Code P0300 Random/Multiple Misfire and loss of power under load

[CamGTP]

Correct, the rear sensors do not do anything for fueling. All they do is monitor the cat function.

[RKCRLR]

I received the Denso O2 sensors, installed them, and took a test drive.  The truck runs a lot better although I don't know if the problem is completely solved.  It didn't set a CEL and there are no pending faults.  The O2 sensor readings seem to make sense under full throttle.  However there are still misfires associated with full throttle and the fuel trims seem high under full throttle conditions.  At no point did the acceleration flatten out although it may not have had the power it should.  

 

One thing of note is that it is a lot cooler now.  The previous problems were occurring when the temperatures were in the ninetys and triple digits.  It was in the low seventies for today's test drive.  

 

Data file attached

 

 

trackLog-2024-Oct-22_12-19-15.ods

Edited October 24 by RKCRLR
Attached the correct log file.

[asilverblazer]

Looking at the latest log, it is still running lean at WOT.

 

Log only included bank 1 upstream and downstream sensors. Bank 2 wasn't logged.

 

Fuel trims are still doing some weird things in the log... its looks like they stop responding to anything.

 

At WOT, presumably during power enrichment and it appears to be only during power enrichment, it runs lean. A log of no WOT and only applying 80% throttle might confirm this.

 

Can you log injector duty cycle? I'd like to see what the injectors are doing vs. being commanded when these lean conditions appear. 

 

A custom tuner would probably be better able to recognize oddities and problems.

 

EDIT: Can you log fuel rail pressure?

 

Edited October 23 by asilverblazer

[RKCRLR]

18 hours ago, asilverblazer said:

Looking at the latest log, it is still running lean at WOT.

 

Log only included bank 1 upstream and downstream sensors. Bank 2 wasn't logged.

 

Fuel trims are still doing some weird things in the log... its looks like they stop responding to anything.

 

At WOT, presumably during power enrichment and it appears to be only during power enrichment, it runs lean. A log of no WOT and only applying 80% throttle might confirm this.

 

Can you log injector duty cycle? I'd like to see what the injectors are doing vs. being commanded when these lean conditions appear. 

 

A custom tuner would probably be better able to recognize oddities and problems.

 

EDIT: Can you log fuel rail pressure?

 

Sorry!!  I screwed up and uploaded an old log.  I've edited my post and attached the latest log.  

 

I have an option listed in the menu to log the fuel pressure but it doesn't have data.  I suspect my truck doesn't have the ability to monitor fuel rail pressure.  

[asilverblazer]

After looking at the latest log, everything looks like I would expect it too. O2 sensors are reporting, fuel trims are adjusting and it looks like everything is doing what you expect. I didn't see the obvious lean condition at WOT either. That's the GOOD NEWS!

 

However, you're still getting miss fires on cylinders 1, 4 and 8 and none on the others, appearing to rule out a fueling issue, air intake issue, exhaust, vacuum leak, etc. I think that is good news too.

 

As isolated as the miss fires are I would suspect an issue specific to the problem cylinders. I don't recall if a compression check was done, but do so to rule out valve issues. That leaves ignition.

 

I never had good luck with rotor caps on that generation of engine, I would inspect that for sure. Including for 'corrosion' on the underside at the terminals and on the rotor tip and top at the coil to rotor terminal. The problem cylinders are also next to each other in firing order - is the cap on securely?

 

I've also had bad plug wires new out of the box. If you still have the old ones, try an old wire on the problem cylinders to see if the miss fire counts drop with a different wire. 

 

What codes are you currently getting?

 

FYI: I'm looking at the data in excel and applying conditional formatting to the columns to better see lean vs. rich conditions. When the PCM commands power enrichment (11.5 commanded AFR) the O2 sensors should show rich. Also, as MAF reading increases so does fuel flow. Dips in O2 voltage (lean) have a corresponding bump in short term fuel trim (adding fuel). 

[RKCRLR]

45 minutes ago, asilverblazer said:

After looking at the latest log, everything looks like I would expect it too. O2 sensors are reporting, fuel trims are adjusting and it looks like everything is doing what you expect. I didn't see the obvious lean condition at WOT either. That's the GOOD NEWS!

 

However, you're still getting miss fires on cylinders 1, 4 and 8 and none on the others, appearing to rule out a fueling issue, air intake issue, exhaust, vacuum leak, etc. I think that is good news too.

