Bad running after manual swap

[1998_K1500_Sub]

What would cause too high of a spark voltage?

A high-resistance path to ground, e.g., an open circuit or very high resistance coil wire to the dizzy... so then the secondary voltage rises atypically high and the spark finds a new path to ground, which I believe is what you've witnessed (with multiple coils?)

Also I have not checked the wires with an ohmmeter, I did just replace them about 2-3 months ago, and none of them seem bad at first glance. Will certainly check them though. How much resistance am I looking for to know they're good?

Perhaps a few hundred to maybe 1000 ohms per foot of wire, just a guess. All wires should measure similar ohms/ft, that's the benchmark.

Check the coil wire carefully.

Perhaps substitute another wire in its place, if you have any lying around. I always keep my old ign wires until I'm sure the new wires are working properly.

 

[JPVortex]

A high-resistance path to ground, e.g., an open circuit or very high resistance coil wire to the dizzy... so then the secondary voltage rises atypically high and the spark finds a new path to ground, which I believe is what you've witnessed with multiple coils.



Perhaps a few hundred to maybe 1000 ohms per foot of wire, just a guess. All wires should measure similar ohms/ft, that's the benchmark.

Check the coil wire carefully.

Perhaps substitute another wire in its place, if you have any lying around. I always keep my old ign wires until I'm sure the new wires are working properly.

I do have almost a full set of brand new wires. I had to replace 2 of them again recently because I had them touching the exhaust recently.

I already tried swapping between the 2 coil wires I have, no difference.

And I'm not sure if the wires would be the problem to be honest. Because the tach glitches out like hell, I would think that'd be a coil problem, not a wire problem, because the tach signal comes from the coil.

Heres the video of the "glitching/bouncing" the tach has when running poorly from the other day:

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[1998_K1500_Sub]

And I'm not sure if the wires would be the problem to be honest. Because the tach glitches out like hell, I would think that'd be a coil problem, not a wire problem, because the tach signal comes from the coil.

The same thought occurred to me... but I reasoned that a voltage (actually, current) irregularity in the secondary will reflect in the primary and cause some oddity in the primary voltage, and possibly affect the tach.

Isn't the tach signal simply the ICM's drive voltage to the coil? I could look it up, but I haven't.

 

[JPVortex]

The same thought occurred to me... but I reasoned that a voltage (actually, current) irregularity in the secondary will reflect in the primary and cause some oddity in the primary voltage, and possibly affect the tach.

Isn't the tach signal simply the ICM's drive voltage to the coil?

I suppose that could be it, I’m going to see what I can do tomorrow, but this truck may end up going to a shop for it. I feel like I have a tail and I’m chasing it in circles right now looking for the problem.

 

[1998_K1500_Sub]

I suppose that could be it, I’m going to see what I can do tomorrow, but this truck may end up going to a shop for it. I feel like I have a tail and I’m chasing it in circles right now looking for the problem.

It would be enlightening to put a 'scope on some of those low-voltage wires and see WTF is happening.

 

[Schurkey]

The original distributor was literally sparking here and there from the body of it into the bracket.

Distributor, or coil? If the coil is sparking from anywhere but the spark output terminal, it's faulty. Insulation has broken down somehow.

Now I am starting to get suspect of the parts store ignition coils. I just swapped out YET another one, and this time it doesn't even run the truck.

My OEM one runs the truck but you can literally see the spark coming out of the side of the coil. So the primary windings probably have a short.

Sounds like the insulation's broken-down somewhere on the coil

Yes. Faulty insulation. But if the coil sparked out the side ALL the time...the engine wouldn't run at all. Seems like only one or maybe two cylinders have enough spark voltage to escape through the side of the coil.

It would be enlightening to put a 'scope on some of those low-voltage wires and see WTF is happening.

I just love using an automotive 'scope on ignition systems. They'll show differences in cylinder-to-cylinder ignition problems quickly and effectively.

Automotive 'scopes are becoming a lost art, largely because distributors are an endangered species; and other ignition types--waste spark, coil-on-plug, etc. are not as easily connected to a 'scope. (They CAN be connected, but often each cylinder separately; and it's a pain in the tuckus and takes a long time.)

and/or there's too-high spark voltage

High voltage will eventually destroy the insulation, but it typically takes awhile.

Another test is to clip a "test" spark plug to the top post of the distributor (or, depending on the cap style, stick it upside down in the hole) and then attach the coil wire to the test plug. The test plug will spark with every cylinder firing, and should spark equally for all cylinders. Yes, the engine will run if you do this.

