Hi Folks,

Been chasing an intermittent mis/vibration for the past two years, off and on. No codes at all. Did some internet searches last night, one recommendation was to use a timing light (i have never used a timing light on this car - I figured, why??). First tried it on 5 and 7 to see if there was the classic inductive crossfire. Definitely on 7, every once and a while there is a "double spark" (you can hear it in the timing light and see it on the balancer). Nothing abnormal on 1,3, or 5. Separated 5 and 7 wires a bit more. Checked #4, also an occasional double hit, also on 6, but not on 2, which was normal. Couldn't get it on 8 as the motor was getting too hot at this point. Wires were changed when I bought the car two years ago (Taylor Pro), but still had same symptoms with stock wires. Plugs are NGK TR55s, which are all good and look normal (pulled them last night). I have never suspected the opti since it always starts right up, even after sitting, never noticed the classic opti symptoms, seems to accelerate fine, etc. Car has 87k on it, can't tell if the opti is original or not (looks original). If this were an HEI, I would suspect a cap and rotor issue. I'm a total newbie on the optispark; could the cap be carbon tracked and crossfiring (or something along those lines)? Any help here would be greatly appreciated. Obviously, I have avoided the opti since I don't want to pull the water pump, but it may be getting down to that.

Thanks!!

 

Did it run this was from day one when you bought it?

 

Yes, I noticed a very slight idle roughness, but smooths out off idle. FYI, this is my first Vette, so I can't compare it to anything (my last smallblock was a 383 with a 222/234 duration at .050 cam, in my 77 Camaro 35 years ago; this car idles like glass compared to that). There is a bit of roughness/miss/vibration around 2000 rpm or so, under no load conditions (in park or neutral), which I have chased on and off since I bought the car. Full throttle acceleration seems fine, doesn't break up, all the way up to the redline, and it always starts incredibly quick, so that's why I have avoided fiddling with the optispark. But like I said, I'm a total Optispark newbie, so maybe there still could be an issue? FYI, I went through the fuel system two years ago to rule out injectors/fuel pressure regulator, etc. and they all checked out fine. Same with vacuum leaks, O2 sensors, fuel trims, etc. I don't notice any roughness or misfiring when driving, just if you have the hood open with the engine running, you can feel something isn't quite right.

 

Optispark issues usually set a high or low res code. Your standard ignition control module should be tested but rule out simple stuff like wires, wires to correct cylinders, fuel injectors are common issue with miles......

 

Is there a specific test for an ignition control module on an LT1?

 

Optispark distributors have a cap and rotor too...

 

Oh, I know, it's just that you can't pop the cap off and check the inside of the cap and rotor in 2 minutes like a Gen 1 smallblock, otherwise that would have been done when I bought the car. I've been trying not to over-react and play into the "oh, it's an LT1, the Opti Spark is the root of all evil" nonsense, since it starts up and runs generally great, no low or high res codes, it just has what feels like a occasional intermittent miss, and now that I finally broke out my timing light and noticed some crossfiring/double sparking, it's leading me down the wires back to the cap and rotor maybe on the Opti. Have I been purposely avoiding messing with the Opti - yep!

Before I go that route, however, how susceptible are Taylor Spiro Pro wires to crossfiring? I am using the factory wire looms, with wires in the factory locations, nothing tie-wrapped together. I have looked at the engine in the dark previously, and misted water over the wires, looked with an inspection mirror, etc, etc, no evidence that I could see of any external sparking to ground or between wires. I had the same wires years ago on my HEI 383 smallblock and never had an issue. Are spiral wound wires more likely to crossfire inductively than the stock wires?

Thanks!