 

As isolated as the miss fires are I would suspect an issue specific to the problem cylinders. I don't recall if a compression check was done, but do so to rule out valve issues. That leaves ignition.

 

I never had good luck with rotor caps on that generation of engine, I would inspect that for sure. Including for 'corrosion' on the underside at the terminals and on the rotor tip and top at the coil to rotor terminal. The problem cylinders are also next to each other in firing order - is the cap on securely?

 

I've also had bad plug wires new out of the box. If you still have the old ones, try an old wire on the problem cylinders to see if the miss fire counts drop with a different wire. 

 

What codes are you currently getting?

 

FYI: I'm looking at the data in excel and applying conditional formatting to the columns to better see lean vs. rich conditions. When the PCM commands power enrichment (11.5 commanded AFR) the O2 sensors should show rich. Also, as MAF reading increases so does fuel flow. Dips in O2 voltage (lean) have a corresponding bump in short term fuel trim (adding fuel). 

Thanks for your help! 

The problem started years ago when I'd get a CEL caused by a multiple random misfire code with no other codes.  In the beginning it would only happen at the end of a long trip going up an incline on a hot day fully loaded.  That is when I started incrementally shooting the parts cannon at it (the engine had good compression).  The engine spun a rod bearing a few years back so I replaced it with a remanufactured long block but the problem still occurred after the engine change.  It started occurring more often and I finally got the low O2 sensor voltage codes, I was happy to get something other than the random multiple misfire codes.  I replaced the O2 sensors with generic O2 sensors but the problem was the same (perhaps worse).  That is when I posted here.  I find it strange that the generic O2 sensors caused the same symptoms as the original sensors but the last run with the Denso sensors definitely was better.  

 

The distributor cap is on securely (but it is a PITA to get to the retaining screws) and and I have already replaced it after I replaced the distributor and cap (I thought I saw a crack in the cap).  Perhaps it is worth looking at again if the misfiring cylinders are on adjacent cap terminals.  The one electrical component I haven't replaced is the ignition coil.  I might have the old spark plug wires but I have checked the new wires with a ohm meter and they were within range as I remember.  

 

There were pending random multiple misfire and all 4 O2 sensor low voltage faults before I replaced the 2 upstream O2 sensors with the Denso units.  I didn't clear the fault codes before I took the last test drive with the Denso sensors.  There are currently no active fault codes and the pending fault codes cleared themselves after the last test drive.  

[asilverblazer]

1 hour ago, RKCRLR said:

Thanks for your help! 

The problem started years ago when I'd get a CEL caused by a multiple random misfire code with no other codes.  In the beginning it would only happen at the end of a long trip going up an incline on a hot day fully loaded.  That is when I started incrementally shooting the parts cannon at it (the engine had good compression).  The engine spun a rod bearing a few years back so I replaced it with a remanufactured long block but the problem still occurred after the engine change.  It started occurring more often and I finally got the low O2 sensor voltage codes, I was happy to get something other than the random multiple misfire codes.  I replaced the O2 sensors with generic O2 sensors but the problem was the same (perhaps worse).  That is when I posted here.  I find it strange that the generic O2 sensors caused the same symptoms as the original sensors but the last run with the Denso sensors definitely was better.  

 

The distributor cap is on securely (but it is a PITA to get to the retaining screws) and and I have already replaced it after I replaced the distributor and cap (I thought I saw a crack in the cap).  Perhaps it is worth looking at again if the misfiring cylinders are on adjacent cap terminals.  The one electrical component I haven't replaced is the ignition coil.  I might have the old spark plug wires but I have checked the new wires with a ohm meter and they were within range as I remember.  

 

There were pending random multiple misfire and all 4 O2 sensor low voltage faults before I replaced the 2 upstream O2 sensors with the Denso units.  I didn't clear the fault codes before I took the last test drive with the Denso sensors.  There are currently no active fault codes and the pending fault codes cleared themselves after the last test drive.  

I think you haven't corrected the original problem, you've only fixed the bad O2 sensors. The generic O2 sensors weren't the original problem but might have exaggerated it.

 

A bad coil wouldn't cause miss fires on only three specific cylinders.

 

It sounds like the original and lasting issue is the miss fire(s) that you can focus on now.

 

Regarding the cap and distributor, again my luck was not good with it. Cap replacement was essentially a yearly maintenance item. Moisture and corrosion would build up inside it on the terminals. Also, the screw to hold the cap down stripped out so it wouldn't stay on securely. 

