1999 5.7 350 vortec P0304 cylinder 4 misfire

[socal k1500]

I took one of your dizzy photos and put some arrows to something that looks like a failure
of the insulating properties between the center conductor and one of the individual runs?

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Anyone else ever see something like this in their Vortec caps?

What does a brand new cap look like in this area?

EDIT: What does the underneath of your rotor look like? Any blow-through evident?

Hmmm...

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Bottom of old rotor and picture of a brand new cap

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[Orpedcrow]

the brass looking cylinder contact thing

I don’t think that is an issue. Should be just the end of the drive shaft. The reluctor ring is pressed onto it, then the rotor screws to the reluctor.

Anyone else ever see something like this in their Vortec caps?

Looks like carbon build up from arcing to me! Stress risers in that valley I would assume.

That should be the #3 spark plug wire. That could cause a backfire if it’s sparking way to early and out of time.

 

[socal k1500]

The old cap your seeing did sit in my garage for about a year but I didn’t see any corossion or anything. Can caps go bad but not have visible damage?

 

[Road Trip]

The old cap your seeing did sit in my garage for about a year but I didn’t see any corossion or anything. Can caps go bad but not have visible damage?

I'm with @Orpedcrow on the photo of the underside of the old cap. To me it
looks like a semicircle of burnt isulator from sustained arcing between the center
terminal vs the #3 spark plug pathway?

Can't tell absolutely if this is what caused the no-start & backfire, but it does
fail a visual inspection. On the other hand, the new cap looks to be good.

As for the underneath of your rotor, that doesn't look to have contributed
to the no-start symptom. (I'll try to find the photo of the rotor that was
blown through to the shaft. (ground)

Good photos. Remember, Spark + Fuel + Compression = must run.

Since all you did was replace the intake manifold gasket, Spark delivery
and Fuel delivery are the areas that we must verify as good.

Gotta be close to running...

 

[Schurkey]

I'd have said
1. A burnable fuel/air mixture
2. Adequate compression
3. Spark across the plug gap, at the right time, and
4. A reasonably free-flowing exhaust system.

More commonly-known as "SUCK, SQUEEZE, BANG, WHOOSH".

If you don't have a way to get the exhaust out of the cylinder, the engine may pop and sputter, but it won't "run", or it runs poorly.

 

[socal k1500]

Well boys add the above picture of the dizzy cap to a list of signs of failure. Slapped the new dizzy cap and rotor on the truck this morning and she started right up easy. I only ran it for a few minutes because I still need to change the oil.

In 5 years of owning this thing I’ve changed the cap and rotor at least 6 times. I think it’s time to get a higher quality one lol.


Now to start driving it and see how she runs and if the rough idle and cylinder 4 misfire are gone. Thanks for your guys help and support !

 

[Road Trip]

Well boys add the above picture of the dizzy cap to a list of signs of failure. Slapped the new dizzy cap and rotor on the truck this morning and she started right up easy.

*YES*

I only ran it for a few minutes because I still need to change the oil.

In 5 years of owning this thing I’ve changed the cap and rotor at least 6 times. I think it’s time to get a higher quality one lol.

With 20/20 hindsight, I think the pinnacle of small block distributors were the
large cap HEI units. This upgrade to the original points/condenser dizzys
occurred right around the time that catalytic converters were introduced
for the '75 model year. (See attached for a pic of the huge high voltage playing
field that the old HEI distributor caps had to work with.)

The GM engineers wanted big spark gaps and a system that would reliably fire
them -- otherwise, the expensive cats would melt down during the warranty
period, and that would be bad juju.

I can personally vouch for the fact that the then-new HEI systems had the suds.
(Previously I had been bit by bad spark plug wires on old points systems, and it was
always a shock...but the *one* time I got shocked by a HEI system, it felt like
a mule had kicked me in the arm. (!) From that point on I wouldn't touch an HEI
wire unless the engine was off. Life Lesson learned. :0)

When the old big cap HEI systems were replaced with the newer crab-cap jobs, it did
make the ignition wire dressing much neater. But that's on the outside. On
the inside of the HEI's big cap, there is plenty of real estate between the individual
cylinders. And the high voltage path from rotor tip to *every* individual spark plug
wire is short, sweet, straight up & out.

And thanks to the inverse-squared law, every time you double the distance between
2 electrical terminals, you end up with 1/4 as much stress on the insulator between
them. (Oversimplification, but when high voltage is concerned, more space is more better.)

On the other hand, due to the design of the new crab cap (featuring spark plug wires
sorted out to Left and Right banks) we now have a high voltage path running
across the inside of the cap? And as your photo clearly shows, the insulator
material that the cap was made of failed, allowing the #3 cylinder spark plug to see
some/all of the 8 sparks for every cam revolution?

And if one of those sparks occurred while the intake valve was open, then it would certainly
fire both the cylinder AND also back through the intake manifold?

