crank hub and the damn rack stub. I took my air grinder and ground the stub just enough to clear the harmonic balancer.
Fun stuff. Couldn't get my wires out from under the steering pump even after loosening the pump. Ripped my wires. Replacing them anyway I guess.
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Old 05-12-2019, 01:08 PM
  #39  
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I can read you have a lot of determination to fix this GS. Well you will gain valuable experience from this even if frustrating. Just wanted to recommend the dialectic grease again as it should help your situation. Dialectic grease is non-conductive and it's main advantage to help plug wire boots slip on and off. Until recently I had thought it was for the terminals also but no it would reduce electrical conduction if used there. I admit I was surprised I had misused it for so many years and neglected to use it when it could have helped the most.

Thx again for sharing this as opti's can become so confusing and disguise other problems.
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Old 05-15-2019, 09:17 PM
  #40  
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So I pulled the Optispark out and checked it on my bench. I ran a cordless drill on the shaft and mounted the opti in a wood vice so I wouldn't damage the casting. First off the bearing is shot! So much play in it and the noises coming from the bearing when rotated aren't good.
Also the disk is warped and hitting my optical sensor. The disk does not sit in the middle of the optical sensor it is close to the bottom half and is hitting the sensor when rotating. I hooked my dual trace oscilloscope to it as well as a regulated power supply set to 5VDC. The 360 slotted track signal (outside sensor output looked reasonable) but the individual cylinders (inside track) looked inconsistent. Looking at the waveform the frequency was changing/shifting even though the drill speed was fairly consistent. I then upped the regulated power supply voltage to see if it would smooth out the waveform but it didn't have much effect.
My optispark is junk, and I am not going to get it rebuilt or replace it with another inferior optispark.
I am taking a chance and going full tilt with the Torqhead 24x setup. Hopefully this will be a reliable replacement, it sure looks like Paul did his homework on this retrofit. He also seems to be a decent guy, he answered all my questions (some were probably stupid) on the phone.
Wish me luck! I am also going to have to figure out how to use HP Tuner Suite to tune my car (I ordered that in the Torqhead package as it was cheaper than ordering it from HP Tuner directly). I do have the JET DTS so I got my feet wet with programming already but HP Tuners is a different animal. I will be putting the JET DTS up for sale at some point after a successful install of the Torqhead 24x setup.
I'll keep everyone posted on the install with pictures and an honest report of any issues.
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