 

The easiest tests/checks I'd do would be the cap inspection and wires followed by a new log. A change in miss fires conditions should be readily apparent.

[RKCRLR]

2 hours ago, asilverblazer said:

I think you haven't corrected the original problem, you've only fixed the bad O2 sensors. The generic O2 sensors weren't the original problem but might have exaggerated it.

 

A bad coil wouldn't cause miss fires on only three specific cylinders.

 

It sounds like the original and lasting issue is the miss fire(s) that you can focus on now.

 

Regarding the cap and distributor, again my luck was not good with it. Cap replacement was essentially a yearly maintenance item. Moisture and corrosion would build up inside it on the terminals. Also, the screw to hold the cap down stripped out so it wouldn't stay on securely. 

 

The easiest tests/checks I'd do would be the cap inspection and wires followed by a new log. A change in miss fires conditions should be readily apparent.

Thanks, those will be the next checks on my list when I get around to it.  Going by memory, all the cylinders had random misfires to some extent stored in the history.  I'll have to look again but I don't know if I'll be able to separate the before and after the generic O2 sensors.  

 

If the truck computer resets and says it is ready for a smog check I may just sell it the way it is.  I've gotten weary chasing this problem and a new truck is on it's way.  

 

Thanks again for your help.  

[lunytnz]

I found that a dirty MAF was the cause of the same code and kind of runability problems in my 99.  I use a K&N filter and I believe that contributed to the problem. So, now, as regular maintenance, I spray down both sides of the MAF with air intake cleaner and that has prevented any P0300 codes from recurring.  I wish you luck!  It can be PITA to solve these kinds of problems.

Edited October 25 by lunytnz
typo

[RKCRLR]

Well, I took my truck for a drive today.  I got a CEL for multiple random misfires.  Pending faults for all four O2 sensors for low voltage.  Back to the same old problem.  And the same strange thing where the fuel trim isn't trying to compensate for the low voltage at full throttle.  Log attached.  

 

Edit: I noticed early on in the log that the long term fuel trim will be in the teens and twenties for the first few seconds of full throttle accelerations and the O2 sensor voltage readings aren't low.  Then it pulls the fuel trim and the O2 sensor voltages go low.  Later on the fuel trims and O2 sensors go low as soon as full throttle is applied.  Then later on the fuel trims and O2 voltages are always low regardless of throttle position.  

 

It's like the computer is getting "stuck".  Perhaps going open loop?  Also of note is there were no cylinder misfires recorded until after the fuel trims and O2 sensors went low at full throttle.  

trackLog-2024-Oct-25_13-43-54.ods

Edited October 26 by RKCRLR
Added more information.

[asilverblazer]

On 10/25/2024 at 6:57 PM, RKCRLR said:

Well, I took my truck for a drive today.  I got a CEL for multiple random misfires.  Pending faults for all four O2 sensors for low voltage.  Back to the same old problem.  And the same strange thing where the fuel trim isn't trying to compensate for the low voltage at full throttle.  Log attached.  

 

Edit: I noticed early on in the log that the long term fuel trim will be in the teens and twenties for the first few seconds of full throttle accelerations and the O2 sensor voltage readings aren't low.  Then it pulls the fuel trim and the O2 sensor voltages go low.  Later on the fuel trims and O2 sensors go low as soon as full throttle is applied.  Then later on the fuel trims and O2 voltages are always low regardless of throttle position.  

 

It's like the computer is getting "stuck".  Perhaps going open loop?  Also of note is there were no cylinder misfires recorded until after the fuel trims and O2 sensors went low at full throttle.  

trackLog-2024-Oct-25_13-43-54.ods 105.2 kB · 2 downloads

Sooooo, I think there might be two issues.

1. There are some data issues - I see some instances where the O2 sensors stop reporting, I think this causes the fuel trims to a 'default' setting. This might be 'open loop'. Short term fuel trims go to 0 and long term stagnate. These 'gaps' in O2 data might be a wiring issue where the PCM doesn't get information from the O2 sensors. This triggers the low O2 sensor voltage. It could also be that the circuit that powers the O2 sensors is cutting out. This problem will cause its own drivability issues.

 

The short term fuel trim goes to 0 when the AFR is in power enrichment. It does seem to get 'stuck' example at line 629. The long term trims stop fluctuating as strongly. In fact, the rest of the drive looks like what ever happened at this point never resolved.

