I have dealt with ICm issues for years. I have had COLD no start issues then I go in the house an hour later and it fires right up. Its the ICM dude

 

No, it's not a convertible.
The ICM and coil were replaced months ago as a first try at fixing this.

93Rubie pretty much hit the nail on the head. It turned out to be (95% sure, keeping fingers crossed)... The opti harness. I dug it out and routed it away from everything, basically hanging in the air away from everything else.
Car started up, I got under the hood and wiggled the harness, the engine died, wouldn't re-start, I wiggled harness again, cranked up without hesitation. Over the 2 days of the weekend I tried starting it several times, and it fired up everytime.

So now i'm just waiting for the new opti harness arriving, then i'm going to take it for a run and see how it goes. I'll update how things turn out at the weekend.

Sara Lou

 

This is a possibility. I recently replaced my opti harness. When I pulled the old one out, I found that some coolant and Lord knows what had leaked in there and started in at the old connectors. I cleaned that all out with some brake parts cleaner and a can of computer duster (compressed air) for drying. Also, if your harness doesn't come with an adapter for the other side, be prepared to get one. It will have to be spliced. This was my experience, the adapter was $40 at the Chevy dealer. This however did not cause an intermittant start problem for me.

 

Subscribed..

 

Harness arrived today.... Has connectors on both ends, so I assume it's gonna be 'plug & play', $24.95 shipped off fleabay.

Should get off work at lunchtime tomorrow, so hopefully will have it installed by end of day tomorrow.
I still have the tail lights out to install an antenna, so it will be one day through the week next week before I get a chance to drive it, also will be dependent on weather (which has been wet and miserable for the last couple of months).

Sara Lou

 

Be sure to check the connector that goes to the harness at the intake. that was the one that got me, make sure they will fit together. If they don't your options are to get the adapter and splice it to the back side, or to splice your old adapter to the new opti harness. I went with the splicing a new adapter as I wanted fresh connections on both sides.

 

Well got the new harness installed today.....


I'm really ready to unload the .45 into this thing now.

Back to a no-start/no spark today, again!!.

Sara Lou

 

Sounds like a lot of people are having the same issues. More than likely you too are having an issue with your security system. When you turn the key to the on posistion. Does you security light stay on steady or flash? If it stays on, you either need to swap out the ignition switch, or bypass the VAT system, which is easier. h

 

Just went through this. Hook up a fuel pressure gauge to check pressure

 

Just to make sure that all are on the same page,

The no-start is engine cranks but does not fire?

When it does fire, it runs well. No stall, no hesitation, no backfire?

You have proper fuel pressure at the rail?

The fuel pressure regulator is good?

During no start, you have no spark? At the coil? At the plugs?

When cranking, do you show oil pressure?

You have replaced, coil, icm, opti harness?

I would assume that you have checked all the grounds?

 

Yes, no start= cranks but no spark.

When it does fire, its slightly rough (guessing a tune-up needed), but no stumble, stalls when it looses spark. It fired today, and it backfired for a couple of seconds then cleared (first time its backfired).

40-42psi fuel pressure at rail.

haven't done any fuel system checking other than pressure at scheader(sp?) valve.

When in a 'no start' condition I have no spark at the plug, i'm testing on my own, so I have no one to crank the car while I test coil, but it has been replaced.

Never checked oil pressure while cranking, oil pressure is good when its running, oil is clean and free of contaminates, and showing full on dipstick.

Correct, replaced coil, ICM, and opti harness.

Checking grounds are on my "to do" list..

Problems i'm aware of that need to be addressed, but I don't think would cause the 'start/no start condition:
code 64, right bank O2 sensor bad.
Alternator bad, using fully charged battery and jump box.

I tried it again today, managed to get it to start, so I hooked up datamaster.
I have a new ODB1 USB cable, fresh install of Datamaster, and a fairly new laptop. But I am constantly getting a dropped connection when communicating with the ECM. It records data for a few seconds, then drops the connection, reconnects for a few seconds, then drops again. I've disabled the 'handshake' in datamaster, tested port communication (tests good), tried different port, but nothing changes.

I'm back to suspecting ECM or opti (previous owner claimed the opti had been recently replaced). Tho' I keep discounting opti due to the fact that it will still start now and again.

Any help at this stage would be very much appreciated before I put this thing out of its misery .

Sara Lou

 

Also forgot to add. The 'security light' flashes when key is in off position, and goes out when key is turned on.

Sara Lou

 

I'm no tech expert by any definition but if I may add my opinion.
It appears that you have isolated the failure to either the opti unit or the connector on the engine harness.
You performed a wiggle test on the opti harness and were able to get the failure consistently. You also indicate the opti was replaced. My gut tells me that there may be a problem with the connector on the opti or the unit itself may be a stinker.
Just a thought. Wish I were there to help.
I just purchased a 96 Collector Edition from a forum member and waiting for delivery. The more I read the more I worry about what I may have gotten myself into but that comes with my age (69 years) we tend to worry about things as we get older.
All my best.

 

I'm having the same problem with mine...

I am going to watch this thread closely. Does someone have a VATS disconnect diagram?

 

Backfire points to ECM or Opti. I've been having similar issues as well and mine just backfired the first time last week but then fired right up. Backfired the next day when I tried to start but didn't fire and timing sounded WAY off as it was trying, so I quit and have a new opti in route!

 

Check your grounds (intermittant electrical problems are often caused by loose grounds)

Replace the alternator.

Then see what it does.

 

I have another alternator I got from my sis-in-law when she replaced her's only to find out it was the battery that was bad. I intend to get it tested before install though.

Sorry for the intermittent replies, i'm also in the middle of renovating the house which is taking up most of my time. I don't get much time to work on the vette between working on the house and working full-time, and the weather hasn't been kind over these last few months.

I will update as and when I get chance to try all the different ideas being offered. I appreciate all your inputs.

Sara Lou

 

Seems like you narrowed it down to the harness, or a connector on the opti, coil, or ecm. Just curious, did the start/no start ever appear again between the time you thought you solved the problem and the time you replaced the harness? The fleabay harness could have added to the problem.

 

The car hasn't left the workshop for over a year because of this problem, I can't trust it to drive anywhere.
After I "thought" I'd found the problem, the car ran fine for the rest of the day, turn it off, crank it up, turn it off, crank it up, etc for the rest of the day.

One week later, the harness arrived, I tried the car before install, no start, after install no start. Once I got it to start, would crank up without problem for the rest of the day.

Hopefully soon, I should have a FSM so I can dig deeper into tracking the problem.

Sara Lou