RKCRLR

I put my original spark plug wires back on and took it for a drive yesterday. You would never know anything was wrong until the CEL came on. There was a single P0304 code for cylinder 4 misfire (not the usual P0300). Reviewing the data everything seemed good other than the cylinder 1, 4, and 5 misfires. Other than some data dropouts the upstream O2 sensors never went low and the fuel trims in closed loop look good although higher than they should be. Today I took it for another drive and managed to get a full throttle acceleration run for ~25 seconds. It drove great and the CEL never came on although the fuel system status went to 16 when I let off the gas but it cleared itself and went back to 2. When I pulled the current faults the only thing there was a pending bank 2 downstream O2 sensor low voltage fault. The upstream O2 sensors never went low and the fuel trims don't look too bad (compared to what they used to be). However the cylinders 1, 4, and 5 misfires are still there. It's almost like it is getting better the more I drive it. I'm going to drive it around normally so I can get it to pass smog and then sell it unless someone has some other suggestions on what to look for. Log files attached. trackLog-2025-Apr-03_13-35-45.ods trackLog-2025-Apr-04_15-59-06.csv

I went for a test drive today and the results are a little baffling. I did a ~15 second full throttle run and the engine felt good and the fuel system status stayed a 4 (Open loop due to engine load) until I let off then the fuel system status went to 16 (Closed loop, using at least one oxygen sensor but there is a fault in the feedback system). It never went to 8 (Open loop due to system failure) like it usually does. The CEL came on but I'm not exactly sure when. When I pulled the codes I had the P0300 misfire and a pending bank 2 downstream O2 sensor low voltage fault. The data shows the same cylinders 1, 4, and 5 misfires I've been having but the upstream O2 sensors never went low like they usually do after several seconds. Looking at the O2 data you wouldn't think I have a fueling problem. Data log attached. trackLog-2025-Apr-02_15-11-40.ods

I thought I'd post an update. I replaced the fuel injectors, fuel pressure regulator, fuel filter (again); and I reinstalled my original distributor. Same problem. I did a compression check for my sanity (130-145 psi with engine cold), checked/adjusted spark plug gaps (some were slightly out at 0.065"), swapped the spark plugs from the cylinders that had the most misfires with the spark plugs from the cylinders that had the least misfires, and adjusted my CMP to -2 degrees (It was about -9 when I reinstalled the distributor). I haven't taken it for a drive again yet but I don't expect any changes. Log file attached. trackLog-2025-Mar-27_14-01-35.ods

Intake manifold vacuum is normal. I purchased an exhaust pressure gauge and measured at 3 psi or less under load.

I took the truck for a drive using the Torque Pro app so I could see individual cylinder misfires, commanded AFR, and fuel system status. Misfires start happening after I'm at WOT for a few seconds. Cylinder # 4 was the worst offender but there were misfires in several other cylinders. For some reason bank 2 O2 sensors didn't record data but previous logs show bank 2 O2 sensor readings are similar to bank 1. When I pulled the faults there were pending faults for low voltage on bank 2 upstream and downstream O2 sensors (but no pending random misfire faults). Log attached. trackLog-2024-Nov-07_14-08-22.ods

The way power enrichment mode was described in the O2 sensor troubleshooting it seems like the O2 sensors are monitored during power enrichment. The Fuel System Description (attached) doesn't mention power enrichment mode but it does mention acceleration mode. It doesn't mention going open loop during acceleration mode but it does describe conditions for going open loop. I think I've figured out how to make Torque Pro monitor Fuel System Status so I might do a run where I can compare fuel system status, commanded AFM, and cylinder misfires along with the other stuff in one log. K3500 Fuel Control Description.pdfK3500 Fuel Control Description.pdfK3500 Fuel Control Description.pdfK3500 Fuel Control Description.pdf

