1997 Chevy Suburban K1500 P0420

[Big_Red9]

Hello GMT400 world,

I am a new poster to this website, but I've been scrolling through these forums for the past ~6 months. After putting countless hours into my newly acquired '97 Suburban, I figured it's time to finally post the issues I've been battling with in lieu of going to a mechanic.

I'll begin with what my problem is: I have been consistently getting a P0420 code thrown (Catalyst System Efficiency Below Threshold Bank 1) as well as other related codes occasionally such as P0131 and P0151. I know this is an indication to replace my cats, but what I am trying to figure out is WHY they went bad. I will also add that if I am driving under heavy load (******* it up a hill) I'll feel the engine missing and usually that's followed by a flashing P0300. This has lead me to believe that I ruined my cats because my engine has misfired many times.

Here is what I've done so far (I will probably forget some things too):
- New plugs/wires
- New distributor cap & rotor
- New coil and module
- New cam/crank shaft position sensors
- Inspected distributor gear; looked mint
- Timing test; timing was good
- Cat temp test; did not read any major temp difference upstream & downstream of cat after driving for a while
- Fuel pressure test on the fuel rail; ~60 psi while driving and held steady through various driving ranges
- Compression test; all cylinders were the same around 170 psi
- Smoke test; no vacuum leaks
- Intake vacuum pressure check; took the brake booster line off and vacuum met criteria outlined in Haynes manual (20.5 inHg)
- Etc. you name it, I probably replaced/checked it

I am done firing the parts canon at it until I really understand what going on here. On my drive into work today, I recorded fuel trims and O2 sensor readings. I'll attach them here - I narrowed it down to a 3 minute snippet of the entire drive just to make it easier to look at, but these snips are a good representation of the rest of the trip.

This is already a long post so I'll wrap it up. It is my understanding that cats don't go bad without some reason. I thought the reason was due to misfires. I can't figure out why it's missing, but I also am not 100% convinced my cats are shot since I didn't seem to have any backpressure on the intake vacuum test and their temp differences were not significant. So what da heck is goin' on?? Any help will be appreciated.

 

[evilunclegrimace]

Did you consider that the cats are 27 years old and have reached the end of their usefull life span?

 

[Big_Red9]

That is certainly a possibility. I just thought I'd see an indication of bad cats with other tests I did. Unless they can just go bad without being plugged. It also doesn't exactly explain the misfires under heavy load.

 

[evilunclegrimace]

That is certainly a possibility. I just thought I'd see an indication of bad cats with other tests I did. Unless they can just go bad without being plugged. It also doesn't exactly explain the misfires under heavy load.

I had to replace the cats on my'98 at 260,000+ miles. They can and do wear out and don't necessarliy plug up when they fail.

 

[Big_Red9]

I had to replace the cats on my'98 at 260,000+ miles. They can and do wear out and don't necessarliy plug up when they fail.

I guess it may be worth getting them tested. Is there a chance this could help my misfire problems? Assuming new cats make my downstream O2 sensors read normal again, but it is my understanding that the computer is reading only the upstream sensors to adjust AFR. This makes me worry that I could replace the cats but then ruin a new set.

 

[Road Trip]

Hello GMT400 world,

I am a new poster to this website, but I've been scrolling through these forums for the past ~6 months. After putting countless hours into my newly acquired '97 Suburban, I figured it's time to finally post the issues I've been battling with in lieu of going to a mechanic.

I'll begin with what my problem is: I have been consistently getting a P0420 code thrown (Catalyst System Efficiency Below Threshold Bank 1) as well as other related codes occasionally such as P0131 and P0151. I know this is an indication to replace my cats, but what I am trying to figure out is WHY they went bad. I will also add that if I am driving under heavy load (******* it up a hill) I'll feel the engine missing and usually that's followed by a flashing P0300. This has lead me to believe that I ruined my cats because my engine has misfired many times.

