The engine in question is a bone stock 127k mile old LT1. The motor was running perfectly fine but a little heavy on fuel consumption, so I took out the rusty, old as heck Autolite AP 106 plugs and plug wires that came with the car and installed some brand new NGK TR55 V-power plugs (part # 3651) along with new Duralast replacement plug wires. Now the motor has developed a violent stumble or misfire under load, shaking the whole car like crazy. It's not throwing any codes, nor is the SES light coming on when this happens. I don't even have to lug the engine - accelerating out of a turn in 2nd gear at 1500 rpm will cause the stumble. As the owner's manual recommends, I set the gap at 0.050".

Ironically, my old LT1 car had the same problem until I switched to the same part # NGK v-power plugs and MSD wires and that fixed it.

What could possibly be causing this? I have a feeling I might have overtorqued the plugs, possibly cracking the ceramic insulator. But wouldn't that result in a permanent misfire regardless of rpm or load?

 

Sounds like your problem is either one or more of the plugs or plug wires. I'd pick up a set of AC Delco plug wires and plugs and install them.

 

The wires may be on the wrong plugs. You may have not tightened them correctly. I had one that seemed tight but when i double checked them it was only partly on. The gap may be off on some or all.

 

+1 to double check the gap, and also that the plug wires are going to the correct cylinders and seated properly.

 

Check gap, wires at the plugs and opti, and speaking of the opti...........

 

Sounds like either the wrong gap, loose plug wire, arcing, or you have them wired in the wrong order.

Make sure they are all in the right order, and that they click onto the opti side and to the plug. Double check your work, sometimes little things cause big problems.