So I put some Techron in and took it for a drive. I figured if it made an improvement in fuel trims that would be more evidence that the problem is the injectors. Here are my observations: During normal driving the fuel system remains in closed loop (Fuel System Status 2 - Closed loop, using oxygen sensor feedback to determine fuel mix). The truck operates normal and the only signs of a problem is the high fuel trims. The fuel trims do seem to be lower but it is hard to tell since the LTFTs seem to be lower but the STFTs seem to be higher. Whenever full throttle is applied it goes to open loop (Fuel system Status 4 - Open loop due to engine load OR fuel cut due to deceleration). I'm not sure it is supposed to do this but it appears to operate fine and the nomenclature suggests it is normal operation. The LTFTs go to 5% and the STFTs go to 0% however the O2 sensors still seem happy. I can hold it at full throttle for 10 seconds and it seems to run fine even with the lower fuel trims. It reverts to closed loop status when I let off the gas and everything goes back to normal (with the exception of the high fuel trims). When I held full throttle for about 15 seconds the fuel system went to status 8 - "Open loop due to system failure." I was monitoring the status and let off the gas as soon as status 8 came up. While the LTFTs remained at 5% and the STFTs remained at 0% the O2 sensor voltages went extremely low and everything stuck there during part throttle operation and I could feel a reduction in power but the CEL didn't come on. I pulled over and restarted the engine. The fuel system status went to 16 - "Closed loop, using at least one oxygen sensor but there is a fault in the feedback system." The fuel trims went back to reading high and the O2 sensors were happy again. Eventually the fuel system status reset to normal closed loop operation. I pulled the fault status when I got home. P300, P0131, and P0151 were pending. However, it is still ready for a smog check. It appears that the OBDLink app does not support cylinder misfires since I didn't see any in the logs but there is the pending random misfire fault. I checked with Torque Pro and there are misfires in the history. What concerns me is if this is a problem with the fuel injectors why were the O2 sensor readings still good when it went open loop due to engine load? And why can I run at full throttle for so long before the problem occurs? If the injectors were causing the problem at full throttle then why doesn't the problem happen as soon as I give it full throttle? Someone on the other forum posted that they had a similar problem and it was fixed when the EGR valve was replaced. But, from what I've read, a bad EGR valve won't cause low O2 readings on a MAF system. And it isn't part of the low O2 sensor troubleshooting tree nor do I have any EGR codes. If it was stuck open I'd expect a bad idle and if it was stuck closed/clogged I wouldn't expect problems at full throttle. Unfortunately my apps don't support monitoring or troubleshooting the EGR system. I've attached the log where it went open loop with system fault and where it corrected itself. CSVLog_20241105_145254.ods CSVLog_20241105_150705.ods

I thought I'd report what I've done to address the recommended diagnostics for P0131/P0151 low O2 sensor voltage fault codes. Under Diagnostics Aids it says to check for the following conditions: A sensor pigtail may be mispositioned and contacting the exhaust system. No sensor pigtails contacting the exhaust system An intermittent short to ground in the signal circuit between the VCM connector and HO2S I checked the harnesses for abrasion, etc. and didn't see any evidence of shorting to ground. A poor VCM to the engine block ground Checked and cleaned the VCM ground wire connection at the engine block. Lean injectors -- Perform the injector balance test. Refer to Fuel Injector Balance Test with Tech 2 This is where I'm leaning (no pun intended) but I currently don't have the equipment to do an injector balance test. Fuel contamination -- Water, even in small amounts, near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a lean exhaust and can also set this DTC. Refer to Alcohol/Contaminants-in-Fuel Diagnosis. I have put many, many, tanks of fuel through the system since the problem first occurred. And when I collected fuel samples during pressure testing there was no evidence of water. And before the problem occurs the O2 sensors aren't reading lean although the fuel trim is high and the fuel trim goes low at an idle. Water wouldn't just disappear at an idle. The way the system is operating it doesn't seem like a water problem. Fuel pressure -- If the pressure is too low, the system will be lean. In order to confirm, monitor the fuel pressure while driving the vehicle at various speeds and loads. Refer to Fuel System Diagnosis. I drove around with a fuel pressure gauge duct taped to the windshield and fuel pressure remained within specification. I'm still suspicious of the fuel system and might do a sanity check by checking fuel flow rates both right out of the tank where it meets up with the filter and in the return line after the regulator. I figure it should be at least 1/2 gal/minute. Exhaust leaks -- If there is an exhaust leak, the engine may pull the outside air into the exhaust and past the sensor. Refer to Exhaust System Inspection in Engine Exhaust. I checked for exhaust leaks where the manifolds bolt to the heads, at the manifold to pipe flanges, where the O2 sensors bolt in, and at the connections to the cat converters. There isn't any evidence of exhaust leaking and I don't hear any exhaust leaks. Leaks downstream of that shouldn't have an impact to the upstream O2 sensors. Vacuum or crankcase leaks can cause a lean condition or a possibly high idle. I checked for loose hoses and don't hear any vacuum leaks. The engine idles smooth and the fuel trims are negative at an idle. The intake manifold pressures are normal. My experience with vacuum leaks is they cause problems at idle and low part throttle conditions but not at full throttle. An intermittent may be caused by any of the following conditions: A poor connection Rubbed through wire insulation A broken wire inside the insulation As mentioned I checked for rubbed through insulation and poor connections. I'm not ready to tear into the harness to check for broken wires in the insulation. And the symptoms aren't really consistent with broken or grounded wires, at least not to individual sensors. Before the problem occurs the HO2S sensors read 86mv (and the upstream sensors have now been replaced twice) so the sensors aren't the problem. It then goes on to say the following in the diagnostics tree: The condition that set this DTC is not present. This DTC may have been set by one of the following conditions: A signal wire intermittently shorted to engine ground or to the HO2S LOW circuit As mentioned already checked HO2S connector water intrusion Connectors are dry. An exhaust leak between the HO2S and the engine Refer to Exhaust System Inspection in Engine Exhaust. As mentioned already checked. An intermittent lean engine condition such as the following: Vacuum leak As mentioned already checked. Incorrect or defective PCV valve or system PCV valve appears to be operating correctly. And my experience is they cause symptoms similar to a vacuum leak when they aren't operating correctly. Incorrect fuel pressure As mentioned already checked. Lean fuel injectors A distinct possibility An inaccurate MAF sensor Checked and cleaned. A lean engine condition during power enrichment operation This is what is causing the CEL and misfires but there isn't any direction to do specific troubleshooting for lean condition during power enrichment.