Here is what I've done so far (I will probably forget some things too):
- New plugs/wires
- New distributor cap & rotor
- New coil and module
- New cam/crank shaft position sensors
- Inspected distributor gear; looked mint
- Timing test; timing was good
- Cat temp test; did not read any major temp difference upstream & downstream of cat after driving for a while
- Fuel pressure test on the fuel rail; ~60 psi while driving and held steady through various driving ranges
- Compression test; all cylinders were the same around 170 psi
- Smoke test; no vacuum leaks
- Intake vacuum pressure check; took the brake booster line off and vacuum met criteria outlined in Haynes manual (20.5 inHg)
- Etc. you name it, I probably replaced/checked it

I am done firing the parts canon at it until I really understand what going on here. On my drive into work today, I recorded fuel trims and O2 sensor readings. I'll attach them here - I narrowed it down to a 3 minute snippet of the entire drive just to make it easier to look at, but these snips are a good representation of the rest of the trip.

This is already a long post so I'll wrap it up. It is my understanding that cats don't go bad without some reason. I thought the reason was due to misfires. I can't figure out why it's missing, but I also am not 100% convinced my cats are shot since I didn't seem to have any backpressure on the intake vacuum test and their temp differences were not significant. So what da heck is goin' on?? Any help will be appreciated.

Greetings Big_Red9, and Welcome to the GMT400 forum!

That was a great introduction & problem description. About the only thing I'm wondering
at this point is if you have a 5.7 or 7.4 engine, for this alters the perspective on the
'misfire under heavy load' problem.

Since you are pretty far along on all this, I'll minimize the theory and share a bulleted list:

* You are correct that you really must troubleshoot/repair the misfires under load
before you even think about replacing the cats. When the SES ("Check Engine") light goes
from steady on to flashing mode this is the computer's way of warning the driver that
continued operation in this state may lead to catalytic converter damage.

* Given your problem description, I think that:

A) your recovery plan needs to be to first fix the misfire 100%.

B) Then ensure that SES light goes from all the way from Flashing > On > OFF. You may find
that after the misfires are cleared that there are other codes that don't set a SES light
are also being logged. Gotta fix them all.

C) And if you still can't clear the SES light because the only code remaining a is P0420, then
proceed with cat replacement. In that order. Cats are spendy, so we need to make sure that
you sequence your repairs so that the new cats are an investment in cleaner air, instead
of just sacrificial exhaust bits that are melted down by extreme temps causes by a still-misfiring
engine upstream?

****

OK, I looked at the live data you shared, and had the following questions:

* Was the 3-minute segment that you shared was logged when the P0131 & P0151 codes
were triggered? Was this the time that the engine was misfiring under load? That would be
the most helpful data of all to review.

* The fuel trims showing at ~2-7% on the left bank, and ~2-8% on the right bank. So
the feedback is working, but at the same time we're not seeing gross changes of 15-25%?
In other words, do we need to explore the possibility that the misfires under load are
not just a gross leaning out due to insufficient fuel delivery, but due to other issues
affecting proper combustion? (Again, knowing if the problem was/was not occurring
duriing the data capture is key.)

* A gentleman named Schurkey stated this succintly: "The O2 sensors are Oxygen sensors,
not Combustion sensors. Bad combustion will lead to a 'False Lean' situation." (paraphrasing)

When combustion is good, Oxygen is inducted during the intake stroke, consumed during
a quality burn, and then oxygen-free gases are pumped out the exhaust valve, past the
upstream O2 sensors before entering the waiting catalytic converters.

When combustion is bad, Oxygen enters the cylinder during the intake stroke, left intact when no
combustion occurs, and all that oxygen is then pumped out past the exhaust valve, sniffed
by the O2 sensors, they react with low voltage generated (no voltage because there is Oxygen
on *both* sides of the sensor) ...and then the unburned fuel + unburned oxygen hits the
cat's already hot honeycomb, and cat meltdown commences. And during this 'false lean' condition,
the computer responds by adding additional fuel, making a hot cat situation even worse. (!)

****

Alright, all that was somewhere between a mouthful and keyboard mush. Check out this
page from the Factory Service Manual where the O2 sensor is described:

You must be registered for see images attach

So where to go from here?

