I've been trying to diagnose some misfire issues. I've noticed that just about each cylinder doesn't have consistent spark when hooking a timing light up to the plug wire. I figured I'd troubleshoot the timing light to make sure it wasn't the problem by pulling the plug boot away from the distributor terminal and watching the spark jump, and voilà! All of a sudden the timing light and spark jumping the gap are consistent. no misses in spark.

I used a second timing light to confirm my findings. And then also hooked up my laptop to collect RPM, and Wideband O2 metrics. (see below)

This is off a 1988 Pontiac Trans Am 350 (L98) that's been sitting for 10+ years. The following parts have been replaced with OEM or better quality.

Spark Plugs, Wires, Cap, Rotor, Ignition Coil, Fuel pump, Fuel filter, Mass Air Flow, TPS, Fuel Injectors, O2 (wideband with standard output)

Timing has been set to factory 6* BTDC

In the below plot, the yellow highlighted sections are when I lifted the boot off the terminal and allowed the spark to jump. while still not running great, there's an immediate difference in AFR, and a stabilizing and slight increase of engine RPMs. (pulling the boot completely off has more or less the same affect as having it connected, as that cylinder seems to spend most of it's time misfiring with the boot connected.)

Has anyone seen this? Or know what could be the cause? The next course of action is replacing the distributor and ignition module. But I'm tiring of shooting the parts cannon without knowing what the cause could be. I would think HEI destributor would either work or it wouldn't.

TL;DR: engine has more consistent spark and less misfiring when pulling plug boot away from distributor terminal and allowing spark to jump.

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  • 2
    The thing I think I'd be checking is to see if you might have cracked a porcelain of the spark plug in the cylinder you're testing. It could be finding it's way to ground. Also, what gap did you use on the plugs? I know originally the HEI was supposed to be gapped something like 0.060", but they later came out and stated to put them at 0.045" as they were burning out the ignition or something with the large gap. Also, you should try moving the spark plug and wire from one cylinder to another and see if it follows the pair or not. Nov 23, 2021 at 2:12
  • Is the coil correct for the ignition system? For example it is not a 6v coil that is meant to have a ballast resistor to run on a 12v system. If so, it may be producing too high an output voltage which may be causing arcing where it shouldn’t.
    – HandyHowie
    Nov 23, 2021 at 11:37
  • @HandyHowie - With the HEI system, the coil is bolted into the cap. I'm pretty sure they are "one size fits all" (pretty sure V6's could use the same as V8's). There's no ballast resister on these to lessen down the voltage while running. Nov 23, 2021 at 16:35


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