Ok, ill try to make this short at first, but im sure the questions will be plentiful.

This car was a clean low mileage 2 owner car (56k) when I got it. After about a year of off and on driving it spun the #1 rod bearing unexpectedly. Full of oil, 40 PSI at idle. Only thing the forum and the machine shop could figure was it was a G-force related oil starvation issue so I bought a Canton road race pan and rebuilt the motor.

My dyno/tuner guy confirmed (before I decided how wild to build the motor) he could tune this one year only ECM and away I went.

Aside from not being able to read/write to the ECM at first (we sent it to his computer tech guy who has old GM equipment and he factory re-flashed it) and all was well after, at first.

A few months ago a weird misfire would occasionally happen around 6500-7200 rpm. It was only milliseconds, like a rev limiter interrupt, and only random. So I had waited to add the Nitrous kit until we could figure it out. Well the other day, right around the same RPM (racing my wifes C6 Z06) the car shut down completely. Letting the clutch back out for a split second to attempt engine brake starting it and all it does is pop and backfire. Thinking I sucked a valve we pulled both valve covers and found nothing wrong. Compression test is great cylinder to cylinder and normal oil pressure. Put new plugs in it (got rid of the iridiums for copper for the Nitrous, plugs are text book tan porcelain).

Opti-spark was fresh build from Opti doctor, Mitsu sensor.
ECM reports -14 degrees timing advance (cheap scanner), but all other stuff looks normal during cranking.
"Boofs" out the exhaust with occasional backfire only, no attempt to light.

Now a little back ground to muddy the waters.

When we fired the car to drive it to where we were going to remove and rebuild the motor last year, the battery was dead from sitting. We put a high power battery charger on it to start it but something was wrong with the charger. The interior lights and all the relays in the car started pulsing several times rapidly, so I removed it and just put a new battery in the car. At that time the proximity keyless system (OEM) and radio (factory) stopped working. Its been that way since as I hadn't got around to fixing (CCM?). A few months ago the A/C system started turning on by itself when you put the key into the ignition. It turns off if you tell it, but if you cycle the key, 9 times out of 10 it turns the A/C back on again. Even had one VATS light a could weeks ago, but the car started fine. I also started receiving "random misfire" engine codes a few months back, but they are far between. Ive read the threads about this being common and often without symptoms (like my case). This car has a big cam and aluminum flywheel so I kind of expected this part. I also considered removing the crank sensor and reluctor, when I re-built the motor but decided not to bother even though the ATI Damper had the spacer.

I know a lot of people are going to say, "opti" as thats par for the course, but like I said its only 1 year and 5 thousand miles old, plus I converted to electric water pump and plugged the cover, not to mention this is already teh Gen 2 with Mitsu sensor. On top of that my symptoms arent consistent with a bad opti for the most part. I have read a few peoples symptoms of a bad ICM and/or coil, however. These are both originals and I opted against the MSD stuff as I heard they are hard on OPTI's and its a lot harder to change the OPTI, lol. Since im only planning on running my 50-175 hp plate system I planned to only go hotter on the ignition if needed (again to preserve the OPTI).

I have a GM service manual but their way of organizing stuff is horrendous. I could only find the removal procedures for each (OPTI, CCM and ICM) but not any Diag? Is there any easy way to check each with a DVOM at the harness plugs? I have a FLUKE 87V.

 

If you never changed the ICM and coil, if that's what you mean by 'original', then do it.

The A/C coming on randomly is probably the A/C Programmer having gotten shot. If you're lucky it's just the five capacitors that are shot. Probably due to age, since they all die eventually.

Are you positive the PKE system does not work at all? Or is it possible that it was merely completely reset and needs to be re-programmed? Have you also made sure your keyfob's battery is both in good working order, and that the clip that holds the battery down isn't broken?

The radio, can't help you there; haven't run into a failed one yet, just bad backlight bulbs. But if it's Bose Gold, there are two separate units. The thing you interact with is just a control panel. The actual 'radio' hardware is in a control box inside the storage compartment area behind the passenger seat.