* Let's start with the everyday versus going straight for the exotic. The ignition systems in these trucks
are highly stressed, and due to the decreasing quality of most of the repair parts we're sold, it's
possible to have failures in the high voltage area not that long after they were installed? Especially
when it comes to the cap, rotor, coil, etc. Look carefully for signs where the high voltage sparks
have located/created an easier path to ground.

* Do you have a cylinder by cylinder misfire count available for us to look at? If we can
focus on which cylinders are failing under load then this will help.

* Could you list ALL the different ways that your vehicle can be operated and NOT
encounter misfires? Sometimes figuring out all the scenarios where the engine ain't misbehaving
will help to contrast when the engine is misfiring, and these differences will help us
decide which way to go.

* If you can capture the data when the misfires are occurring it would be good to see.
And any photos that may support the ongoing diagnosis (spark plug coloring, etc) are
welcome. Being remote, we only get to see what you share.

****

Have you taken advantage of downloading the Factory Shop Manuals for your truck yet?
(I'm assuming that you have since you've been reading the forum for awhile.)

Let us know what you discover. Full focus on the misfires under load. Actually misfires
at any time, but especially when the flashing SES is occurring. Then we'll play it by
ear once that's solved.

Best of luck.

Cheers --

 

[Road Trip]

I guess it may be worth getting them tested. Is there a chance this could help my misfire problems? Assuming new cats make my downstream O2 sensors read normal again, but it is my understanding that the computer is reading only the upstream sensors to adjust AFR. This makes me worry that I could replace the cats but then ruin a new set.

Just wanted to jump back in and tell you that your understanding is correct. The feedback loop for the computer is indeed the upstream O2 sensors
as you described. And your last sentance is dead on correct. Fix the misfires first, and only then go for the cats if they are too far gone for
the P0420 code to quit occurring. (No prediction at this time, but I have seen P0420 codes disappear once the malfunctioning engine upstream
has been fully sorted out. And by the same token, I've also seen the P0420 persist after the engine was fixed, and the cats had to be replaced.)

It all depends upon how long this has been going on, and are the cats just deeply offended or already destroyed?

 

[Big_Red9]

Greetings Big_Red9, and Welcome to the GMT400 forum!

That was a great introduction & problem description. About the only thing I'm wondering
at this point is if you have a 5.7 or 7.4 engine, for this alters the perspective on the
'misfire under heavy load' problem.

Since you are pretty far along on all this, I'll minimize the theory and share a bulleted list:

* You are correct that you really must troubleshoot/repair the misfires under load
before you even think about replacing the cats. When the SES ("Check Engine") light goes
from steady on to flashing mode this is the computer's way of warning the driver that
continued operation in this state may lead to catalytic converter damage.

* Given your problem description, I think that:

A) your recovery plant needs to be to first fix the misfire 100%.

B) Then ensure that SES light goes from all the way from Flashing > On > OFF. You may find
that after the misfires are cleared that there are other codes that don't set a SES light
are also being logged. Gotta fix them all.

C) And if you still can't clear the SES light because the only code remaining a is P0420, then
proceed with cat replacement. In that order. Cats are spendy, so we need to make sure that
you sequence your repairs so that the new cats are an investment in cleaner air, instead
of just sacrificial exhaust bits that are melted down by extreme temps causes by a still-misfiring
engine upstream?

****

OK, I looked at the live data you shared, and had the following questions:

* Was the 3-minute segment that you shared was logged when the P0131 & P0151 codes
were triggered? Was this the time that the engine was misfiring under load? That would be
the most helpful data of all to review.

* The fuel trims showing at ~2-7% on the left bank, and ~2-8% on the right bank. So
the feedback is working, but at the same time we're not seeing gross changes of 15-25%?
In other words, do we need to explore the possibility that the misfires under load are
not just a gross leaning out due to insufficient fuel delivery, but due to other issues
affecting proper combustion? (Again, knowing if the problem was/was not occurring
duriing the data capture is key.)

