Haha, no smoking gun as of yet.

I'll know more later this week once I get the replacement ECU + BCU and spare coil pack in the mail, generously donated to my cause by Zuke. We're going to clean the plugs up, and run some new numbers at that time.

I'm interested in replacing the coil pack, as that's the one ignition component we haven't touched yet.

I did hook up the scanner at the end of the day today with my original ECU + BCU plugged in. The scanner did not look much different, if at all, from when I had the donor ECU + BCU installed. I also took a video of the O2 sensor graphs with the vehicle running, as Extinct suggested. Voltage looked normal at varying RPMs.

Fault codes on the UG today were the following: P0300; P0308; P0304; P0306; P0305; P1884; and, one pending P1668

 

Hey, folks!

It's been awhile. Here's where we're at:

We determined that the OEM wires from the 02 sensor connectors to the ECU were crap. They were thin and brittle, and were not conducting very well anymore, which seemed to make sense as a possible cause of my misfire issues, given that we'd tried basically everything else. We re-wired both upstream O2s from the connectors, through the firewall to the ECU with fresh (better quality) wiring. What a colossal pain in the **** that was, BUT... It seems to have been mostly worth it. I now have a consistent P0175 code, and intermittent P0306 and P0308 codes. We also moved the coil packs from the OEM location using the relocation kit, which I'm sure you all are familiar with. During that process, we re-wired the injectors and re-wired the ground wires for the coil packs, as well.

I've driven about 25 miles so far, mixed city and highway. I do not have a consistent flashing CEL, and there is only a slight stumble just above idle. I can't seem to determine any pattern as to why the CEL flashes when it does. It doesn't flash at idle; it just seems to flash randomly. Acceleration and power delivery is fine, and there is no rich or sulfur-like smell from the exhaust. If the CEL wasn't on, I honestly don't think I'd notice an issue. I filled the tank up with 93 earlier tonight, cleared the codes with the UG, and hopped on the highway. It took about 10 minutes before any codes popped up, and when they did, I got the P0175. A few minutes later, still on the highway, I got the P0306 and P0308. Those have been the only codes all day today since we buttoned it back up.

As always, I'm open to any ideas you may have. The truck is running better now than it ever has since I got it, but it's not perfect. Should I drive it for a bit, and see if the ECU "learns" that the wiring has been fixed? Or, avoid driving it until it's truly 100%? Thanks again for your help.

 

I would recommend focusing less on the codes and more on understanding how the engine is running and what the codes are telling you. The misfire codes may or may not be related to fuel mixture and/or the oxygen sensors. I would start by charting the oxygen sensor voltages and the MAF values. Both can be done with an elm327 bluetooth obd2 reader and the free torque app. If the oxygen sensors are cycling normally, you can rule out mixture issues. Then your misfires are likely caused by plugs, wires, coils, or blown hg. You can swap plugs and wires with other cylinders to rule out those, move 6 and 8 to 2 and 4. 6 and 8 are not on the same coil so the coil is unlikely the problem. They are adjacent cylinders, so it is possible you have the beginning of a blown hg between those two cylinders. You can check the plugs on those two cylinders for signs of coolant intrusion but sometimes they are so gradual it is hard to detect.

 

I only scanned the previous posts, but the question I have. What brand are your O2 sensors? When I did my exhaust system and bought "just as good" aftermarket sensors from roverparts, my truck hated them. I had to replace the replacements with sensors from the dealer (ouch!) to get it to run right and not throw all sorts of codes.

 

I'm using Bosch sensors all around at the moment. I've read that Bosch's quality control can be suspect, such that even brand new O2 sensors can malfunction out of the box. I'm going to do as Extinct suggested and swap plugs before I spend money on Walker O2 sensors.

I did some more extensive driving yesterday and today. I noticed the following behavior during some "testing", very consistently: The flashing SES light is not constant, only occurs below about 2,100 RPM, and only when the engine is under light load. There is a noticeable stumble when it is flashing, but the stumble goes away under heavier load (accelerating from a stop, going uphill, overtaking, etc.). When the SES light begins to flash, a boot-full of throttle seems to "clear" the misfire condition. Cruising at highway speeds between 60-75, the light never once flashed, and there was no hesitation. It would maintain 75mph uphill with ease. This was during about 50 miles of highway cruising. I never encountered a scenario where the SES light began to flash while accelerating, maintaining 70MPH uphill, or cruising at speed. If I slowed down from highway speed to 25-40 MPH, the SES light began to flash briefly, nearly every time I did that.

No matter what the cause is, I don't understand why the RPM and/or engine load seems to alter the misfire condition so consistently. Why wouldn't the misfire be constant across all driving conditions, particularly if I had a leaking head gasket into cylinders 6 & 8? When I give it the beans, the engine sounds perfectly healthy and pulls smoothly, and the truck cruises like a dream at speed.

 

The flashing CEL is a warning that you have a CAT damaging misfire i.e. unburnt fuel going to the cats. Only under light load and only under 2100 rpm does not indicate wires, plugs, or coils as those usually get worse with load. Another test you can do is to unplug the oxygen sensors and see if it happens. Different test is to unplug the MAF and see if affects it.

 

@BLucare Not necessarily, lower rpm means more time between spark and if you had a leak technically more coolant could get in to affect the mixture. Head gasket leaks are painful if they are small ,mine never stumbled or overheated literally just over pressured to cooling system a bit.

BUT it could be the coil packs as they start to go you will get misfires, quite often a low RPM. They only way to eliminate them is to replace them but they are cheap about 50.00, maybe a hour to swap.

 

Thanks, fellas. I have spare coil packs, which I can easily swap in now that I've relocated the coils. I'll also do as Extinct suggests with the MAF and O2 sensors.

Regarding the unburnt fuel going into the cat, because the SES is not flashing constantly, I assume that would mean the fuel is being burnt fully when it's not flashing (i.e at higher RPM, and highway cruising)?

As far as the head gasket symptoms, compression is good, and there are no signs of coolant in the oil or out the exhaust pipe, nor signs of exhaust gasses going back into the system in the overflow tank. What else can we check for that?

 

@BLucare Given all of that I would push head gasket to the bottom of the list.

Start with coil packs and work your way through everything else

 

So, I think some progress was made today. I did not have a chance to swap plugs or coils, but I did fix what I believe was a slight vacuum leak from the oil separator-to-manifold connection on Bank 2. The plastic insert at the manifold end was clearly flexing and cracked, so I took the hose off entirely, cleaned some gunk out of the hose and separator hole on top of the valve cover, and replaced the plastic connection with a brass connection and solid hose clamp. Following that repair, I cleared the codes and took it for a drive. The P0175 Bank 2 "Rich" code disappeared and has not returned since, and the slight engine stumble + flashing SES was happening less, but still occurring between about 1,200 and 2,100 RPM. I was hopeful that possible leak may have caused my cylinder 6 and 8 misfires, because the separator is on that side, but no such luck.

Then, I cleared the SES with the UG again, unplugged the MAF and idled it for a minute. There were immediate P0102 and P1884 codes, the UG showed an "error" where the MPG usually would be, and there was a strong rich/unburnt fuel smell from the tail pipe. I assume those symptoms mean the MAF is working correctly, right? On the other hand, it seemed to idle alright, or at least not a whole lot worse than it did with the MAF plugged in, and it didn't die... What else should be expected when the MAF is unplugged? I don't want to drive it with the MAF unplugged, right?

I'm hoping to find time to look at the plugs tomorrow.