Misfires, fuel/air issues. Where to start?

[Zimmerly]

Hey guys.
98 K1500 ext cab, 5.7 Vortec.
Background of myself, I own a truck identical to it that I rebuilt a lot of. No schooling, self-educated mechanic with fairly good knowledge of the GMT400.

Now on to the truck... I just bought it a week ago. It runs a rough idle, smells rich, and has noticeable misfires. Looking around, I notice a new nylon main fuel line from the tank to the engine bay. The tubes going into the injectors look newer as well. I can tell the evap tubes have been messed with, because the upper one isn't routed the way it should be around the upper rad hose. The lower tube looks to have been replaced. Looking back on the driver side frame rail, I notice the fuel vent tube has been cut out, from the drivers door to about 1ft before it becomes rubber near the tank, so there is no vapor going into the evap canister. I have a feeling this isn't my main issue, but then again I'm not a pro. The previous owner said he just installed a new cap and rotor, and I know he did (or at least someone did), because it's black, while the rest of the distributor is still the stock gray color. He said he [thinks] it needs a new ignition coil. I've already replaced the PCV valve because it was rattling while running, and I thought there was a long shot that might've been what I needed, but of course not.
I used my Bluetooth scanner with Dashcommand, and found lots of misfires. A little on bank one, but a LOT on all of my even numbered cylinders, which I found interesting.
Since my other truck and engine are identical in model and year, I plan on swapping coil units to see if that's the case, but I'm having my doubts. I don't want to just throw parts at it based on personal "hunches", so I came here for help. I've turned to this forum so many times, and to be honest, I would not have been able to complete a LOT of projects on my build, if it wasn't for this great community. Anyway, enough of the sappy crap.
I just don't know where to start. I do know it's getting too much air and not enough fuel. I checked all air connections and tubes. Everything seems put together. It's leaking oil, and looking in back of the intake I see some oil next to the distributor, so I suspect it needs a gasket.

Here we go with the codes-

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These are the codes I'm getting:

PO300 -Engine Misfire Detected
P0452 -Fuel Tank Pressure (FTP) Sensor Circuit Low Voltage
PO463 -Fuel Level Sensor 1 Circuit High Voltage
P0174 -Fuel Trim System Lean Bank 2
PO401 -Exhaust Gas Recirculation (EGR) Flow Insufficient
P1153 -HO2S Insufficient Switching Bank 2 Sensor 1
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If someone could lead me in the right direction, it would be great. I'm ordering some tubing to patch up my vent to canister line and I'll go from there. Thanks in advance!

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[GoToGuy]

Get out the your fsm and start diagnostics for O2 sensor, fuel tank, EGR testing. Fuel pressure test. What do plugs look like? That's what I'd try first. I'm sure others will have good info also.
Did you pull upper to see injector lines? Or you mean fuel supply lines.

 

[Schurkey]

98 K1500 ext cab, 5.7 Vortec.

It runs a rough idle, smells rich, and has noticeable misfires.

Spider injection and intake manifold gaskets are immediately suspect. They're not the only possible causes, but the small-block Vortecs are infamous for those issues.

Looking back on the driver side frame rail, I notice the fuel vent tube has been cut out, from the drivers door to about 1ft before it becomes rubber near the tank, so there is no vapor going into the evap canister.

There's your P0452.

The previous owner said he just installed a new cap and rotor, and I know he did (or at least someone did), because it's black, while the rest of the distributor is still the stock gray color.

Verify ALL the usual tune-up items and procedures, including the "new" distributor cap. Vortecs are known for distributor and distributor cap failures.

He said he [thinks] it needs a new ignition coil.

TEST the coil using a spark-tester calibrated for HEI. There are many styles and suppliers, this is my favorite kind:
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

found lots of misfires. A little on bank one, but a LOT on all of my even numbered cylinders, which I found interesting.

More evidence of a failed intake gasket. Not guaranteed...but worth looking into.

I plan on swapping coil units

Ignition coils affect ALL cylinders.

TEST BEFORE REPLACING. "New" coils may be lower-quality than the one you have already.

I just don't know where to start. I do know it's getting too much air and not enough fuel.

HOW do you know that? Have you tested fuel pressure at prime, at idle, and under load? Are the O2 sensors functioning properly? Remember that O2 sensors report FALSE "Lean" signal when there's misfire.

It's leaking oil, and looking in back of the intake I see some oil next to the distributor, so I suspect it needs a gasket.

More-likely, an oil-pressure sending unit/switch, or perhaps the extension tube is leaking or cracked.

MAYBE valve cover gaskets, the RTV Silicone seal at the rear of the intake manifold, or some other leak, or combination of leaks.

PO300 -Engine Misfire Detected

Is this the REAL code, or is your scan tool not capable of displaying individual cylinder misfire codes--P0301--P0308.