* A gentleman named Schurkey stated this succintly: "The O2 sensors are Oxygen sensors,
not Combustion sensors. Bad combustion will lead to a 'False Lean' situation." (paraphrasing)

When combustion is good, Oxygen is inducted during the intake stroke, consumed during
a quality burn, and then oxygen-free gases are pumped out the exhaust valve, past the
upstream O2 sensors before entering the waiting catalytic converters.

When combustion is bad, Oxygen enters the cylinder during the intake stroke, left intact when no
combustion occurs, and all that oxygen is then pumped out past the exhaust valve, sniffed
by the O2 sensors, they react with low voltage generated (no voltage because there is Oxygen
on *both* sides of the sensor) ...and then the unburned fuel + unburned oxygen hits the
cat's already hot honeycomb, and cat meltdown commences. And during this 'false lean' condition,
the computer responds by adding additional fuel, making a hot cat situation even worse. (!)

****

Alright, all that was somewhere between a mouthful and keyboard mush. Check out this
page from the Factory Service Manual where the O2 sensor is described:

You must be registered for see images attach

So where to go from here?

* Let's start with the everyday versus going straight for the exotic. The ignition systems in these trucks
are highly stressed, and due to the decreasing quality of most of the repair parts we're sold, it's
possible to have failures in the high voltage area not that long after they were installed? Especially
when it comes to the cap, rotor, coil, etc. Look carefully for signs where the high voltage sparks
have found an easier path to ground.

* Do you have a cylinder by cylinder misfire count available for us to look at? If we can
focus on which cylinders are failing under load then this will help.

* Could you list ALL the different ways that your vehicle can be operated and NOT
encounter misfires? Sometimes figuring out all the scenarios where the engine ain't misbehaving
will help to contrast when the engine is misfiring, and these differences will help us
decide which way to go.

* If you can capture the data when the misfires are occurring it would be good to see.
And any photos that may support the ongoing diagnosis (spark plug coloring, etc) are
welcome. Being remote, we only get to see what you share.

****

Have you taken advantage of downloading the Factory Shop Manuals for your truck yet?
(I'm assuming that you have since you've been reading the forum for awhile.)

Let us know what you discover. Full focus on the misfires under load. Actually misfires
at any time, but especially when the flashing SES is occurring. Then we'll play it by
ear once that's solved.

Best of luck.

Cheers --

Greetings Big_Red9, and Welcome to the GMT400 forum!

That was a great introduction & problem description. About the only thing I'm wondering
at this point is if you have a 5.7 or 7.4 engine, for this alters the perspective on the
'misfire under heavy load' problem.

Since you are pretty far along on all this, I'll minimize the theory and share a bulleted list:

* You are correct that you really must troubleshoot/repair the misfires under load
before you even think about replacing the cats. When the SES ("Check Engine") light goes
from steady on to flashing mode this is the computer's way of warning the driver that
continued operation in this state may lead to catalytic converter damage.

* Given your problem description, I think that:

A) your recovery plant needs to be to first fix the misfire 100%.

B) Then ensure that SES light goes from all the way from Flashing > On > OFF. You may find
that after the misfires are cleared that there are other codes that don't set a SES light
are also being logged. Gotta fix them all.

C) And if you still can't clear the SES light because the only code remaining a is P0420, then
proceed with cat replacement. In that order. Cats are spendy, so we need to make sure that
you sequence your repairs so that the new cats are an investment in cleaner air, instead
of just sacrificial exhaust bits that are melted down by extreme temps causes by a still-misfiring
engine upstream?

****

OK, I looked at the live data you shared, and had the following questions:

* Was the 3-minute segment that you shared was logged when the P0131 & P0151 codes
were triggered? Was this the time that the engine was misfiring under load? That would be
the most helpful data of all to review.

* The fuel trims showing at ~2-7% on the left bank, and ~2-8% on the right bank. So
the feedback is working, but at the same time we're not seeing gross changes of 15-25%?
In other words, do we need to explore the possibility that the misfires under load are
not just a gross leaning out due to insufficient fuel delivery, but due to other issues
affecting proper combustion? (Again, knowing if the problem was/was not occurring
duriing the data capture is key.)

