Misfires, fuel/air issues. Where to start?

[Road Trip]

PS: I'll do the O2 vs misfire "false lean" twisting up the feedback loop in a subsequent reply. This one is already too long.

Before diving down the misfire/false lean rabbit hole, I will first make the following statement:

If you have a misfire and your fuel trims & O2 sensor graph data are off, you have to take the
resulting fuel trims & O2 graphs with a grain of salt. Not saying to discount it entirely, but be
aware that the engineers who set up the Closed Loop would be the first to tell you that a misfire
is an illegal condition, and that the feedback loop can't correct for this...and might actually make
a bad situation worse.

It can be doubly hard to troubleshoot a twisted up A/F feedback loop on top of a misfire. If possible,
divide and conquer by getting the engine to run misfire-free Open Loop first, and only then reestablish the
Closed Loop and continue troubleshooting if needed/as necessary.

NOTE: When these trucks were new/nearly new these problems compounding each other was a rare thing.
24-36 years later, it's not out of the realm to have a marginal feedback loop (ie: tired O2 sensors) being
tormented by one or more misfiring cylinders.

****

Our GMT400s implement the ability to fine-tune the A/F ratios to stay very close to a 14.7:1 Air/Fuel
ratio despite changes in elevation, fuel formulation, rpm, load, aging injectors, etc. At the time, the
biggest bang for the buck was to use the narrowband O2 sensors described in the previous reply.

During normal operation, the feedback loop constantly hop-scotches back and forth endlessly across the
450 millivolt ideal. The healthier the feedback loop, the faster/tighter that the O2 sensor(s) on one end
and ECM/PCM/VCM on the other end can do this A/F ratio dance.

Let's assume that the #2 spark plug wire burns against the adjacent header tube, resulting in a loss of
spark and a misfire. Instead of the oxygen in the intake charge for the #2 cylinder being consumed by
a successful combustion, it instead gets pumped out the open exhaust valve and past the waiting O2
sensor. Of course the O2 sensor, seeing oxygen on both sides of the zircon membrane, generates
a matching lower (lean) voltage and sends that up to the computer.

In response, the computer attempts to 'correct' the too lean reading by steadily enriching ALL cylinders
on that bank. (Since there's only 1 upstream sensor per bank, they are all treated the same, with the
implied assumption that all cylinders *are* the same. So, thanks to the burnt spark plug wire on the
#2 cylinder, cylinders 4,6, & 8 are all being fed a too-rich mixture.

Depending upon just how sideways everything gets, once we cross the +150% emissions threshold,
the SES light will come on. And if the issue is bad enough that the engineers were concerned that the
cat will be damaged, the SES light actually starts flashing, alerting the driver to a serious condition
requiring immediate attention.

****

Years after the GMT400s were manufactured, more modern systems would identify a misfiring cylinder
that could damage the cat with unburnt fuel, so the computer would protect the cat by disabling the
fuel injector to that misfiring cylinder. Pretty cool. But not on our old trucks.

Misfires are bad juju. If Bank #1 has pretty upstream sensor graphs and reasonable fuel trims, but
at the same time the Bank #2 upstream sensor is not crossing the 450 millivolt threshold and the
fuel trims are stuck against the stops, then even if all you are getting is the P0300 misfire, you can
still tell that the problem is on the right side of your engine. ('96+ Vortec owners. The OBD1 TBI
system doesn't identify which cylinder isn't pulling it's weight -- you have to troubleshoot them the
old-school way. :0)

Hopefully I was able to explain this in such a way that you can use the 'goofed' live data to help
you narrow down where to look for the root cause. Maybe not pinpoint the problem, but still better
than nothing.

Food for A/F and misfire thought.

Cheers --

 

[L31MaxExpress]

Before diving down the misfire/false lean rabbit hole, I will first make the following statement:

If you have a misfire and your fuel trims & O2 sensor graph data are off, you have to take the
resulting fuel trims & O2 graphs with a grain of salt. Not saying to discount it entirely, but be
aware that the engineers who set up the Closed Loop would be the first to tell you that a misfire
is an illegal condition, and that the feedback loop can't correct for this...and might actually make
a bad situation worse.

