1997 Chevy Suburban K1500 P0420

[evilunclegrimace]

After seeing everyone's input here today I decided to pony-up and buy a United Motor Products premium cap and rotor from RockAuto. It'll be here by Monday and I will follow the installation with some road testing. Before then (maybe tonight) I'll post some pictures of my spark plugs. Stay tuned!

Make sure that you order the VENTED cap.
PN. DCR820X

 

[Big_Red9]

Good evening folks,

I have all the spark plugs out as you can see in the attached photo. #4 was the only one that came out looking a bit wet. The rest were pretty much in the exact same condition. They all had a gap of 0.06" which is no different than when I put them in. I don't think their state is alarming to me, but let me know what you guys think. I'm only a bit worried about #4. It's been a while since I've had a P0300 code and I don't ever think I've had a misfire code for #4. Maybe the new cap & rotor will clear this up?

 

[Big_Red9]

Make sure that you order the VENTED cap.
PN. DCR820X

I made sure to get the vented one. I should've also included that info.

 

[Road Trip]

Good evening folks,

I have all the spark plugs out as you can see in the attached photo. #4 was the only one that came out looking a bit wet. The rest were pretty much in the exact same condition. They all had a gap of 0.06" which is no different than when I put them in. I don't think their state is alarming to me, but let me know what you guys think. I'm only a bit worried about #4. It's been a while since I've had a P0300 code and I don't ever think I've had a misfire code for #4. Maybe the new cap & rotor will clear this up?

Thanks for posting your plugs in a remote-friendly manner, keeping track of
which cylinders they came out of, and also arranging them geographically.
It helps!

****

Speaking of working remotely like this, it's easy for me to give off the wrong impression,
especially about troubleshooting philosophy. While I may be a completist when it
comes to considering *all* the theoretical possibilities, by the same token I am a firm
believer in a measured response implemented with a conservative building block approach.

My personal perfect-world unattainable troubleshooting goal always is: 1 problem, 1 part, 1 fix.

At one troubleshooting extreme, zero parts changed while analysis leads to paralysis.
At the other extreme, 100% empirical testing without trying to prove/disprove a theory.
(aka: parts cannon approach - swap parts out and see what happens.)

And with a lab full of calibrated test gear + a rolling road dyno we can figure out what the
problem is down to an individual component. The next best thing is on the road with a
multichannel storage scope where the data channels can be graphed against each other
and then be run back and forth in time around the areas of interest. Really cuts down
on the guessing.

Next best is capturing data during a failure and upload it like you are doing.
Much, much better than just a list of stored codes retrieved with a code reader.

So what am I trying to describe with all that jibber-jabber? We may be forced
to analyze the data / change a part based on the analysis / evaluate / build
on the base by adding a new component / evaluate / repeat until the
driveability goal is obtained. It may sound inelegant, but some empirical
testing based on the best data we have to work with will eventually get
us there. (Even if the local dealerships or garages have given up on it.)

****

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.

Back to your spark plugs. On (6) of the plugs the color is as expected on
a closed loop Vortec motor. The worst-looking plug (#4) just happens to
be physically adjacent to the next worst (#6) plug? Is this coincidence, or
do we need to pay close attention to this during our ongoing troubleshooting?

I think the latter is the most prudent approach.

At this point I am going to ask you to replace the plugs, and further I am
going to ask you to discontinue using the old factory spark plug gap spec of
.060" and instead run a .045" gap. (The thought process here is give the
Ignition System the best possible chance of delivering a robust spark even
when you are working your engine towards it's maximum torque production,
which = the highest cylinder filling = the highest probability of blowing the
spark out. More to support this request later.)

With .045" gaps and the rest of the ignition system in a fresh state of tune,
now we have 2 ways of collecting the data. The first (ideally) will be with a
live data capable Scan tool that can differentiate between individual cylinder
misfires vs just aggregating them all under the random-misfire P0300.
(A genuine P0300 is an indicator of a global issue affecting all cylinders, versus
a P0304, where we have an issue specific to one hole in the block. (!)

And the second piece of data? That would be the ability to periodically pull
that #4 plug 500 or 1000 miles down the road and compare it to one of
the pretty cylinders. Does it still have an unexplained difference in coloring
as compared to the good cylinders? Are you using *any* coolant? Zero
coolant used is the allowable limit.

If you are using *zero* coolant, then we need to look at the fuel delivery
system closer, and figure out if cylinder #4 is being fed by a drooling
injector?

One more easy check. I'm assuming that your ignition timing is correct,
but *in addition* what is the state of your CMP <> CKP 'Cam Retard'
signal? It's need to be as close to -0- degrees as you can get it. Especially
since we're troubleshooting a misfire under heavy load. (If you can't get
this reading into spec we'll discuss this more when we cross that bridge.)

****

Conventional wisdom would be to focus on insufficient fuel flow during heavy
demands. But in your original posting you mentioned the fuel pressures that
you had while driving, and they look alright. More importantly, if I were to blame
the fuel delivery I would have to explain why the fuel trims look so good, especially
during the 2nd set of graphs you shared.