* A gentleman named Schurkey stated this succintly: "The O2 sensors are Oxygen sensors,
not Combustion sensors. Bad combustion will lead to a 'False Lean' situation." (paraphrasing)

When combustion is good, Oxygen is inducted during the intake stroke, consumed during
a quality burn, and then oxygen-free gases are pumped out the exhaust valve, past the
upstream O2 sensors before entering the waiting catalytic converters.

When combustion is bad, Oxygen enters the cylinder during the intake stroke, left intact when no
combustion occurs, and all that oxygen is then pumped out past the exhaust valve, sniffed
by the O2 sensors, they react with low voltage generated (no voltage because there is Oxygen
on *both* sides of the sensor) ...and then the unburned fuel + unburned oxygen hits the
cat's already hot honeycomb, and cat meltdown commences. And during this 'false lean' condition,
the computer responds by adding additional fuel, making a hot cat situation even worse. (!)

****

Alright, all that was somewhere between a mouthful and keyboard mush. Check out this
page from the Factory Service Manual where the O2 sensor is described:

You must be registered for see images attach

So where to go from here?

* Let's start with the everyday versus going straight for the exotic. The ignition systems in these trucks
are highly stressed, and due to the decreasing quality of most of the repair parts we're sold, it's
possible to have failures in the high voltage area not that long after they were installed? Especially
when it comes to the cap, rotor, coil, etc. Look carefully for signs where the high voltage sparks
have located/created an easier path to ground.

* Do you have a cylinder by cylinder misfire count available for us to look at? If we can
focus on which cylinders are failing under load then this will help.

* Could you list ALL the different ways that your vehicle can be operated and NOT
encounter misfires? Sometimes figuring out all the scenarios where the engine ain't misbehaving
will help to contrast when the engine is misfiring, and these differences will help us
decide which way to go.

* If you can capture the data when the misfires are occurring it would be good to see.
And any photos that may support the ongoing diagnosis (spark plug coloring, etc) are
welcome. Being remote, we only get to see what you share.

****

Have you taken advantage of downloading the Factory Shop Manuals for your truck yet?
(I'm assuming that you have since you've been reading the forum for awhile.)

Let us know what you discover. Full focus on the misfires under load. Actually misfires
at any time, but especially when the flashing SES is occurring. Then we'll play it by
ear once that's solved.

Best of luck.

Cheers --

Thanks for the warm welcome and in-depth response!

I should have included originally that this is a 5.7. Also, that 3 minute data snip was just from my drive to work today without any misfires or other problems triggered. I have cleared my codes many times while I've been dealing with these issues over the past many months. Each time hoping I made the correct fix.

I agree that collecting this data would be more useful while I'm experiencing a misfire event. On my way home tonight I will be sure to log the portion of my trip where, if I really give her the beans, I can make the engine miss. I'll post those snips on here for everyone to ponder.

To address your concern about the high voltage areas: all of those related components were the first things I replaced when all of this started. Since then, I've had many opportunities to assess what state they are in, and I have not seen any sign of arcing or any other electrical damage. That being said, I intend to check my cap and rotor again tonight anyway.

I am using a scan tool that does not allow me to detect which cylinder is misfiring. My only indication of a misfire event is feeling the car shake and/or getting the dash flash (which when scanned is almost always P0300 - multiple cylinder misfire).

All the ways that my vehicle can be operated and NOT encounter misfires: Any driving condition that does not put too much load on the engine. This is a problem that I get when I'm going up a steep hill real fast. I've also had it happen where I towed a trailer up a steep hill. To be clear, I am by no means putting this car through anything beyond what it's designed for. I have had this problem occur going the speed limit on the highway (70mph) on a particularly steep portion of said highway. I have not seen any other condition cause a misfire.

I will try to get some spark plug pics to post here this week, but probably not tonight unfortunately.