If the CCM were shot, your dash would not work at all, and you would not be able to pull codes using the paperclip method: https://tech.corvettecentral.com/201...trouble-codes/

 

Ok I cant say I checked the remote for the keyless, since it happened at the same time as the battery. Ill go through that since its easy. Yes the ICM and coil are the originals that came when the car left the factory. I ordered a new Delco from the dealer, I planned to test the coil since thats usually a very simple ohm test but hadnt decided on which replacement. If its in fact true that a hotter coil is really hard on the opti ill source a stock one. That really sucks if true, as like I said this is a nitrous car, albeit a small system, but you can never have too much spark on the bottle.

The A/C unit has been flashing an error code for a long time but if I recall correctly its the common, "mode door" one. But the capacitors you mention are located inside the head unit and not somewhere else correct?

Thanks!

 

If there was an actual error code flashed, then it's probably the blend door motor as you suspect. But the A/C coming on randomly for no reason and/or the display reading "---" is definitely the programmer.

And unfortunately, no, that's not the head unit. The head unit, much like the 'radio', is not the actual computer. It's just a dumb control panel. The A/C Programmer is located in the driver's footwell, bolted with a single 7mm bolt to the transmission tunnel. It's a PITA and a half to get out. I can just barely do it, and I'm a fairly skinny guy with deft fingers. If you do decide to try to remove it at some point, I would recommend removing the PCB from the plastic case rather than trying to remove the vacuum block if you can help it. There are four 7mm screws that hold it in--two hold the vacuum solenoid board to the PCB, and two hold the PCB to the case. Pull the four vacuum solenoid hoses off the vacuum solenoids (they're held together as a unit, but will easily slip off), then remove the two boards and just leave the case dangling.

If you have to remove the vacuum block you'll be in there for an hour cursing up a storm at why GM decided to put a one-way lock washer on there to hold it on.

Good luck!

 

New ICM actually had a negative effect. No longer any boofs from the exhaust. It just cranks over with out any activity besides one single event where there was a minor back fire in the intake side. Pulled the coil wire to check for spark and its sparking nice and hot and jumped an inch gap to the valve cover bolt. Sounds like its indeed the ECM or Opti at this point. ECM reports -14 degrees timing to my buddies SNAP ON scanner (same as my cheapo scanner). Never looked at cranking timing before when the car ran fine, is that normal?

 

It's very, very rare for a PCM on the 1994-1996 Corvette to just 'fail'. They don't just 'fail'. Especially if you can still communicate with it via a scanner. Unless you have personally connected to it and mucked around with the tune via JET DST, I wouldn't lay the blame on the PCM.

As to the -14 degrees, that depends on what the scanner is claiming. If that's 14 degrees advanced, then yes, that's correct. If it's claiming the spark is retarded 14 degrees, that cannot be the case. The absolute minimum as programmed is 0 degrees. So that would be either an instrumentation error on the part of both your scantools, or...well, no clue. It's not possible to run 14 degrees retarded on an opti. 0 is as low as it goes.

 

Have you physically checked the ignition timing? IOW, used a timing light? It sounds an awful lot like the timing is way off. Did you check the opti rotor? They're known to fall off on occasion.

 

This car is 400hp at the rear wheels, the tune is heavily modified.

2 different scanner reported the same -14 degrees.

 

I don't have a pointer installed for the ATI balancer. I plan to pull the water pump and inspect the OPTI next as i went for the low hanging fruit first, lol.

 

Ask whoever tuned the car if they changed the “minimum spark advance” table. I’m not sure if that table actually allows negative numbers, but stock it’s set to all 0.

Best of luck with the opti-pulling. That’s always a fun job.

 

Just an FYI, when I was trying to diagnose a random misfire on my LT4 I replaced the ICM. I had one hell of a time finding a new one that would not set a code. Delphi was a no go as well as the AC Delco. The engine would start and run but it would set a code almost immediately.
The Borg Warner CBE122 worked correctly for me.
Although the random misfire turned out to be the Opti in my case not the ICM.

 

Also do you have fuel pressure? Maybe the fuel pump went bad?

 

3 month old Walbro 255, Hadn't checked, but with the flames out the exhaust, it's not consistent with that...

 

Timing.

 

Agreed.

Hopefully I'll have time to get it apart this week.

 

Finally got around to pulling the Opti. The rotor screws fell out. FML

1 year and 5k miles

 

Bam.


Good job getting to the bottom of it.

 

nailed it!