I spent some time checking things. I removed the distributor cap (and I have blood stains on the truck to prove it) and inspected it. There were no signs of arc traces or cracks on the interior. The posts had some minor corrosion that I removed but no erosion. Also no signs of anomalies on the rotor or spark plug wire ends that attach to the cap. I swapped plug wires 2 and 4 (since they are easy to swap back) to see if the cylinder 4 misfire moves to cylinder 2. I removed all for connectors from the VCM and inspected for corrosion, moisture, etc. and didn't see any problems. I sprayed the connections with electrical contact cleaner and reinstalled the connectors. I also removed the MAF and sprayed the sensor wires with contact cleaner while I was at it. I inspected wiring harness for areas of rubbing, breaks, etc. and didn't find anything. I cleaned the VCM harness ground connection at the front lower passenger side of the engine block (I'm pretty sure this is where the VCM and O2 sensors ground but I also checked the ground connections at the thermostat housing and back of the passenger side head). Unfortunately, I can't find an option in Torque Pro to log fuel system status however my OBDLink app does have that ability (however it can't log commanded AFR). When I looked at the fuel system status (before resetting the fault) it showed 16 which indicates "Closed loop, using at least one oxygen sensor but there is a fault in the feedback system". Also, when I pulled the freeze frame data for the P0300 fault it reported that the fuel system status was in open loop due to a system fault. I'd like to do a fuel injector balance test but my equipment doesn't support it. Also, it won't show a problem if all injectors are flowing low but close to each other. I'd have to get a OBD scanner that has that ability or pay someone to do it but I may be able to borrow one. Yesterday I reset the faults and took it for a drive into town and back (about an hour round trip) and didn't purposely try to reproduce the problem. I just drove normally and never used full throttle, only about 70% max. Long term fuel trim hits 25% (I think that is maxed out) under part throttle conditions but the O2 sensors are happy and there are zero misfires during the entire drive. The truck ran good, no CEL, no pending faults, and the fuel system remained closed loop during driving. Also of note is the it reports that the only things preventing from being ready for a smog test is the evaporative system (which always takes a long time to reset) and the O2 sensor heater. I've attached the log file. I believe there is a fuel delivery problem somewhere in the fuel system. This is causing the high fuel trim values to keep the O2 sensors happy. When I apply full throttle it exceeds what the fuel trims can provide to keep the O2 sensors in the correct range. This causes the the VCM to go open loop, the O2 sensors are ignored, and the fuel trim goes close to zero. This causes a lean misfire which is the abrupt loss of power I feel just before the CEL goes on and causes the low O2 sensor fault. The challenge now is figuring out where the fuel delivery problem is. I'm leaning towards the fuel injectors but I'd like to be sure before I dig into them. I might try putting some Techron through the system. While this probably won't cure the problem, if the fuel trims get better it would give me more confidence the injectors are the problem. I suppose the problem could also be that the VCM isn't driving the injectors properly. Also, do the fuel injectors have internal screens? CSVLog_20241101_132845.ods

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

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

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

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.

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.

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.