Hopefully this explains why I am currently trying to get the Ignition System to be in
as good condition as your fuel delivery. And assuming you are not being goofed on
by a new (bad) coil and/or control module, then thanks to the lower KV required to
jump the .045" gap, we will end up in 1 of 2 places:

* No codes, no misfire, NO coolant usage at all, proceed to Easy Street & enjoy a victory brew.
* Still misfiring, codes to work with, cylinder specific misfire data, and fresh plugs to read.

...And only after we're sure that we've got the problem found/fixed...see if the P0420 has
disappeared or not.

More on justifying the .045" recommendation in a bit.

Cheers --

 

[Road Trip]

More on justifying the .045" recommendation in a bit.

Normally it seems prudent to not discuss politics, which is the best oil, religion, or spark
plug brands & gaps. Seems like Belief System stuff is the 3rd rail of most forums. :0)

But since we're going to methodically troubleshoot this misfire, I gotta go there.

So as to write a chapter instead of a book, here's the bulleted version of
what I found in the Vortec spark plug gap rabbit hole:

* '88-'95 TBI V8s left the factory with spark plugs featuring a .035" gap.
A lot of them still run around just fine on .035" gaps.

* The '96+ Vortecs left the factory with plugs gapped to .060".
Jumping this larger gap necessitates much higher KV (KiloVolts) to do so, and
the higher the psi in the combustion chamber the more KV it takes to reliably
jump the gap. This puts additional stress on the *entire* high voltage
generation/distribution circuit. (Coil > Rotor > Cap > Wires.) And as evidenced
by what we are all reading in this forum, lots & lots of tough to troubleshoot intermittents
stem from this aggressive gap spec.

* Back in '03, GM released a TSB that changed the recommended gap from
.060" to .045". (TSB 03-06-04-060, dated 04-Jun-2003) Note: This took
some digging. Interestingly, this TSB was aimed primarily at the LS motors?

* If you look up your '97 K1500, you will be shown the NGK TR55. Pre-gapped to: .059"

* The plug that was released for the TSB above is the NGK TR5. Pre-gapped to: .039"
Note: According to the NGK website, all other parameters besides the gap are the same.
And NGK says that either plug can be safely gapped +/- .010" from as-manufactured.

* I read with interest about a dyno tuner who would open up spark plug gaps in the
hunt for mo' power until he encountered heavy throttle misfires, and then close
them down .010-.015". And he ended up in the .045" neighborhood. (!) His
commentary was intelligent, and I liked the fact that he let the engine tell him what
was working best instead of just going with some spec. (He was wanting to open
up the gaps as much as possible without making the engine fussy or finicky.)

* To see what the heavy hitters in the Vortec world were doing to maximize their toys,
I read a few offshore/marine/boating and a couple of RV sites where the motorhomes are
based on the P30 chassis. (Normally 7.4 or 8.1) I found several anecdotal stories, and
this one is typical of what I read:

You must be registered for see images attach

* NOTE: In the FSM it's documented that the SES light won't illuminate until the issue raises the emissions by an additional
50% over the original emissions specification. (This is the EPA rule that all manufacturers coded to.) So when I read of someone
experiencing misfires that they can feel but the light doesn't come on, I believe them. Shoot, when the chore truck first
followed me home I went from SES constantly on, to SES intemittently on, to SES off but I could still feel intermittent misfiring,
to finally no light & no misfire. (Took 4 passes to sort it all out.)

* The 8.1 story above references AC-Delco plugs. In the dozens of screens that flew by, I thought I saw that back in the '09
timeframe that Delco-Remy sold off the actual spark plug production to NGK? I don't know what changes if any have occurred
since then, but what I do know is that I've used all manner of V-groove NGKs since the '90s and have been very satisfied with them.
(And I'm pretty fussy about this stuff. :0) So I don't know if AC-Delco plugs come out of the same plant as the NGKs or not?

* Before I forget, there was an interesting saga in a boating forum about a freshly rebuilt Vortec 350 (Mercury marine?)
where it would run a steady 4000 rpm full throttle for ~20 minutes, and then suddenly bad sounds would happen, the power would
fall off, and and then after waiting a couple of minutes, the owner would throttle up, and the engine would start running full throttle
normally again for a bit, but fail again after an ever decreasing amount of time for the 2nd, 3rd, and subsequent failures?
After trying many things, the problem was troubleshoot to the Ignition Coil heating up after being run for many minutes
at full power, and then shorting out through the side of the case? And the arcing to ground was the cause of the sound?
(This particular boat owner was tenacious, for in most of the threads like this the boat owners with similar issues (and after
sinking boatloads of money into the troubleshooting :0) ...would just give up and sell their toys for a loss.

* Last & least. For the past year my '99 454 C2500 has been running cheerfully on a set of plugs gapped to .045",
replacing a set to worn plugs that measured in the ~.064" range, plus or minus a handful. Pleased as punch with
the results.

****

Where do we go from here?

* Install a fresh set of NGK TR5 plugs. Ensure that the spark plug wires are dressed just like the
Factory Service Manual has it drawn out. Or cheat and look at @L31MaxExpress's dressed wiring
harness photos & emulate.