 

Line 1085-1595 looks like the engine was off.

Line 1620 short term fuel trim comes back on, not sure why.

 

If you take out the data below line 630 - everything looks reasonable. Only miss fires on cylinders 1 and 4. This is roughly the first 10 minutes of your log/drive. So what happened? Loose wire connection? Maybe a power or ground cable? I would typically be hesitant to say the PCM is the problem, but it might be worth considering.

 

EDIT: It could be that the O2 low voltage code triggers the PCM to stop referring to the O2 data and subsequently stop using short term fuel trims. If the O2 low voltage happened at around line 617 then the rest makes sense. Line 617 - 630 should have been a power enrichment event but the long term fuel trims are using a pretty weak enrichment amount compared to the drive so far. Long term trims are adding up to around 20% but at power enrichment it is only adding 2-3%. Engine also goes lean at this time (low O2 reading). Why is the long term fuel trim so weak at this data point?

 

2. Miss fires are still occurring on cylinders 1, 4 and 8. Miss fires are reported on other cylinders but its a small fraction of what are reported on 1, 4 and 8. Probably because of issue 1.

Edited October 28 by asilverblazer

[RKCRLR]

7 hours ago, asilverblazer said:

Sooooo, I think there might be two issues.

1. There are some data issues - I see some instances where the O2 sensors stop reporting, I think this causes the fuel trims to a 'default' setting. This might be 'open loop'. Short term fuel trims go to 0 and long term stagnate. These 'gaps' in O2 data might be a wiring issue where the PCM doesn't get information from the O2 sensors. This triggers the low O2 sensor voltage. It could also be that the circuit that powers the O2 sensors is cutting out. This problem will cause its own drivability issues.

 

The short term fuel trim goes to 0 when the AFR is in power enrichment. It does seem to get 'stuck' example at line 629. The long term trims stop fluctuating as strongly. In fact, the rest of the drive looks like what ever happened at this point never resolved.

 

Line 1085-1595 looks like the engine was off.

Line 1620 short term fuel trim comes back on, not sure why.

 

If you take out the data below line 630 - everything looks reasonable. Only miss fires on cylinders 1 and 4. This is roughly the first 10 minutes of your log/drive. So what happened? Loose wire connection? Maybe a power or ground cable? I would typically be hesitant to say the PCM is the problem, but it might be worth considering.

 

EDIT: It could be that the O2 low voltage code triggers the PCM to stop referring to the O2 data and subsequently stop using short term fuel trims. If the O2 low voltage happened at around line 617 then the rest makes sense. Line 617 - 630 should have been a power enrichment event but the long term fuel trims are using a pretty weak enrichment amount compared to the drive so far. Long term trims are adding up to around 20% but at power enrichment it is only adding 2-3%. Engine also goes lean at this time (low O2 reading). Why is the long term fuel trim so weak at this data point?

 

2. Miss fires are still occurring on cylinders 1, 4 and 8. Miss fires are reported on other cylinders but its a small fraction of what are reported on 1, 4 and 8. Probably because of issue 1.

I did turn the engine off and restart it, that is probably lines 1085-1595.  If you're referring to data issues around rows 95-163 and 173-191 I see that too.  I'm not sure if it is actual data issues with the truck or data is being dropped from the Bluetooth connection between my OBDLink MX+ and my phone.  I also noticed that the timing advance data appears to have been dropped.  

 

Speaking of timing advance, do the values seem low?  From what I've read I should be seeing more timing advance at full throttle.  

 

It would be nice to know what "[GM]H2OS Sensor" is supposed to be.  It doesn't correlate with any specific O2 sensor nor an average of any sensors but it seems to have a general correlation to the individual O2 sensors.  I wonder if it has anything to do with the HO2S Sensor Return in the attached schematic.  It is interesting that all O2 sensors go to that common reference junction, I wonder if anything is going on there.  I also included the Fuel Control Description from my service manual for reference.  

 

Thanks for your time.  

K3500 Heated Oxygen Sensor Schematic.pdf K3500 Fuel Control Description.pdf

[asilverblazer]

19 hours ago, RKCRLR said:

I did turn the engine off and restart it, that is probably lines 1085-1595.  If you're referring to data issues around rows 95-163 and 173-191 I see that too.  I'm not sure if it is actual data issues with the truck or data is being dropped from the Bluetooth connection between my OBDLink MX+ and my phone.  I also noticed that the timing advance data appears to have been dropped.