I have not downloaded the Factory Shop Manuals yet... I was not aware that was something you can do on here but I will certainly do that - thanks! Thus far I've been using the Haynes manual and YouTube for help.

One last note: this may be a separate issue, but my idle is usually getting close 1000 rpms. I have heard my IAC valve mysteriously clicking while I have the key on but not ignition on. The IAC harness has 11V which is lower that the other solenoids that consistently have darn near perfect 12V. Seems to be some very small voltage drop somewhere between the end of the harness and VCM. Is this related to the misfires? - seems like a stretch. But I thought it was worth mentioning nonetheless.

 

[Big_Red9]

So I drove her pretty hard on my way home from work. I floored it up a hill on the highway and, to my disbelief, I could not get it to misfire. However, the data shows some odd things. As you'll see in the attached pictures, I marked the sections where I floored it up the hill with red vertical lines. The time between the lines is about 1 minute for scale. I have a couple more minutes of recorded data but I shortened it to make reading it easy.

What I see from this data is that the fuel trim is within an acceptable range even while I've got the pedal to the floor. It appears that the upstream sensors are doing their jobs (for the most part) although maybe tending to read more rich. The downstream sensors are trying to hang in there but appear to follow the trends of the upstreams - very much like the cats aren't working quite right.

The abnormality is what happens directly after I get to normal highway driving after this heavy load event. The O2 sensors trail off to be almost the exact same readings that suggest an almost nonexistent level of O2. It's not shown in the attached pictures, but the O2 readings return to fluctuating after a few minutes.

Real strange! Hopefully this data is screaming something that someone with more technical know-how can hear. Thoughts?

P.S.
On this trip ol' Big Red also reached 160,000 miles so wish her a happy 160.

 

[Road Trip]

Thanks for the warm welcome and in-depth response!

I should have included originally that this is a 5.7. Also, that 3 minute data snip was just from my drive to work today without any misfires or other problems triggered. I have cleared my codes many times while I've been dealing with these issues over the past many months. Each time hoping I made the correct fix.

No problem, new members do this all the time. These GMT400s look all the same on the outside,
but there are unique 'features' with each drivetrain variation. Us old guys are never offended if you
refresh our memory of what you're driving and/or what you are working on.

I agree that collecting this data would be more useful while I'm experiencing a misfire event. On my way home tonight I will be sure to log the portion of my trip where, if I really give her the beans, I can make the engine miss. I'll post those snips on here for everyone to ponder.

When troubleshooting an 'intermittent', you really can't truly fix it *until* you figure out how
to break it at will. (!) So reading the above is good news, for you have figured out how to
stimulate the fault at will. For a lot of really tough problems, this is the hardest step of all.

To address your concern about the high voltage areas: all of those related components were the first things I replaced when all of this started. Since then, I've had many opportunities to assess what state they are in, and I have not seen any sign of arcing or any other electrical damage. That being said, I intend to check my cap and rotor again tonight anyway.

By coincidence we've been huddling around another member's truck with a similar issue.
Because of the fuel trims the decision was made to take the running truck down and
replace the intake manifold gaskets. (Common Vortec issue.) Everything went to plan,
he stuffs the distributor back in, but now a hard no start?

What he did shouldn't have caused any starting issue? Sure enough, after some head
scratching & a hard look at all the bits, if you follow this link you can see a subtle-looking
failure that was the root cause: (No start traced to dizzy cap failure)

And swapping in another cap cured the no-start, and he's back in the hunt. Don't know
what brand cap & rotor you are running, but there's lots of reported disappointment in this area.
(As in, high voltage stuff goes bad way too soon, Vortec dizzys cause weird problems and
are always suspect, etc.)

I am using a scan tool that does not allow me to detect which cylinder is misfiring. My only indication of a misfire event is feeling the car shake and/or getting the dash flash (which when scanned is almost always P0300 - multiple cylinder misfire).