P0452 -Fuel Tank Pressure (FTP) Sensor Circuit Low Voltage

Missing tube to canister.

PO463 -Fuel Level Sensor 1 Circuit High Voltage

In-tank fuel sensor issues? I'd clear it and see if it comes back.

P0174 -Fuel Trim System Lean Bank 2

Probably FALSE lean due to misfire. How old are the O2 sensors?

PO401 -Exhaust Gas Recirculation (EGR) Flow Insufficient

Verify ENTIRE EGR system from the exhaust manifold through the corrugated tube, all the passages in the intake manifold, and the EGR valve itself. Easy to get a plugged EGR passage that restricts flow.

"I" would do this during the removal/replacement of the intake manifold, which you're taking off to change the gaskets.

P1153 -HO2S Insufficient Switching Bank 2 Sensor 1

Again, false report due to misfire. But replace the O2 sensors (ALL of them) if there's any question about their age/mileage.

 

[Zimmerly]

Wow. I did not expect such quick responses. I also still have a lot to learn when it comes to diagnostics and using specialty tools to do so, but with that said, I am familiar with all the unique quirks to these trucks, and common issues with them and the 5.7 in general. I'm NOT familiar, however, with things like what false codes are thrown, or what process to take in troubleshooting more complicated issues like this.
Thank you guys. I will try to reply the best I can.

Get out the your fsm and start diagnostics for O2 sensor, fuel tank, EGR testing. Fuel pressure test. What do plugs look like? That's what I'd try first. I'm sure others will have good info also.
Did you pull upper to see injector lines? Or you mean fuel supply lines.

I don't have an fsm. I haven't come across an issue that required one until now. Ignorance on my part. I will do my research on what I need to do to test these things.
The plugs look new, but I will have to take them out and inspect them. And the nylon supply line is new, and injector lines are also newer.

Spider injection and intake manifold gaskets are immediately suspect. They're not the only possible causes, but the small-block Vortecs are infamous for those issues.

-Yes, I considered intake gasket being a possibility.

There's your P0452.


Verify ALL the usual tune-up items and procedures, including the "new" distributor cap. Vortecs are known for distributor and distributor cap failures.

I had also wondered about this, and if maybe he even installed it incorrectly.

TEST the coil using a spark-tester calibrated for HEI. There are many styles and suppliers, this is my favorite kind:
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

I will have to order this right away, thank you. When it comes to diagnostic tools, I'm clueless

More evidence of a failed intake gasket. Not guaranteed...but worth looking into.

If it comes down to it, I goota do what I gotta do.

Ignition coils affect ALL cylinders.

TEST BEFORE REPLACING. "New" coils may be lower-quality than the one you have already.

Luckily I have an identical truck and can swap the units over if need be. By what I'm hearing, it's sounding like it could be something else. Bank 1 misses but not nearly as much as bank 2 (photo in original post).

HOW do you know that? Have you tested fuel pressure at prime, at idle, and under load? Are the O2 sensors functioning properly? Remember that O2 sensors report FALSE "Lean" signal when there's misfire.

I guess I don't know that for a fact, I think my ignorance and conclusion-jumping just saw that the lean code, pressure code, and egr code were thrown, so I admittedly was just assuming based on those. I'm not the most familiar with false codes and what could cause them.

More-likely, an oil-pressure sending unit/switch, or perhaps the extension tube is leaking or cracked.

I just replaced my oil pressure sensor in my other truck that was spewing 1qt every 10mi, so I inspected that first, and it isn't wet. It's the most wet all the way around the base of the distributor shaft.

MAYBE valve cover gaskets, the RTV Silicone seal at the rear of the intake manifold, or some other leak, or combination of leaks.

I did the intake on my other 5.7 and this did cross my mind. Would it make sense to try tracing down the leak first to see if it's the intake? Would a simple intake vaccuum leak be the cause of all the issues, do you think? I know it might be wishful thinking for that to solve this, but it would be nice.

Is this the REAL code, or is your scan tool not capable of displaying individual cylinder misfire codes--P0301--P0308.

I couldn't say for sure if it has the capability. I do know I used this scanner and app because it had the ability to read my cam retard offset when reinstalling the distributor in my other 5.7, and can also read individual cylinder misfires in real time. 2 4 6 and 8 all misfire like crazy, the odd cylinders misfire occasionally comparatively.

Missing tube to canister.

Well, that checks that code off the list. I'll be installing a new vent line to the evap canister asap.

In-tank fuel sensor issues? I'd clear it and see if it comes back.

I've cleared the codes, and they came back during a trip to the store.

Probably FALSE lean due to misfire. How old are the O2 sensors?

It's hard to say how old they are on this dirty Wisconsin truck.

Verify ENTIRE EGR system from the exhaust manifold through the corrugated tube, all the passages in the intake manifold, and the EGR valve itself. Easy to get a plugged EGR passage that restricts flow.

