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

RKCRLR · Saturday at 06:06 PM

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

Edited Saturday at 06:11 PM by RKCRLR

RKCRLR · Saturday at 10:23 PM

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.

asilverblazer · Monday at 08:56 PM

I'm pretty sure I'm following everything with you, and I think the conclusion of fuel injectors makes sense too.

Everything is fine and is confirmed by the logs until you need full power, power enrichment, lower AFR, or simply an injector duty cycle (fuel amount) that they cannot achieve (deliver) that results in lean condition that causes other problems... I might be inclined to say that you don't need to replace all 8 injectors because of the miss firing only occurring on some cylinders. A full new set would probably be just as easy though. Its what I would do.

RKCRLR · 2024-11-06T21:25:32Z

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.