It can be doubly hard to troubleshoot a twisted up A/F feedback loop on top of a misfire. If possible,
divide and conquer by getting the engine to run right Open Loop first, and only then reestablish the
Closed Loop and continue troubleshooting as necessary. NOTE: When these trucks were new/nearly new
these problems compounding each other was a rare thing. 24-36 years later, it's not out of the realm
to have a marginal feedback loop (ie: tired O2 sensors) being tormented by one or more misfiring cylinders.

****

Our GMT400s implement the ability to fine-tune the A/F ratios to stay very close to a 14.7:1 Air/Fuel
ratio despite changes in elevation, fuel formulation, rpm, load, aging injectors, etc. At the time, the
biggest bang for the buck was to use the narrowband O2 sensors described in the previous reply.

During normal operation, the feedback loop constantly hop-scotches back and forth endlessly across the
450 millivolt ideal. The healthier the feedback loop, the faster/tighter that the O2 sensor(s) on one end
and ECM/PCM/VCM on the other end can do this A/F ratio dance.

Let's assume that the #2 spark plug wire burns against the adjacent header tube, resulting in a loss of
spark and a misfire. Instead of the oxygen in the intake charge for the #2 cylinder being consumed by
a successful combustion, it instead gets pumped out the open exhaust valve and past the waiting O2
sensor. Of course the O2 sensor, seeing oxygen on both sides of the zircon membrane, generates
a matching lower (lean) voltage and sends that up to the computer.

In response, the computer attempts to 'correct' the too lean reading by steadily enriching ALL cylinders
on that bank. (Since there's only 1 upstream sensor per bank, they are all treated the same, with the
implied assumption that all cylinders *are* the same. So, thanks to the burnt spark plug wire on the
#2 cylinder, cylinders 4,6, & 8 are all being fed a too-rich mixture.

Depending upon just how sideways everything gets, once we cross the +150% emissions threshold,
the SES light will come on. And if the issue is bad enough that the engineers were concerned that the
cat will be damaged, the SES light actually start flashing, alerting the driver to a serious condition.

****

Years after the GMT400s were manufactured, more modern systems would identify a misfiring cylinder
that could damage the cat with unburnt fuel, so the computer would protect the cat by disabling the
fuel injector to that misfiring cylinder. Pretty cool. But not on our old trucks.

Misfires are bad juju. If Bank #1 has pretty upstream sensor graphs and reasonable fuel trims, but
at the same time the Bank #2 upstream sensor is not crossing the 450 millivolt threshold and the
fuel trims are stuck against the stops, then even if all you are getting is the P0300 misfire, you can
still tell that the problem is on the right side of your engine. ('96+ Vortec owners. The OBD1 TBI
system doesn't identify which cylinder isn't pulling it's weight -- you have to troubleshoot them the
old-school way. :0)

Hopefully I was able to explain this in such a way that you can use the 'goofed' live data to help
you narrow down where to look for the root cause. Maybe not pinpoint the problem, but still better
than nothing.

Food for A/F and misfire thought.

Cheers --

The injector shut down with misfire would be a benifit of the 0411 swap although I feel like the service manual states my 97s black box did the same. The PCM will shut the injector off, then intermittently turn it back on in the event it is a intermitent condition. If the misfire went away naturally it will keep spraying fuel, if the misfire remains it will turn the injector off again for the set time period before attempting to enable it again. The 0411/P59 will both disable closed loop due to misfire as well.

 

[Zimmerly]

Hey, thank you guys so much.
I appreciate the input and the learning lessons.

PS: I'll do the O2 vs misfire "false lean" twisting up the feedback loop in a subsequent reply. This one is already too long.
:0)