* And even if there's no P1345 code, reverify that your CMP<>CKP synchronization is in spec. We
don't want to risk crossfiring inside the cap to the wrong spark plug terminal because the rotor tip/terminal
physical phasing is off.

I'm sure I could scrape up some more trivia and load it in here, but this feels
like a good place to stop for now.

Q: And why am I being so fastidious about all this?

A: Because if we have Solid fuel delivery + a Robust, blowout-resistant spark,
IF it still goofs up then we will have to start looking at the remaining
member of the triad: Dynamic Mechanical Compression.

If you have any questions about the above, please don't hesitate to ask.

Cheers --

PS - I attached a couple of Advance Auto screens. Try as I might, it
wouldn't let me request a TR5 plug for a '97 K1500...but did come
back with a .060" gap Ruthenium job for ~$14 each? Hey, the tuna is
fine, I don't want the lobster, but thanks for offering. :0)

 

[Schurkey]

How old are the upstream O2 sensors?

Maybe the fuel trims look so wonderful because the O2 sensors are "telling stories" to the computer that may not be true.

I was going to suggest putting downstream O2 sensors in before condemning the catalysts. But you've got that handled already. Not sure I'd have bought cheapie, off-brand O2 sensors, though.

Yes, cap, rotor, smaller-gap spark plugs. See what happens. Assure the ignition coil can reliably fire a spark tester calibrated for HEI, with the coil hot from being run for 20+ minutes, and preferably misted with water from a squirt-bottle.

And definitely a better scan tool.

 

[barry_g]

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.

EGR.

 

[Big_Red9]

Hello again,

I apologize for not checking in sooner; I've been plenty busy between work and school. I have some updates for the community:

1.) I've been driving around with my new distributor cap & rotor. It may be in my head, but I feel like she likes it better than the last one. I don't have anything quantitative to back that up, but I know I won't be changing that anytime soon.

2.) I have more data to share. I floored it on the way home from work today going up the usual steep hill. I was able to get a misfire which is bittersweet news. I don't notice anything different from this new data and the other data I posted last week. And I do still notice the car go into Power Enrichment mode (thanks again for the knowledge Road Trip). Still seems like there may be a possible spark problem which brings me into my next point.

I am not too keen on buying 8 spark plugs for $15 a piece. However, I do think it would may be worth a try to shorten my spark plug gap. I am willing to buy the following spark plugs from RockAuto for only ~$3: DENSO 4511 Platinum TT Preset gap = 0.04". I would then gap them to 0.045".

I know this may make many of you sigh in disappointment, but my thinking is if I put those in and they fix my problems then I'll definitely upgrade to some good ones.

Let me know what you guys think. If there's something too flawed with my logic or I'd be better off doing something else first then I'd like to hear it.

 

[Big_Red9]

EGR.

I have been able to verify that my EGR valve works. I hooked the pins up to 12V while I had it out and saw that it actuates. I also cleaned it up and checked the wire harness with a multimeter. I don't think this is what's causing my problems.

 

[Road Trip]

Hello again,

I apologize for not checking in sooner; I've been plenty busy between work and school. I have some updates for the community:

1.) I've been driving around with my new distributor cap & rotor. It may be in my head, but I feel like she likes it better than the last one. I don't have anything quantitative to back that up, but I know I won't be changing that anytime soon.

2.) I have more data to share. I floored it on the way home from work today going up the usual steep hill. I was able to get a misfire which is bittersweet news. I don't notice anything different from this new data and the other data I posted last week. And I do still notice the car go into Power Enrichment mode (thanks again for the knowledge Road Trip). Still seems like there may be a possible spark problem which brings me into my next point.

I am not too keen on buying 8 spark plugs for $15 a piece. However, I do think it would may be worth a try to shorten my spark plug gap. I am willing to buy the following spark plugs from RockAuto for only ~$3: DENSO 4511 Platinum TT Preset gap = 0.04". I would then gap them to 0.045".

I know this may make many of you sigh in disappointment, but my thinking is if I put those in and they fix my problems then I'll definitely upgrade to some good ones.

Let me know what you guys think. If there's something too flawed with my logic or I'd be better off doing something else first then I'd like to hear it.

Hello Big_Red9,

I'd definitely try the Denso plugs for $3 each. Putting those $$ per plug Ruthenium jobs in our Vortec motors is like
putting caviar on a hotdog. The common sense rule of thumb is you go with the affordable copper plugs if
you are troubleshooting a new (to you) engine or still doing the iterations on a custom tune. Especially if you can get to the
plugs without major disassembly. (What really matters is the heat range, not so much how exotic the metallurgy is,
especially for our naturally-aspirated pushrod V8s.)

On the other hand, if you are running a factory turbo or supercharged engine you have to run whatever the factory specifies. Or
if you have to pull the intake manifold to get to the plugs, then by all means you step right up to the 100K mile platinum or iridium plugs.

****

Given all this, those $3 Denso platinum plugs will be more than good enough to see if the .045" gap
is setting you up for success or not.

Make it so.

:0)