Unfortunately, this is really at the limits of my knowledge of understanding log data and potential problems with the data set. A wired scanner, would eliminate some of those possibilities, to what end I'm not sure. For simplicity sake, looking only at the data that doesn't appear questionable might be easiest.

 

Also, there are essentially two things that are known. Low O2 voltage and miss fires (1, 4 and 8). What progress can be made by not looking at the data? 

 

 

What does you FSM suggest for diagnosing the Low O2 voltage? Does anything in the FSM for that diagnostic path correspond with the random miss fire diagnostic path? 

 

19 hours ago, RKCRLR said:

Speaking of timing advance, do the values seem low?  From what I've read I should be seeing more timing advance at full throttle.  

I wouldn't focus on that yet, might be pulling timing because of the above conditions. I can posit a condition where the timing is occurring too late and leaving an incomplete combustion causing a low O2 reading...

 

I expect the fix to be much more simple than the diagnostics, the miss firing cylinders might lead you to the O2 voltage issue. Assuming a quick check of the ignition, compression, fueling of those cylinders doesn't reveal anything, then a more in depth inspection might reveal something causing the O2 reading issues; something like an intake manifold leak, exhaust leak, etc. You might also while in that process find a wiring issue that explains the data problem and O2 sensor issues. 

 

Another option, reach out to a competent tuner that looks at data logs everyday, Justin at Blackbear is the go to on this site.

 

[RKCRLR]

7 hours ago, asilverblazer said:

Unfortunately, this is really at the limits of my knowledge of understanding log data and potential problems with the data set. A wired scanner, would eliminate some of those possibilities, to what end I'm not sure. For simplicity sake, looking only at the data that doesn't appear questionable might be easiest.

 

Also, there are essentially two things that are known. Low O2 voltage and miss fires (1, 4 and 8). What progress can be made by not looking at the data? 

 

 

What does you FSM suggest for diagnosing the Low O2 voltage? Does anything in the FSM for that diagnostic path correspond with the random miss fire diagnostic path? 

 

I wouldn't focus on that yet, might be pulling timing because of the above conditions. I can posit a condition where the timing is occurring too late and leaving an incomplete combustion causing a low O2 reading...

 

I expect the fix to be much more simple than the diagnostics, the miss firing cylinders might lead you to the O2 voltage issue. Assuming a quick check of the ignition, compression, fueling of those cylinders doesn't reveal anything, then a more in depth inspection might reveal something causing the O2 reading issues; something like an intake manifold leak, exhaust leak, etc. You might also while in that process find a wiring issue that explains the data problem and O2 sensor issues. 

 

Another option, reach out to a competent tuner that looks at data logs everyday, Justin at Blackbear is the go to on this site.

 

I had a long response typed out but accidentally closed the window and lost it all.  The short of it is I think there might be a fuel delivery problem (although I don't know how that can be with good fuel pressure).  From what I've read the 25% long term fuel trim (before the VCM gives up) is too high.  It seems like the O2 sensors are reporting correct information but the VCM isn't responding correctly or the fuel isn't getting there.   

 

I've attached the upstream low O2 sensor voltage troubleshooting from the FSM if you're interested.  

K3500 DTC P0131 & P0151 HO2S Circuit Low Voltage Sensor 1.pdf

Edited October 31 by RKCRLR

[asilverblazer]

19 hours ago, RKCRLR said:

I've attached the upstream low O2 sensor voltage troubleshooting from the FSM if you're interested.  

K3500 DTC P0131 & P0151 HO2S Circuit Low Voltage Sensor 1.pdf 223.04 kB · 1 download

Have you followed the diagnostics in this section?

 

Reading through points to many of the same things we are talking about... wiring conditions, unmonitored air in the exhaust... Also, noted that miss fires will interfere with this codes test cycles. 

 

I didn't see it but it implies to resolve the miss fire condition first. Notice that injector balance test is included. I may be incorrect in my above idea that there are two separate issues. It could be that one problem is presenting itself to the computer in two ways. 

 

So, I posit this, it could be that there is a lean condition at cylinders 1, 4 and 8. Lean enough at times to trigger the low voltage code, but not so lean to cause a miss fire. 

 

Looking forward to what you find during that diagnostic.