Not at all! This is why I come to this forum. Until 2020, I knew very little about vehicle work. I bought my first GMT400 with the idea of just having "a truck". I needed new cv axles and had the idea of replacing the drivers seat. One thing literally led to another, mostly powered by the concept "while I'm already doing this, I might as well do this too".
Before I knew it, a year had passed, and I had replaced everything down to the heads of the engine, replaced at least 75% of the engine bay, rebuilt the entire front end suspension, swapped to the F44 12b rear end, took my frame down to bare metal (painted + RP-342 inner frame), replaced my entire interior, did some of those gmt400-specific uprades (things like nbs master cyl, temp/compass mirror), lots of other things in-between... My point is, I just wanted "a truck", specifically like my dad had growing up. Once I got involved though, I kinda fell for the truck, along with the gmt400 model. A LOT of that can be credited to this forum.
The information you can find about literally anything involving these trucks just blows my mind, and the intelligent people we have here who welcome other gmt400 owners with open arms is just incredible. I could not have done half of the work on my 1st 98 if it wasn't for this forum, and I'm completely serious about that. Youtube and google are helpful, but I've tried that with other vehicles in the past (and still try to) -and don't get me wrong, there are some other helpful vehicle-specific communities out there- but the endless information on this page, along with the good-hearted folks here who take chunks of time out of their life to share their own experiences and properly educate other truck owners is simply unmatched.
I've posted here before once in awhile, but this is the first real involvement I've had with the forum that involves my own problem, and I have to say, you guys didn't let me down. I will take as much information and education I can get, and I really appreciate the time you guys have taken to address this total stranger's truck problem. It literally makes my day.

I will take the advice given and test the system with those two sensors taken out.
I'll be replacing them all either way.

Thanks again! I hope you all have a great weekend!
-Jacob

 

[Road Trip]

I will take as much information and education I can get, and I really appreciate the time you guys have taken to address this total stranger's truck problem. It literally makes my day.

Good to hear! It is GMT400 owners with your mindset that is the target audience of all this stuff.

These trucks are too well engineered to be sent to the scrapyard before they are all used up, but
as time goes by fewer & fewer garages will have the requisite talent to fix them. (So many places are
slowly regressing to the point where if there's no DTC there's no fixing them. And the places that
can still troubleshoot have more business that they can handle. (!)

So if you are willing to add to your skillset & become more self-sufficient, that's savvy.

I will take the advice given and test the system with those two sensors taken out.
I'll be replacing them all either way.

It may seem counterintuitive to pull the rear O2 sensors first, but in the past we've had
scan tools that mixed up these sensors when reporting, so I want to see 2 sensors, each
identified as:

* Bank 1, Sensor 1
* Bank 2, Sensor 1

And once they warm up, (assuming no/small handful of misfires) we want to see them both
dancing like the green graph in the previous reply. I half expect that you will have 1
active, but the other one not crossing the 450 millivolt threshold. And if we're just dealing with
aged O2 sensors, then installing new ones will cause both sensors to once again work similar to
each other.

And this is when you install the new rear sensors, and with any luck the scan tool (or app)
display will now be rational for all 4 sensors.

And if this doesn't happen, then the fact that you are posting live data means that as a
group we can come up with a diagnostic strategy that will sort this out in the fewest moves.

Good stuff. Looking forward to your next status update.

Cheers --

 

[Zimmerly]

And this is when you install the new rear sensors, and with any luck the scan tool (or app)
display will now be rational for all 4 sensors.

Thanks, Road Trip!
After doing some reading, I decided to go with the Denso sensors. I've read so many bad things about Bosch, and ACDelco brand is just a tad more expensive than I can comfortably buy at the moment. (Some unforseen obstacles in life have me pinching pennies). I still want a decent brand. I see some good reviews on Denso sensors, and looks like they are OEM for Toyota, so I feel good about them.
What do you think?

Also, taking a look at the engine bay this morning, I thought I'd troubleshoot my "Oil guage at max" issue. The sensor was unplugged entirely. When I bought it, I noticed there was no hold-down nut for the upper intake tube, and the AC compressor was unplugged. I think maybe I'll dive into it tonight and see what other fun easter eggs I can find.
I'll let you know how it goes!

Thanks again!
-Jacob

These are the oxygen sensors I'm looking at. Let me know what you think:

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[Road Trip]

Thanks, Road Trip!
After doing some reading, I decided to go with the Denso sensors. I've read so many bad things about Bosch, and ACDelco brand is just a tad more expensive than I can comfortably buy at the moment. (Some unforseen obstacles in life have me pinching pennies). I still want a decent brand. I see some good reviews on Denso sensors, and looks like they are OEM for Toyota, so I feel good about them.
What do you think?

I've personally had good luck with Denso sensors over the years. And I've also had unpredictable results
with the Bosch sensors, (some good, some incompatible?) so I tend to avoid those. Hopefully others can
chime in with their first-hand experience in this area.

