Need help finding cause of Strange missfire

[user-23911]

I always find it amusing when someone says....change this.
Someone else says .....change that.

Swapping part using guesswork never works.
You need to learn how to measure things using the right tools.

If you do random swapping of parts you're more likely to make the problem worse, not better.
Then what do you do?
But if you DO fix it with random swapping.....it's nothing more than luck.

[pjc360]

I always find it amusing when someone says....change this.
Someone else says .....change that.

Swapping part using guesswork never works.
You need to learn how to measure things using the right tools.

If you do random swapping of parts you're more likely to make the problem worse, not better.
Then what do you do?
But if you DO fix it with random swapping.....it's nothing more than luck.

Not too mention it gets expensive replacing things specially when it doesn’t solve the issue.
That’s why I haven’t bought a new ignition box yet because I haven’t been able to prove without a doubht that it’s causing the problem.
That’s why I’m here sharing this information because I’d love to find out how to diagnose this correctly.

[murre]

op. describe when problems started . Has the car been working properly? Then suddenly began to bother?Have you done anything right before the problems started? I have two ideas about what might be wrong. if the sound is very awful. so you can hardly believe your ears.
1. a piece of head gasket is missing between two cylinders. Sounds normal until you get a certain load
2.The ignition coil operates to a certain rpm. then it sparks completely wrong

[pjc360]

op. describe when problems started . Has the car been working properly? Then suddenly began to bother?Have you done anything right before the problems started? I have two ideas about what might be wrong. if the sound is very awful. so you can hardly believe your ears.
1. a piece of head gasket is missing between two cylinders. Sounds normal until you get a certain load
2.The ignition coil operates to a certain rpm. then it sparks completely wrong

I can’t even say for sure when the problem occurred but it’s been happening for a long time.
For the longest time I thought it was a drivetrain issue because it feels a lot like a drivetrain vibration when your driving it and it starts missfiring.
So i spent a lot of time and money on new driveshafts and U-joints, new springs, new engine and transmission mounts.
All of that did make it smoother but never eliminated it completely.
Then I realized it wasn’t drivetrain because it would occur at the same rpm while in park or nuetral.
So I knew it wasn’t drivetrain at that point.
That’s when I bought a new distributor, new coil, new plugs and new wires.
Again it helped smooth things out a little but didn’t eliminate it.
Everything I’ve done seems to slightly improve it but never eliminates it.
Then I realized the issue is much worse when the vacuum advance is plugged inn, so I started looking at rotor phasing.
The rotor phasing seems to be within the proper range from what I can see.
So then I thought maybe it’s getting too much advance too quickly. The springs that came in the distributor had the timing all in at 2500 rpm.
I swapped springs to bring it in a little slower to all in by 3200-3400 rpm.
Again that seemed to help a little but never eliminated it.
Then I decided to pull all the plugs and noticed cylinder 8s plug looked like it was struggling to fire compared to the other 7 cylinders.
So I did a compression test and every cylinder is putting out 155psi.
So then I thought ok maybe it’s got a bad valve spring on cylinder 8, pulled the valve cover and couldn’t see anything wrong with the springs on cylinder 8.
So then I decided to check every cylinder at 2500 rpm with my timing light and I could see that it wasn’t just cylinder 8 missing at 2500 it’s all 8 cylinders missing.
So that led me to believe maybe it’s the ignition box.
But what’s throwing me off is if it were the box wouldn’t it continue to miss at 2500 rpm with or without vacuum advance? Because it seems to clear up once I take the vacuum advance away.
So now I’m back to thinking it’s ether rotor phasing causing the spark to scatter inside the cap, or the reluctor tooth to pick up coil post air gap is ether too close or too wide at 2500 with vacuum advance.
But what I don’t understand is I have checked the pick up coil air gap with and without vacuum advance with my hand held mity vac pump and it’s at a solid .008 which is the correct spec.
So now I’m basically Just back to the same old situation of not knowing exactly what it is.

[Tuner]

Have you separated the mag trigger and coil primary wires yet? That was first suggested last Tuesday and a few time since.

[MadBill]

Try fitting a length of metal (rod, bolt, or whatever) into the coil wire connector, with the other end contacting the cap tower. Fire the engine, run it up to the problem revs then move it away from the cap and see if the spark will consistently jump a 1/2"-3/4" gap. If it does, repeat with one or more cylinder wires.
Realistically, it's high time that you swapped out the box; maybe you could borrow one?

[Circlotron]

If your vac advance gives say 10 deg, what happens if you remove and plug the hose and then bump up the initial timing by an extra 10deg? Run it at the problem rpm and see if it is still bad. That should tell you if it is a distributor phasing problem or not.

[murre]

where do you take vacuum to vacuum canister? and use a vacuum gauge with a T-connector between the carburettor and the vacuum canister.Look at the gauge while driving.a friend to me has a"misfire" at 30mph".he uses ported vacuum.We measured with two vacuum gauge at the same time.one meter shows 20hg unported .the other one at ported vacuum depending on the butterfly angel showed mostly 0 hg or 20hg but at 30mph it was fluttering around 5hg.that fluttering affected ignition.

[Circlotron]

Pull the distributor out and put the shaft in a variable speed drill. Run a wire from the coil secondary with a 1/2 inch gap to ground. Run the drill speed up and down and see how you go.