"I" would do this during the removal/replacement of the intake manifold, which you're taking off to change the gaskets.

Again, false report due to misfire. But replace the O2 sensors (ALL of them) if there's any question about their age/mileage.

Would you recommend just doing intake gaskets, O2 sensors and egr cleaning right away, or should I test coil spark and fuel pressure before then? I have to get my hands on the tools before I do that.

Thank you guys very much for your quick responses!

 

[Zimmerly]

Well, another day gone by. I hooked up my scanner this morning and cleared my codes before driving to work, and hooked it up again before the drive home from work. I had a few misfires, but nothing like before. After the 10min drive to work and 10min drive back home, I never threw a code for misfire. I'm still idling rough.
The only codes I have now are related to the evap system and the EGR (along with my stupid wheel sensor).
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P0174 Fuel Trim System Lean Bank 2
P0452 Fuel Tank Pressure (FTP) Sensor Circuit Low Voltage
PO463 Fuel Level Sensor 1 Circuit High Voltage
C0225 Left Front Wheel Speed Sensor Circuit Open
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My questions now ------
1- Is the fuel level/gauge problem related at all to the lack of input to the evap canister, or lack of pressure in the tank due to the open vent? Or is more than likely a float problem in the tank?
2- Could the lack of evap from the vent ALONE be the cause of my rough idle?
3- Could my previously failed PCV be the reason for the misfiring?

After changing the PCV, I only drove a couple blocks and didn't notice anything at the time, but I have to say, the quality of the drive and the acceleration response seem to be MUCH better today, and I hope I'm not overly excited about not throwing a misfire code after 20min drive time. The thing was throwing codes left and right, and in the 5min I idled in the parking lot at work, I had maybe 6 misfires. I forgot to check the count when I got home though, I had a lot going on at the time.
Thanks again for all your help. I still plan to change plugs, patch my vent/evap input line and inspect things a little further. It's been a long Monday, so I might relax tonight. In the meantime, it would be nice to know what you guys think.
Thanks!
-Jacob

 

[Zimmerly]

I just drove across town with my scanner plugged in. Still no misfire codes, but misfiring, nonetheless. Still getting the same codes as I was previously today.
I thought I would post a screenshot of my stats after I came home and pulled in the driveway. Is my manifold pressure high? My MAF airflow rate is also measuring .81lb/min (translates to 7.56 g/sec). If my googling is correct, I should be between 6 and 7 grams per second.
Could this be another sign of intake gasket failure? Other possibilities?

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Thanks again.
-Jacob

 

[Zimmerly]

Hello again. I was able to configure my Dashcommand scanner app to display a line graph of my O2 sensor voltage. I did a little research on how to make sense of them. IF they are working properly, and if my research is correct in understanding these stats, Bank 2 Sensor 1 is in fact running lean, while bank 1 Sensor 1 seems to be running very rich...

I've attached 2 screenshots of my Dashcommand O2 sensor data.
The first screenshot is during my drive home from work after a 10min warm-up.
The second screenshot is after coming home from work and putting my truck in park on the driveway.

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My questions:
1- Why isn't Bank 2 going through the same drastic ups and downs as Bank 1?
2- Do any of my sensors look "shot" at first glance? Should I change them?
3- Are we still suspecting an intake gasket leak? Or has my Bank 2 Sensor 1 failed?

Thanks,
-Jacob

 

[Schurkey]

I have completely no f'n idea how the #2 downstream (yellow) sensor can be MORE rich than the #2 upstream (blue) sensor.

Even if the catalyst was totally non-functional, they'd be equal.

1. The scan tool is goofy.
2. The O2 sensor(s) are goofy.
3. The computer is goofy.

A second scan tool to confirm might be helpful. Beyond that, I'd cram in four new O2 sensors.

 

[Zimmerly]

I have completely no f'n idea how the #2 downstream (yellow) sensor can be MORE rich than the #2 upstream (blue) sensor.

Even if the catalyst was totally non-functional, they'd be equal.

1. The scan tool is goofy.
2. The O2 sensor(s) are goofy.
3. The computer is goofy.

A second scan tool to confirm might be helpful. Beyond that, I'd cram in four new O2 sensors.

That was what I was thinking as well. Something seems screwy. I wish I knew more people in the area, or I'd have another scan tool to check. I'm just going to do the O2 sensors and be done with it. I will report back.
Thanks Schurkey!
-Jacob

 

[Road Trip]

I'm NOT familiar, however, with things like what false codes are thrown, or what process to take in troubleshooting more complicated issues like this.

My questions:
1- Why isn't Bank 2 going through the same drastic ups and downs as Bank 1?
2- Do any of my sensors look "shot" at first glance? Should I change them?
3- Are we still suspecting an intake gasket leak? Or has my Bank 2 Sensor 1 failed?

