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I took my ‘Vette for an extended run before Christmas, ran fine. Went on some errands yesterday and everything was fine until I made my first stop. I came out started it up and she ran real rough. Pulled out onto the highway and the fun started. The car pinged heavily and backfired through the mufflers whenever I stepped on the gas. Had to pop it into neutral (automatic) and keep it revved at a stoplight to prevent stalling. I parked it after about a 5 minute drive went into the store, came out started up, revved it a couple of times and drove home. The drive home was smooth sailing as if nothing had happened. Is this what happens when the Opti Spark is getting ready to kick the bucket?
Read the codes.I have a similar issue and I have narrowed mine down to a possible bad ECM/PCM and have it with a local tech running diagnostics on it now. I am getting code #36 (Faulty High Resolution or Extra Low Resolution Pulse) and code #42 (Ignition Control Circuit Shorted or Grounded). My car is a '94 and the Opti was at some point upgraded to the MSD "Vented" type. I had a local tech check the voltages thru the harness and read the codes while running with an older Snap On OBD-I reader. Opti voltage checked out on mine until the underhood temp reached a point that the ECM started breaking down. As I said I have my ECM being diagnosed professionally as I write this. Stay tuned for results.
If you tach reads zero or bounces around when the misfiring happens, you may be having an opti sensor failure problem. Otherwise, I doubt the opti is at fault here.
Last edited by MatthewMiller; Jan 7, 2019 at 09:06 AM.
My car's a 1994 base model, I bought it last March from a local used-car dealer with a clean Carfax sheet. The only history I have is limited to this sheet. According to it, one of the heads was replaced but no wrecks or other repairs I know of. I did not see the tach bouncing around when this happened and have not read the codes yet. I'm not too far from the Greenville SC area but have not found a Corvette specialist yet other than my local Chevy dealer. Do I need an OBD-1 reader or can I read the codes using a jumper wire and counting the flashes? I looked at my Opti and I believe it to be original and do not see a vent hose.
Newtomevette,
Chances are that even if the Opti isn't the main problem in this case, it will become a problem somewhere down the road, being non-vented. If I were you, I would plan on converting it to a vented style as soon as practical. The cap and rotor are probably in rough shape due to the ozone and moisture from the lack of a vent. I converted a 94 Opti to add the venting, and it wasn't too difficult. The hose harnesses are now available again in reproduction, although a bit pricey (~$85, IIRC). If you pull your Opti off, check to make sure it has the Mitsubishi sensor. It so, rebuild it , don't replace it. Good Luck!
Coil and ICM are cheap enough if you have high mileage to swap out and see if it fixes the issue. But before that I would recommend getting it up to operating temp at night and in the dark open the hood, disable the lights inside the hood. Look for arcing around the plug wires. Some people even will take a spray bottle and mist a little water onto the wires. Use you hand to operate the throttle if you don't see any at idle.
In our area, North Hills Automotive is the go to. That's where Kevin Whittaker Chevrolet sends their older vettes.
What you describe could be other things. My left fuel rail went into open loop on me and acted much like that. Until I found my lost 14mm wrench sitting across the wires behind the intake.
As mentioned, check the ICM and Coil. If you take them to Autozone, they can test 'em.
Here's a link to the GM tech video on ABITS (optispark) operation and diagnostics. Unfortunately, yours would be the un-vented Optispark which is more apt to fail. It can be retrofitted with ventilation.
Thanks Renfield, and others who have chimed in to help. Work has been crazy (and my daily’s in the shop) so I will be able to dive into things on Saturday. I’ll report back on what I find. A vent kit, sounds like a good idea, I’ll check some of the vendors to see what’s out there. I had been thinking about the cap and rotor, and the OEM plug wires have probably seen better days too.
I used a paperclip to jump across terminal 4 and 12 on the ALDL to pull the diagnostic codes on my ‘94 base model. Both were preceded by H (which the FSM lists as historical) which makes sense as this initially happened around May of last year, but I ignored it as some sort of hiccup and it hadn’t happened again. Until now. Evidently code 13 is a problem with the bank 1 heated oxygen sensor #1 and code 41 is an open ignition control circuit. Time to dig a little deeper.
I used a paperclip to jump across terminal 4 and 12 on the ALDL to pull the diagnostic codes on my ‘94 base model. Both were preceded by H (which the FSM lists as historical) which makes sense as this initially happened around May of last year, but I ignored it as some sort of hiccup and it hadn’t happened again. Until now. Evidently code 13 is a problem with the bank 1 heated oxygen sensor #1 and code 41 is an open ignition control circuit. Time to dig a little deeper.
I used a paperclip to jump across terminal 4 and 12 on the ALDL to pull the diagnostic codes on my ‘94 base model. Both were preceded by H (which the FSM lists as historical) which makes sense as this initially happened around May of last year, but I ignored it as some sort of hiccup and it hadn’t happened again. Until now. Evidently code 13 is a problem with the bank 1 heated oxygen sensor #1 and code 41 is an open ignition control circuit. Time to dig a little deeper.