[pjc360]

Pull the distributor out and put the shaft in a variable speed drill. Run a wire from the coil secondary with a 1/2 inch gap to ground. Run the drill speed up and down and see how you go.

What am I looking for here?
Am I looking at rotor phasing position or the reluctor tooth to pick up coil post air gap?
Here is a picture of the rotors position with vacuum advance connected at idle.
So 18 initial, with an additional 12 degrees from vacuum advance.
It swings back a little bit but not a whole lot, not enough to be causing a missfire imo.
But the strangest part of this issue is when I remove the vacuum advance the missfiring clears up.
So the vacuum advance does 3 things, increases timing obviously, it changes rotor phasing position, and it changes reluctor tooth to pick up coil post gap.
I’ve checked timing with two lights both are Within 1/2 of a degree of each other.
I’ve measured the reluctor tooth to pick up coil post gap with and without vacuum advance.
The air gap remains at .008 with and without vacuum advance.
I did this with my hand held mity vac pump and a brass feeler gauge.
So that pretty much leaves rotor phasing.
But in the picture the position of the rotor looks to be within range.

63E78597-8C81-4122-825B-7DF29B677502.jpeg

[Circlotron]

Pull the distributor out and put the shaft in a variable speed drill. Run a wire from the coil secondary with a 1/2 inch gap to ground. Run the drill speed up and down and see how you go.

What am I looking for here?

Just looking to see whether the sparks occur in a regular manner or erratically at some critical rpm. Rotor and cap not involved in this test. Just looking at spark behavior as they come out the top of the coil; no need to send them to the cap and rotor.

[John Wallace]

Have you tried a different distributor?



Don't know what this one looks like inside.
Possibly when the advance plate moves it rubs a wire?
(shorting it?)
A Wire has an internal partial break?
Or it provides or breaks a ground?

Does the plate move smoothly?

I think I would be inspecting the inside of that distributor very closely?

[pjc360]

Have you separated the mag trigger and coil primary wires yet? That was first suggested last Tuesday and a few time since.

I haven’t had a chance to work on the truck at all because it’s in the shop.
Over the summer I had re-geared the 8 3/4 rear axle from 3.55s to 4.56’s.
With how much I’ve been working and not having a shop at the moment I decided to take it to a shop to have the front Dana 44 axle re-geared to 4.56s to match the rear.
In the last couple of days we have gotten over a foot of snow here otherwise I would have grabbed it today and brought it back to the house to separate the trigger wires and to perform a couple a more tests on it.
But I don’t think it woukd even make it to my house and then bsck to the shop without 4wd.
So I’m basically stuck waiting for the front axle to get geared to match the rear axle so I can have my 4wd back.
I’m moving into a new house at the beginning of January that does have a small garage.
So once I get the truck back and I’m in the new house I’ll be able to mess around with it in the garage where I’m out of the snow and weather.
I’m also starting a brand new job January 2nd.
So I’ve had Christmas to worry about with the wife and 3 kids, I’ve had the truck to worry about with the strange missfiring issue andbthe gearing not matching.
And I’ve had packing to worry about and renting a u-haul, and I’ve had to give my notice at my current job and stress about the new job.
I’ve had a lot on my plate this month.
I’m hoping once we get moved and I get the truck back and ive started the new job that on The Weeknd’s I’ll be able to narrow this issue down with the missfiring.
It should help to be in a garage out of the elements.
At my current house whenever I work on it I’m outside on the side of the road feeezing.

[pjc360]

So I got the truck back from the shop, the front axle is now geared with 4.56s to match the rear axle.
So I have 4wd back which is nice.
Still fighting the missfiring and overall rough running and plug fouling mess.
I did a little experimenting with my timing light while it was running the other day.
I notice that when I increase the rev limiter on the dials on the crane hi-6 the missfiring at 2500 is limited to cylinder 6 and cylinder 8.
It was no longer missing on the other 6 cylinders like it was when the rev limiter was set at 6k rpm.
Also noticed that when it’s idling I can see a miss in the timing lights strobe on a few different cylinders.
So I’m back to thinking the hi-6 cd box is causing the issues with the missfires and the plug fouling.
I don’t know how or why it’s affecting cylinder 8 the most, but number 8s plug is always fuel/carbon fouled and looks likes it’s struggling to fire compared to the others and I do see missfires on other cylinders ranging from idle to 2500 rpm.
The cd box is the only part of the ignition that isn’t brand new, so I’m thinking I’m going to cross my fingers and pull the trigger on buying a new msd digital 6 and wiring it up and separating the trigger and coil wires on opposite sides of the truck with new wiring all the way around on the ignition and prey that it solves this problem.

[Tuner]

The RFI from solid wire plug wires or a bad suppression plug wire will cause a rev-limiter to act at earlier RPM than it is set (or the chip is marked in the MSD that use a plug-in "chip") and cause random misfire.

Use a DVM and check the resistance of each wire. Leave the wires in the cap and measure between each plug terminal up in the boot and the terminal inside the cap. Different brands and types of resistance wires will have different resistance per foot, but wires in a set should have similar resistance per inch of length. Longer wires should have more resistance than shorter wires.