My 93 z28 4A had the opti (GM non vented), coil, plugs & wires replaced 3,000 miles ago. The car ran fine until I had to use on on some very short runs from cold.
The car is now very reluctant to start but can eventually be coaxed into life on about 4 cylinders, gradually creeping up to 8 with a lot of black smoke from the tail pipes (unburnt gas I presume).
The car will then run fine for about 20 miles and will then start stalling at idle and missing. It seems like the opti again but can they fail this quickly? The opti hasn't got wet or any oil on it
Any cheaper ideas than replacing the opti again would be welcome. Help please as I'm beginning to think that the LT-1 is a pos!

 

Black smoke usually means oil in the cylinders. This would be a bad piston ring but I doubt this would occur on more than one cylinder in such a short time. Does the exhaust have a rich gas smell to it?

Things to check are the IAC, coil, wires, and the opti itself. When you reattached the plug wires were you sure to get the firing order correct?

Also, how do you know it's only working on four cylinders?

 

from everything i've seen/heard black smoke is running rich not burning oil. oil makes the smoke blue so i don't think you have to worry about that. the easiest way to tell whats wrong with your car is to throw it on a scanner rather then guess and replace the wrong and expensive part.