IME, the O2 sensors don't cause problems from a cold start because the car is in open-loop mode, ignoring most of the sensors' information. Once it reaches near operating temps, the ECM will start paying attention to the O2 sensor, and that will make it run badly if the sensor is bad. You can data-log with something like EEHack to see if the signal from the sensor looks bad. But honestly, on a car that's ~25 years old I'd probably just replace both sensors. They do get old and go bad over time and with use, and the replacements aren't very expensive. That code 41 sounds like some kind of short that may be heat related, either internally in the ICM or coil, or in the wiring to/from it.
I purchased a new coil (Autozone lifetime warranty) had the ignition module checked, passed 3 times. Need to figure out whether I have a splined Optispark or pinned type, so I can order a vented unit from the Optidoctor. Going to order new ThunderVolt direct fit Taylor 8.2 plug wires from Summit Racing. Naturally, that will mean new/modified wire looms. Not particularly thrilled that they’re red, but have had excellent performance from Taylor wires in the past. When I removed the OEM coil wire it seemed to come off way to easy. New oxygen sensors on the way also as they're probably original to the car.
I purchased a new coil (Autozone lifetime warranty) had the ignition module checked, passed 3 times. Need to figure out whether I have a splined Optispark or pinned type, so I can order a vented unit from the Optidoctor. Going to order new ThunderVolt direct fit Taylor 8.2 plug wires from Summit Racing. Naturally, that will mean new/modified wire looms. Not particularly thrilled that they’re red, but have had excellent performance from Taylor wires in the past. When I removed the OEM coil wire it seemed to come off way to easy. New oxygen sensors on the way also as they're probably original to the car.
The red ones add about 15 hp.
How many miles on your '94? Apologies if I missed that nugget.
Don't forget the timing cover gaskets. Are you also doing hoses, coolant, water pump, knock sensors?
Post some pics if you get a chance. I sold my '95 in Polo Green with Tan interior a couple years back, but I never tire of seeing them.
That code 41 sounds like some kind of short that may be heat related, either internally in the ICM or coil, or in the wiring to/from it.
Heat might have been a problem, here's all I could see of the ICM when I began removal.
Here's a picture of it on my bench after extraction. The cooling vanes of the heat sink are pretty clogged with grease.
Another view.
After cleanup.
So it seems that with all the grease the ICM might have been getting a wee bit warm. I had it tested and it passed, so I'll hold off on replacing it for now. Lifetime warranty ICM $85 at Autozone, my unit looks to be original, I have no doubt the replacements come from the "pacific rim" so I will give the OEM module a shot now that its cleaned off. New coil/wires are next.
How many miles on your '94? Apologies if I missed that nugget. Don't forget the timing cover gaskets. Are you also doing hoses, coolant, water pump, knock sensors?
When I bought it it had 63k on the odometer (only about 65k now), one of the reasons I brought it home. Judging from the grease on the heat sink, new gaskets are probably a good idea. I just bought a DeWitt radiator so I will also be doing the hoses when springtime comes. Once I figure out if I need spline or pin optispark I will upgrade to vented unit with the new water pump. What a can of worms!
So it seems that with all the grease the ICM might have been getting a wee bit warm. I had it tested and it passed, so I'll hold off on replacing it for now. Lifetime warranty ICM $85 at Autozone, my unit looks to be original, I have no doubt the replacements come from the "pacific rim" so I will give the OEM module a shot now that its cleaned off. New coil/wires are next.
Interesting with all that crud on the heatsink. That definitely would be a problem. Just making sure: did you use thermal paste between the ICM and heatsink when you reassembled everything? That's important to get the heat to transfer from the ICM to the sink.
One thing I always found weird is that GM mounted a heat sensitive device to the engine block. This likely saved them .0004 seconds on the line, but I doubt that it is ideal. Bean counters usually win in a game of will with engineers.
A while back someone here showed it remounted to the radiator shroud. I've been too lazy to do it, but I really like the idea.
My FLIR camera attachment shows the issue pretty clearly.
View straight down from above ICM. Point reading is on heat sink. ICM reads about 40 degrees less.
Relocating the ICM sounds like a job that’s worth the effort. If anybody’s done that, or has a link with ideas, please post!
Thanks,
John
Extending the harness is the only real difficulty. It has been a long, long time since I saw the post, and it wasn't a step-by-step. He just went straight forward with the ICM and mounted it heat sink and all at the most convenient spot on the radiator shroud. Some liked his idea, others picked at it. The usual stuff. No one ever posted a follow up with their version of his idea.
I'll dig back through my replies for you John, but that's basically it.
I'd bet the FLIR would show a whole different picture.
Okay. After trying every search variant possible inside corvetteforum, one google search got it.
My guess is that the front of the cylinder head is not nearly as hot as the back of the ICM in operation. That's just a guess, since I've never measured either, but it would mean that even the head is a sink for the ICM. That said, on mine the previous owner just mounted the ICM with some washers as a standoff so it wouldn't contact the head, and it does very well. You don't have to mess with the harness at all.
Opti Spark going bad?