Greetings Jacob,

It's looks like you might have a combination of problems, interacting in such a way that your spaghetti graph
resulted, which defies straightforward interpretation. Goofy squared, not the least of which is that it looks like:

* Your scan tool is mis-identifying a couple of sensor outputs, -or-
* there is some crosswiring between the (4) O2 sensors and the VCM.

In order for us to reason our way through this, I'd first like to explain at a high level how O2
sensors work, followed by how upstream (of the cat) & downstream (of the cat) O2 sensor
graphs will look like in a healthy feedback loop.

Once we understand these, then disconnect both rear O2 sensors and see if we get proper
transitions on both Bank 1 & Bank 2 upstream sensors. (ie: prove/disprove what the app is
showing us.)

Finally, I'll discuss the special case where a misfiring cylinder on one bank distorts the feedback
loop, causing that entire bank to be fed a too rich A/F ratio in order to fix a phantom (false overall)
too lean condition.

****

Alright, let's take a look at the voltage output of a GMT400-era narrowband O2 sensor vs the Air/Fuel ratio
it is inferring based upon what's in the exhaust gases:

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(credit: Tomorrowstechnician.com O2 wideband vs narrowband article)

Note that the output of the O2 sensor swings over a small range. (Nearly 0 volts to almost 1 volt.) And that a
stoich (chemically correct) A/F mixture = ~450 millivolts. (See graph above.) This signal comes from measuring
how much oxygen is on the exhaust side of a zircon membrane vs the atmospheric side. Here's a simplified
diagram of a narrowband O2 sensor:

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What's easy to remember is that IF there's oxygen on both sides of the zircon membrane, no voltage is developed = Lean condition reported.
(Keep this in mind when we consider the special case misfire scenario later on.)
But if there is no oxygen in the exhaust stream, this oxygen quantity difference across the membrane causes a voltage to be developed = Rich condition reported.


So the way they implemented an A/F feedback loop using these narrowband O2 sensors is elegantly simple.
Let's say that the Open Loop (factory fueling table) is slightly rich when the O2 sensors fully warm up and
start sending out an ~1 volt signal to the computer.

In response, the computer in turn immediately cuts back the fueling in small increments, until the O2 sensor now
senses excess oxygen (lean condition) in the exhaust. (ie: oxygen on both sides of zircon membrane = 0 volts.)

As soon as the computer sees this voltage transition from high to low, it immediately responds to this change by
adding additional fuel in small increments...and again, the O2 sensor senses a lack of oxygen in the exhaust, and
signals this by the output voltage going back towards 1 volt -- and in a healthy engine bay, this 'a little lean > a little rich
> a little lean > a little rich' zigzag (in the close proximity of a perfect stoich A/F ratio) ...this ~1 volt dance repeats nonstop
for the duration of this drive cycle.

And since all this Closed Loop activity is well within the operational window that the cats can handle, we extract maximum
efficiency with minimum emissions while that sawtooth shaped feedback signal is occurring. Pretty cool stuff.

Of course all this is going on before (upstream of) the catalytic converter. On our V6 & V8 engines, these A/F feedback
sensors will be identified as Sensor 1 for Bank 1 and Bank 2. So look for the Bank 1 Sensor 1 graph in this chart:

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As you can see, the green (Sensor 1) is switching up and down up to one cycle every second. NOTE: The healthier the
O2 sensor, the more responsive it is, and the faster the overall A/F feedback loop will operate.

At this time I'm not going to dive down the rabbit hole as to why the downstream (of the cat) O2 sensor is relatively
steady during normal operation, but just remember that once the cat-monitoring O2 sensor warms up, these Bank 1 (& 2)
Sensor 2 output(s) stays pretty steady IF they are behind healthy cats. (See blue graph above for Sensor 1, Bank 2.)

****

Alright, I dragged everyone through the basic sensor theory just so that we can easily visually identify the graphs of healthy
upstream (A/F feedback loop) vs downstream (cat efficiency monitor) O2 sensors. And here is where I would like to verify
that we aren't being lied to by the app. My suggestion would be to temporarily physically disconnect both rear (downstream)
O2 sensors electrical connectors, and see if the remaining 2 upstream graphs are sensical. (ie: both varying rhythmically between
0 & 1 volts.)

In English, let's figure out if the all-important upstream o2 sensors are being correctly identified. And once we have that,
then we get both Bank 1 & Bank 2 sensors working as they should. And *then* we can reconnect the downstream sensors
and see if we can't get those working correctly as well. (The divide and conquer approach.)

For what it's worth, I've attached the graph that was giving us all the most heartburn, for it wasn't making sense.

Hope this helps. I like the fact that the OP is capturing data, so I want to use this opportunity in order for all of us
to be able to come up with a good analysis strategy for these waveforms. It's my sincere hope that we are able to
get this one over the finish line.

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)