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 don't know much about anything, and even less about high voltage ignition... but could it be that the extra gap is causing higher build up of spark energy? and that maybe there is something wrong with one of the ignition components? I noticed this a few days ago, and put new plugs and wires on, same problem. The cap\rotor\coil are all new as well, though all economy grade. Next up on my list I guess would be higher output coil and a new distributor. Though the motor is pretty much stock. (and had been sitting for 15 yrs... injectors were replaced a few months ago.)

 

I'm suspicious of anything that isn't oem grade or better, and I've had problems with cheap parts in the past. I'd recommend sending that junk to the bin and replace it with at least some AC Delco gear before getting busy with a distributor swap.

 

off to summit I go tomorrow! Looks like they have some MSD in stock

 

welp. the new MSD ignition coil didn't change anything. I guess my next step is replacing the distributor and module. I thought maybe that injector could be soaking the plugs, so I unplugged the fuel injector for the #1 cylinder (which seems to be one that's acting up more than the others) and no changes there either. This is an odd one.

 

Nothing is 'soaking" anything that would cause this.

I suspect if you used a different timing light, it might react dfferently.

 

I'm not ready to buy a new timing light just yet (though this one is old) as the engine is definitely running poorly. Connecting the light to the coil wire, you can feel the engine miss when there's periods of no spark. I'll see if I can find somewhere to borrow one, looks like the auto stores don't rent them.

Does the ECM have any control over ignition that could it could prevent spark from firing? (not saying that's what it's purposely doing, but if perhaps the ECM is going wonky, could it cause no spark occasionally?)

EDIT: one thing to clarify, with the timing light connected, pulling the plug wire away and allowing the spark to jump. The timing light is consistent then.

 

update...

I had to end up buying a new timing light as i broke mine closing the hood on it. I also put the original injectors back in, they don't have the best flow, as the car runs lean until closed loop kicks in, and the BLM is able to get it to stoich (according to Wideband O2)... Oddly enough the problem is still occurring. In fact it's a noticeable difference when you pull the plug boot away from the terminal and the spark jumps, you can hear the engine pick up a bit. I'm completely flummoxed at what's going on... cap, rotor, plugs, wires, coil are all brand new. compression is consistent across all cylinders. The only other thing I can think of is replacing the distributor\module.

 

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 missfiring with the boot connected.)

 

I'm not understanding this diagnostic procedure of messing with the boot. What happens when you replace the plug wire? Perhaps you have faulty new parts.

 

great question! Sorry I could have done a better job at explaining.

Engine is misfiring pretty badly, I was hoping I could connect a timing light to each plug wire, just to make sure each cylinder was at least getting spark. What I noticed was that just about all of cylinders that will only fire (according to the timing light) less than 50% of the time. I thought that was pretty odd, so instead of relying on the timing light, I wanted to pull each boot away from the terminal to see if I could actually see spark jumping. I just so happened to still have the timing light connected while doing that procedure, and noticed that when I pulled the boot away from the distributor terminal, all of a sudden the timing light would flash consistently, and (according to the chart) the engine actually runs better when you pull the boot away and force the spark to jump.

a google search pulls up similar post.

https://www.460ford.com/threads/spar...m-plug.158680/
and
https://www.yesterdaystractors.com/c...alk&th=2129312

(which I didn't find until just a bit ago) which has some links of what's called a "Spark Intensifier"
https://www.collectorsweekly.com/sto...g-intensifiers

Plug wire are brand new, but exact same problem occurred with the pervious wires. I've even played around with plug gap, no change.

 

It does sound like the plugs may be fouling. However, if they look good and there’s no evidence of that, then it has to be something else.

Did you do ANYTHING right before it started acting up or was it running perfectly one day and suddenly started acting up?

First I’m assuming that it’s a stock TPI, no vortec heads as they require a different plug than stock. Did you use the stock heat range and not go with an absurdly cold plug? Didn’t you recently swap transmissions? Were any wires pinched during the swap? When I bought my iroc I was chasing electrical gremlins for about a month. When I finally broke down and ripped off all the wire looms there were a number of wires nearly cut through where the guy had bolted up the transmission with the wires caught. I guess he eventually noticed and just replaced the wire loom. I’m still pissed about that. Replaced the bad sections of wiring and bam - all was well for a while.

Anyway assuming no wiring harness issues double check your grounds especially at the back of the head and the one going to the firewall. Make sure it’s there and if so they are good and tight.

I’ve seen something similar when a guy swapped the wires around in an in cap coil but yours is external - doubt that’s it. Also seen something similar (miss) with an in cap coil there the guy stripped the screw holding down the internal ground strap. Again yours is external but there still could be a ground issue.

Anyway. That’s all I have at the moment. Hopefully it helps.