Plain & simple, the only 2 realistic solutions to this design issue are to A) someone has to
sell a Vortec distributor cap with superior high voltage insulator material, or B) some
entrepreneur needs to design an old-school sequentially-wired distributor cap
that can be retrofit on top of the Vortec distributor base? There's no reason that
this can't be done that I can see? Sure, you would need custom-length wires, but
I would be the first in line to try one out?

In English, I think that when the Vortec crab cap designers attempted to improve
how the spark plug wires were routed to the original distributor caps, they
created a new problem. The layout of the new Vortec crab cap design puts a greater
stress on the high voltage insulator material that the cap is made from?

Now to start driving it and see how she runs and if the rough idle and cylinder 4 misfire are gone. Thanks for your guys help and support !

Fingers crossed that the exploratory intake manifold gasket renewal + the
discovery/replacement of the bad cap will be just enough to allow the engine
to run as designed.

But don't forget to get that cooling system flushed at your earliest opportunity.

And if that P0304 comes back after the cooling system flush then the only thing
left is the valve stems being too tight in the valve guides while climbing a grade
as described in that old TSB.

Let us know what happens. And thanks for keeping us in the loop like you have been.

Misfire-free travels!

Cheers --

 

[HotWheelsBurban]

*YES*



With 20/20 hindsight, I think the pinnacle of small block distributors were the
large cap HEI units. This upgrade to the original points/condenser distributor
caps occurred right around the time that catalytic converters were introduced
for the '75 model year. (See attached for a pic of the huge playing field that
the old HEI distributor caps had to work with.)

The GM engineers wanted big spark gaps and a system that would reliably fire
them. I can personally vouch for the fact that the then-new HEI systems had
the suds. (I had been bit by bad spark plug wires on old systems, and it was
always a shock...but the *one* time I got shocked by a HEI system, it felt like
a mule had kicked me in the arm. (!) From that point on I wouldn't touch an HEI
wire unless the engine was off. Life Lesson learned. :0)

When the old big cap HEI systems were replaced with the newer crab-cap jobs, it did
make the ignition wire dressing much neater. But that's on the outside. On
the inside of the HEI's big cap, there is plenty of real estate between the individual
cylinders. And the high voltage path from rotor tip to *every* individual spark plug
wire is short, sweet, straight up & out.

And thanks to the inverse-squared law, every time you double the distance between
2 electrical terminals, you end up with 1/4 as much stress on the insulator between
them. (Oversimplification, but when high voltage is concerned, more space is more better.)

On the other hand, due to the design of the new crab cap (featuring spark plug wires
sorted out to Left and Right banks) we now have a high voltage path running
across the inside of the cap? And as your photo clearly shows, the insulator
material that the cap was made of failed, allowing the #3 cylinder spark plug to see
some/all of the 8 sparks for every cam revolution?

And if one of those sparks occurred while the intake valve was open, then it would certainly
fire both the cylinder AND also back through the intake manifold?

Plain & simple, the only 2 realistic choices are to A) someone has to sell a Vortec
distributor cap with superior high voltage insulator material, or B) some
entrepreneur needs to design an old-school sequentially-wired distributor cap
that can be retrofit on top of the Vortec distributor base?

In English, I think that when the Vortec crab cap designers attempted to improve
how the spark plug wires were routed to the original distributor caps, they
created a new problem. The layout of the new Vortec crab cap design puts a greater
stress on the high voltage insulator material that the cap is made from?



Fingers crossed that the exploratory intake manifold gasket renewal + the
discovery/replacement of the bad cap will be just enough to allow the engine
to run as designed.

But don't forget to get that cooling system flushed at your earliest opportunity.

And if that P0304 comes back after the cooling system flush then the only thing
left is the valve stems being too tight in the valve guides while climbing a grade
as described in that old TSB.

Let us know what happens. And thanks for keeping us in the loop like you have been.

Misfire-free travels!

Cheers --

The Achilles heel of these systems (original coil in cap HEI) is the rotor. When it is allowed to burn through in the middle, under the spring contact, it will take out the module and other parts in the distributor. Standard/Blue Streak finally came out with a DR318X rotor that is made of stouter plastic with higher dielectric strength, and these will last longer under heavy use. Just like the cheap Vortec rotors, the cheap HEI rotors don't last very long under extreme use.
Back in my parts store days, in the late 70s, our store sold the HEI rotors by the case, to several garages that had lots of fleet business. Fleets of '77 Impalas and '78 LeMans', in Houston, Texas area summer traffic in the boom times, when every company has peripheral connections to the oil bidness, will teach a lot about parts and vehicle durability.

 

[socal k1500]

Summit sells this one.

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rock auto has this one that i see people say is good

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[Orpedcrow]

Glad it was sorted!

Word on the street is the UMP is the way to go. The D.U.I stuff is also supposed to pretty great. That’s what I’ll be getting when I’m ready.