RebelRider

Yes, this is another lt1 no fire situation.
I'm currently working on my 95' C4 LT-1. It's all stock under the hood.
The only repairs I have made to the car, have been a brand new gm performance opti I installed about 2k miles ago along with 2 new upstream 02s.I also bypassed the vats system with resisters. As most of you will know, if you are having vats problems on the cars it will not even let the engine spin over.

So here leads to my problem. Recently whenever the car gets nice and warm, it will miss from time to time at idle. It will also hesitate while barely on the throttle, but otherwise run fine all day. Well as I was slowing down to pull I to my drive a few days ago, (roads a little rough) I the car just quit. The car has no codes. After the wife and I pushed about 900ft to my shop, I discovered I have no spark from the coil, going to the opti. I swapped out the coil, with no success. Still no spark. I really don't want to start parts changing but I'm a little stuck on what to do next. I brought my icm to have it tested where at first it failed but then it passed 4 times. They claimed they had wires touching when it failed. Am I supposed to get a pulse reading to the icu while the engine is turning over? I'm getting power to it but no pulse. Do I have another opti problem? Something more?
Thanks in advance....

aminnich

Did they cycle the ICM enough times to get it hot? (about 8 times). It sounds more like an opti problem, but it should be throwing a code if it is bad enough to shut down the engine.

RebelRider

Thanks for the quick response. They mentioned that it wasn't even warming up while they tested it. It took every person in the store to figure out how to test it. I might try a different parts house. No codes at all at this point. That's what has me stumped. It was no sputter, it just quit on me...

vetteoz

http://www.mainstreamtopics.com/foru...rt-diagnostic/

RebelRider

^^great write up. Thanks
I'm going through the steps now.....

pcolt94

You may test components all day but may never find anything concrete.

It does have opti symptoms and lots of work to change one. It also have indications of the ICM might be bad under actual operating conditions.

I think it might be worthwhile to replace the ICM. You have a chance it might be defective. Buy a good GM unit. Foreign ones people have problems with. And even if it does not fix the problem, it's not bad to have a spare ICM. It's a relatively quick item to change.

If you're not fixed after that, then you would have a higher confidence factor the opti is probably the problem.

RebelRider

I'm suspecting the icm myself considering I recently changed the opti and coil. I just hate to take a $100+ shot in the dark. Yes, I know I'm a cheap a**....
I also forgot to mention that I am getting good fuel pressure with no bleed down....

jaa1992

ICM or the cable from opti to ECM

RebelRider

Are you referring to the one that goes from the opti, and then plugs in underneath the Injector cover on the intake?

RebelRider

Small update.....
I removed the icm and brought it to another parts house today. They tested it several times so that it would get warm. It passed with flying colors. That makes two different parts stores that have tested it. Passed every time.....

bjm206

The ECU (computer) also uses the timing signal from the Opti to fire the injectors. If the injectors are also not firing while cranking that would point to the opti (or wiring, ECU, etc.) as the source of the problem. A noid lights can be use to see if the injectors fire while cranking.

RebelRider

I just went through all of the test. I'm not get a pulsing light at the injectors. I'm getting the right volts and ground to the opti like I'm supposed to also. Looks more and more like the opti......
I'm getting power to the injectors, just not pulsing.....

aminnich

Power is constant thru fuses to each bank, pulsing is when the PCM provides a ground. I would next check the short harness from the opti to the harness under the pass side fuel rail cover.

RebelRider

I replaced this harness when I swapped the opti. I also checked both ends of the opti harness and it checked out ok.....
It's going to really irritate me if it's another opti. I did like everyone told me and paid the extra for a good one, just to have it go bad in a couple thousand miles. Oh well, it's got to be done......
Any other suggestions? Should I just go ahead and start tearing into the opti?

RebelRider

I also meant to clarify, I am checking for a pulsing GROUND at the injectors with no success....

aminnich

If you get to the point of replacing the opti I have a take off Gen II AC/Delco opti that I am willing to sell. If you need it send me a PM and we can work out any details.

But first I wonder if it is related to the VATS bypass?

RebelRider

Ok, thanks. I will let you know.
I also thought of the vats problem. When I first had issues with the vats, it would not even spin the engine over. The vats bypass isnt complicated at all. Its just a series of resistors taking the place of the one in the key. So I'm about 99.2% sure it's not the issue. Plus, I figured even if it did it would cut power to the opti. ( which I do have power )
The vats will cut off fuel after about 2 seconds of running, and not the spark....

aminnich

You are right there. It would still have spark, but not fuel. It was just a thought, otherwise I am about out of ideas.

RebelRider

You and me both. Lol
Thanks for your help.....

bjm206

If you hook a voltmeter (or better yet oscilloscope) into the back of the opti connector shell from distributor reference low to high res or distributor reference low to low res without dis-connecting it from the rest of the harness you can see if the opti is sending signals.

With the key in the run position turn the engine over slowly by hand. The high res signal will be rapidly pulsing as the crank moves and the low res will see a pulse of variable duration every 90 degrees of crank rotation. If you see these pulse trains then the opti is very likely working and the problem lies elsewhere (ecu, wiring etc.).

Be careful as shorting across pins could cause electrical carnage.