These are the oxygen sensors I'm looking at. Let me know what you think:

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The only caveat I see in the above is that this is a Amazon listing. Elsewhere in the forum there was a discussion of just how many counterfeit parts
are showing up on Amazon & ebay auctions. Essentially if the part is available on Alibaba then you need to be extra cautious.

The recommended course of action is to either purchase from a brick-&-mortar store, or a dedicated auto parts website like Rock Auto. I just checked,
and they have the Denso listed for your vehicle under the Daily Driver category:

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(LINK)

So there you have it. Denso has been good for me, and as long as you don't get
a counterfeit one it should work for you. Wish I could find the thread where the
discussion of how Amazon practices the 'binning' of the same part number parts
from different sellers into a common spot, working on the theory that all things
with the same part # are all equally legit.

But we are discovering that today things are not the same as it ever was. FWIW
I've been a Prime member for a long time, and I appreciate the convenience...but
now I tend to use Amazon only for non-mission-critical bits. :0)

Hope this didn't muddy up the waters too much.

Best of luck --

 

[Zimmerly]

Wish I could find the thread where the
discussion of how Amazon practices the 'binning' of the same part number parts
from different sellers into a common spot, working on the theory that all things
with the same part # are all equally legit.

Hope this didn't muddy up the waters too much.

I was actually waiting for some feedback before I hit the "Check out" button, and I'm glad I did.
I did read another post on the forum regarding the sellers themselves shipping counterfeit products. These sensors were both sold by "Amazon.com" as the seller, so I dug into that and felt assured that they're forbidden to sell counterfeit parts... but if they don't KNOW they're counterfeit (or at least play dumb about it), that's another story.
And yes, the reason for Amazon was the convenience and their usually lower prices. This information is really an eye-opener.
I will think twice about buying crucial parts through them from now on.
I actually couldn't find an upstream Denso sensor on Rockauto, and the downstream one states "exc. Canada built", and mine was built in Canada.
What about universal sensors? Are there any you would recommend, preferably comparable price? Or should I get vehicle specific? Sorry so many questions, I've been reading a lot of contradicting things on pages lately.

 

[Zimmerly]

Well, I crawled underneath the passenger's side of the truck a little while ago and immediately noticed some funky stuff going on with the wiring to the fuel tank. There is 1 ground wire screwed to the crossmember, and a connector left over beside it with no wire going to it. There's a black and a gray wire coming from the tank that have been cut. I found another post on this forum regarding the wiring colors. I have to crawl back under and see if the wire colors make sense according to the photos I found from someone's delphi pump instructions. I am hoping the fuel level issue is simply a wiring problem.
I need to figure out what's going on with what seems to be extra wiring down there.
someone obviously replaced the pump unit. There are 2 grays. One is connected, one is cut, but had been previously spliced, telling by the shrink connector. Something tells me someone was having issues figuring things out.

Question-
Do you think any other issues could have been caused by this wiring?

I'll be warming up the truck soon to test the sensors after pulling the 2 downstream. I'll report back.

Thanks!
-Jacob

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[Schurkey]

There's a black and a gray wire coming from the tank that have been cut.

Does the fuel pump run? Those are the colors for the fuel pump power (gray) and fuel pump ground (black) wires according to my '97 C/K service manual.

Purple, and Orange/Black stripe should be the fuel gauge sending unit--although I'm getting kinda confused looking at the wiring diagram in that '97 C/K manual.

At some point in time, the fuel level system in the tank stopped driving the gauge directly. Instead, it sent a signal to the computer, the computer sent a signal to the gauge in the instrument cluster. I think this changed in '97. When it did, the gauges became stepper-motor type instead of the older balanced-coil type, and the stepper motor gauges are infamous for failure.

 

[Zimmerly]

After taking another look, it seems as though someone ran a seperate wire for the fuel pump feed and ground, and I assume it's because they weren't getting power to the pump.
Would I be out of line to assume the purple and orange fuel level wires are dead too, and they only hooked up the pump to simple get it running?
I'm going to take a test light to the harness and see if I even have power going to the level sensor. If not, I'll start at the other end of the plug and work my way back. Any other suggestions?

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