This issue crops up from time to time. The affordable stuff calls *all* misfires a P0300.
A true P0300 is a valid, global fail, and sends us down a specific path. At the same time,
an accurate P0301/302/303/304/305/306/307/308 allows us to really focus on what's right
vs what's wrong. At some point, the decision becomes whether to load the parts blunderbuss
for the all-inclusive scatter shot vs. the one-time investment in stepping up to a 'report a
misfire down to an individual cylinder' capable scan tool.

All the ways that my vehicle can be operated and NOT encounter misfires: Any driving condition that does not put too much load on the engine. This is a problem that I get when I'm going up a steep hill real fast. I've also had it happen where I towed a trailer up a steep hill. To be clear, I am by no means putting this car through anything beyond what it's designed for. I have had this problem occur going the speed limit on the highway (70mph) on a particularly steep portion of said highway. I have not seen any other condition cause a misfire.

Solid diagnostic info - copy all.

OK, so I don't want to be guilty of leading the witness, but then again just to let you know
what is over the troubleshooting horizon, there is a very specific scenario where a SBC in
a GMT400 (among other GM lines) will misfire, but *only* under heavy loads, like climbing a
grade, pulling a trailer, etc. And this specific failure footprint only applies when every other
normal tune up part is as new, the engine never misfires at any other time, the compression
test and leakdown tests are good, and in short there's no reason for this to occur.

If this sounds like what you are up against, then the GM TSB PIP3081, dated 7/3/2004**
would be worth a read for comprehension:
(GM Technical Service Bulletin on valve stems sticking when SBCs are flogged, leading to misfire)

And for a hint as to how this could possibly happen, check out how the SBC cylinder heads have an
architectural hot spot designed into the center of the cylinder heads:

You must be registered for see images attach

(For comparison purposes, check out the BBC cylinder head symmetrical exhaust layout photo attached below.)


IMPORTANT NOTE: This is off the troubleshooting table until such time as every other possible
misfire under load possibility has been eliminated. But it's always good to know what the 'over the horizon'
worst-case scenario is out there. With any luck we will figure out a fix short of going there.

I will try to get some spark plug pics to post here this week, but probably not tonight unfortunately.

Excellent. If you could organize them exactly as they are arranged geographically in your
engine bay this would help us prove/disprove typical failure patterns. (Example)

I have not downloaded the Factory Shop Manuals yet... I was not aware that was something you can do on here but I will certainly do that - thanks! Thus far I've been using the Haynes manual and YouTube for help.

The FSMs are the definitive baseline for most everyone in here. Not perfect, but 99.9% better
than just winging it or using the off-brand manuals. Note: The '97 manual might be a few replies
in, but the '98 Manuals are there right at the beginning, and should prove to be helpful.
(GMT400 FSM cache)

One last note: this may be a separate issue, but my idle is usually getting close 1000 rpms. I have heard my IAC valve mysteriously clicking while I have the key on but not ignition on. The IAC harness has 11V which is lower that the other solenoids that consistently have darn near perfect 12V. Seems to be some very small voltage drop somewhere between the end of the harness and VCM. Is this related to the misfires? - seems like a stretch. But I thought it was worth mentioning nonetheless.

Just to give you a taste of how nice it is to have the shop manuals downloaded, here's what the (same) '99 FSM shows
for the electrical wiring between the VCM & the IAC. I would give every inch of those wires a careful visual inspection.

You must be registered for see images attach

This is definitely one of those cases where you always fix the obvious first, and oftentimes that will
also clear up the exotic failures. Even if we can't explain exactly why it happened. :0)

Summary: I'm throwing out all of the above purely as food for troubleshooting thought.

And we will need to verify that everything in the engine bay is good to excellent, including headroom
for insurance. (Think cooling system's ability to get rid of even more heat than you are generating
while flogging the old girl up the mountain pass with your trailer full of toys on the back.)

And at some point we may need to up our diagnostic toolage in order to crack this difficult nut.

Hope I was clear with what I'm trying to share. If you have any further questions, don't hesitate.

I'm off to read what popped up while I was writing my term paper.

Cheers --

(**Located and shared by @socal k1500 - tip of the hat in his general direction :0)