Yep, I can imagine that. Never tried it...but I will. I learned something from this. I've just attached the spark tester to the coil, and then grounded the spark tester with a jumper wire.

Old-style coil-in-cap HEI distributor--but connecting to a separate coil or the coil wire works as well.

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What would cause too high of a spark voltage?

Really, enormously high resistance in the plug wires will sometimes do it. Not often, though.

Typically it's an excessively-large gap somewhere in the system. Spark plug gap being too wide is common. Rotor tip-to-distributor cap terminal is less common.

Also I have not checked the wires with an ohmmeter, I did just replace them about 2-3 months ago, and none of them seem bad at first glance. Will certainly check them though. How much resistance am I looking for to know they're good?

Depends on the plug wire construction. Anywhere from ~35 ohms per foot of wire, to 4,000 ohms per foot. I'd be suspicious of anything beyond 4,000 ohms per foot, but I've seen plug wires on running vehicles that have 20,000 ohms per foot; and plug wires with ~100,000 ohms--but the cylinders were misfiring badly.

 

[JPVortex]

Distributor, or coil? If the coil is sparking from anywhere but the spark output terminal, it's faulty. Insulation has broken down somehow.



Yes. Faulty insulation. But if the coil sparked out the side ALL the time...the engine wouldn't run at all. Seems like only one or maybe two cylinders have enough spark voltage to escape through the side of the coil.


I just love using an automotive 'scope on ignition systems. They'll show differences in cylinder-to-cylinder ignition problems quickly and effectively.

Automotive 'scopes are becoming a lost art, largely because distributors are an endangered species; and other ignition types--waste spark, coil-on-plug, etc. are not as easily connected to a 'scope. (They CAN be connected, but often each cylinder separately; and it's a pain in the tuckus and takes a long time.)


High voltage will eventually destroy the insulation, but it typically takes awhile.


Yep, I can imagine that. Never tried it...but I will. I learned something from this. I've just attached the spark tester to the coil, and then grounded the spark tester with a jumper wire.

Old-style coil-in-cap HEI distributor--but connecting to a separate coil or the coil wire works as well.

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You must be registered for see images attach

Really, enormously high resistance in the plug wires will sometimes do it. Not often, though.

Typically it's an excessively-large gap somewhere in the system. Spark plug gap being too wide is common. Rotor tip-to-distributor cap terminal is less common.


Depends on the plug wire construction. Anywhere from ~35 ohms per foot of wire, to 4,000 ohms per foot. I'd be suspicious of anything beyond 4,000 ohms per foot, but I've seen plug wires on running vehicles that have 20,000 ohms per foot; and plug wires with ~100,000 ohms--but the cylinders were misfiring badly.

Yeah I meant coil not distributor to answer your first question lol.

And yeah I agree that it’s not all the cylinders. Probably one or 2. They’re not huge sparks just a small arcing between the housing of the coil and metal on the engine like the intake, which is when it starts misfiring and stumbling bad. It actually did die a couple times when close enough to the metal.

The truck did have a couple misfires in the passenger bank for a while here and there, I think I may just replace the rest of the spark plug wires to see what happens.

 

[JPVortex]

So really even if a couple of the spark plug wires have enough voltage to escape the coil, if I had a good coil it won’t escape it no matter what?

 

[JPVortex]

So really even if a couple of the spark plug wires have enough voltage to escape the coil, if I had a good coil it won’t escape it no matter what?

I ohm tested a couple spark plug wires. They seem to be crazy high resistance.

I had to set my ohm meter to read 200k because 20k was reading infinite. I tested one that was 2 feet long, and it measured at 29.5

Wouldn’t that mean it had 29,500 ohms of resistance?!?! That’s crazy high.

 

[1998_K1500_Sub]

I ohm tested a couple spark plug wires. They seem to be crazy high resistance.

I had to set my ohm meter to read 200k because 20k was reading infinite. I tested one that was 2 feet long, and it measured at 29.5

Wouldn’t that mean it had 29,500 ohms of resistance?!?! That’s crazy high.

29.5kΩ sounds high (~15kΩ/ft) at least compared to numbers @Schurkey quoted.

I've got some wires downstairs, I'll go measure them.

Do keep in mind, it's a low-current, high-voltage system, so resistance values that don't make any sense in a 12V system are just fine.