I know I'm completely new to this forum, but I was recommended here by someone as a better possibility in getting more help with my issue.

Hopefully someone will be able to make heads from tails on this for me. I've got my LT1 that has run fine for the last 2 years in my Trans Am, and now as soon as it hits closed loop, she just dies like I turn the key off. She will immediately start back up with the turn of the key, but unless I'm driving down the road, she will not stay running. A little back story...

I've had my 88 Trans Am now for I believe 5 years, two years in the making, one year with the crappy 305/t5 combo, and the last 2 years in full LT1/T56 trim. I had replaced 99% of sensors in my rebuild of the motor before dropping it in, but did not replace the Opti. Half way through the first year in LT1 trim, I got the deadly Opti codes, which prompted me to get a reman lifetime unit from advance auto, and I just happened to have an MSD opti cap laying around, not that it's relavant, but I used it. I left the unit without a rotor and sealed it really well with RTV sealant. From there I installed the LTCC unit along with 8 coils and made my own wires to the plugs. Everything ran beautifully and so much smoother IMO.

That brings us to the end of the summer last year, day I was going to be taking this vehicle off the road, she starts to just cut out. Now I'm doing 60 mph, so the vehicle would just lose all power, lose a little speed, and then 2 seconds later kick back on full strength and drive off again (because I was in gear and it would "pop start"). I managed to get the vehicle to storage where she spent the winter. Got her out recently, brought her back to my house, and got the same little cut out on the way but shut it down and left it till now that I finally have free time to figure this thing out.

I have attatched the data log I just took, and from the looks of my amateur eyes, I cannot see anything strikingly out of place. As far as programing, I haven't changed much except tire size, and disabiling VATS and any automatic transmission codes so they would stop popping up with my T56 installed and so it would idle correctly. I'm not running A/C, as far as the LTCC unit, she is working perfectly, as soon as it starts up, solid green light from the unit and the red error indicator light does not come on, which means it's getting signal from the OPTI, and the only time the light goes off is when the engine just cuts off. I have watched it on my laptop, and with free scan, and the second the light for open loop goes out, so does the engine. Need help, let me know what I can do. I've got full diagnostic tools, sensor tester, fuel pressure tester, ect ect, I just don't know where to go first with no ICM to blame it on. And for what it may help, I'm running OBD1 on this.

Could not attach the file here, so here at this link the zip file is hosted :
Switch to open loop and LT1 dies - Third Generation F-Body Message Boards. Any and all help would be greatly appreciated, and will try any idea at this point. Also, if there is a more thorough way to collect data, I'd be interested to know how and do just that. Thanks in advance!

 

Went to 3rd Gen, tried to download, wouldn't let me because I am not a member of 3rd Gen.

You need to upload to an ftp site where anyone can download, not just members of 3rd Gen.

 

If it runs good in open loop and then dies like you say when it goes to closed loop, it's not an opti issue. The opti issue won't care what temperature the engine is at. Are they the original o2 sensors? They do go bad over time. A temporary fix would be to simply unplug the o2's. Most higher end fuel injection systems recommend running in open loop anyways, so the o2's can't suddenly adjust fuel trims whenever they see a spike. If you ever watch a oxygen sensor reading with a really fast gauge it will bounce around a LOT. The exhaust doesn't hit the sensor in a uniform manner.

 

The O2 sensor "bounce around a lot" in closed loop because the PCM is intentionally toggling the A/F ratio back and forth from slightly above 14.7:1 to slightly below 14.7:1. That is required for the monolithic cat(s) used in the 4th Gens to store O2 from the NOx reduction reaction when the mixture is slightly lean, and use it to oxidize the CO and HC when it is slightly rich. That's why the O2 mV's swing back and forth between 0xx and 9xx mV's. It's not "spikes" or the exhaust not hitting the sensor in a uniform manner.

 

Have you ever watched a carbed engine on a dyno with o2 sensors in the exhaust? It bounces around a lot and it has NOTHING to do with adjusting the fuel mixture. It's way worse when they do it with individual sensors in each tube.

 

I'm explaining why a fuel injected engine, set up to operate in closed loop, has wildy swining O2 mV's. I doubt that you would see anywhere near the stability with a carbureted application, but no I've never watched a carbed engine on a narrow-band O2 sensor.