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The back story is over the summer I swapped a 2001 5.3l LM7 into my 1998 K1500 Truck. I rebuilt the engine before the install. Ever since the engine was installed I’ve been chasing a misfire. Nearly all the sensors have been replaced, plugs and wires etc. I finally gave up trying to find the misfire and took it to a repair shop. The first shop gave up trying to fix it and it was taken to a second who seems to have narrowed it down to a closed loop misfire. The engine runs fine at WOT. The shop unplugged one of the o2 sensors and the truck also runs fine. With both of the o2’s plugged in it runs terrible. The DTC is P0300. The engine does misfire all over the place on all cylinders.
I’ve done a ton of internet searching and it seems that others with this problem have fixed it by replacing the o2 sensors (or they never post the resolution to their problem which is frustrating).
The shop said the wiring for the o2’s all seems good, the o2’s are new and the old o2’s prior to the swap never caused a problem.
The tuner said the tune is fine. The truck is running solely off the 411 PCM (I swapped the PCM before the LS swap so I know it ran fine prior). Other than one of the pins in the PCM being damaged I have no idea what else could be the problem. Any thoughts on how to resolve the closed loop misfire? It’s been a couple months since the engine was set in and I’m ready to put this behind me.
Are you sure the O2s are not crossed L&R? Do you have any scanning tool to look at real time data?
I'm sure. I didn't move the o2 harness connectors and I'm pretty sure they wouldn't reach to the opposite sides if I tried. I have an AutoCal but don't have the truck at home to check anything, it's still at the second shop waiting on return emails from the tuner (tuner only communicates via email).
Misfire is subject to the PCM's interpretation. Any mod
that lightens the rotating mass will raise misfire counts.
Lightweight converters and clutches / flywheels, do it.
Anything that makes the motor "edgier" (like cam) will
also. Your solution may lie with the detect criteria more
than any further tweaking of fuel/spark or any component
swapping. If it runs good and just bitches, fix the bitching,
like.
I picked up my truck from the second shop yesterday. They fleeced me for almost $400 without fixing anything. Drove the truck home with the O2s unplugged and it ran pretty well. I’m going to completely rewire the O2 harness and see if that helps. If not, I’ll have it tuned to run without the O2s in open loop all the time.
PS. Anyone in the Waterloo, Iowa area that wants to know the name of the second shop so they can avoid it feel free to send me a PM.
Why did you pay if the problem wasn't fixed?
I own a shop and I don't charge a dime until the problem is fixed and the customer happy
They threatened to hold the vehicle hostage if I didn't pay. I'd be happy to provide you with their phone number MontecarloDrag so you can give them a lesson in business ethics and customer service.
Two years later and I'm still chasing this problem. New harness/wiring has been run to both front O2 sensors and a new 12v source was run to each. Sensors were replaced again. Still have the same problem. Tuner says the bank 2 sensor is sending max voltage and that's what is causing this misfire. Any thoughts out there?
You can tune out the sensitivity of the misfire table. Set it to like max values across the board (I recommend doing this). I have to with the MAF enabled. Stock values detect a misfire with a cam.
Or you could run it in SD... SD disables the misfire tables. When the PCM detects a misfire it keeps the converter from locking.
I want to update this thread with the solution that was finally found a couple of weeks ago. I broke down and took the truck to CJ Tunes in St. Paul, MN. Carl needed less than thirty minutes to diagnose and resolve the problem that had stumped me for three years.
The problem was with O2 sensors. I had installed and ran a 411 ECM in the truck for awhile prior to the LS swap. Carl said that bank 1 is on the passenger side of an L31 5.7L. The ECM was still reading the O2's as though bank one was on the passenger and bank two on the driver side. Carl reconfigured the banks to an LS orientation and it runs properly in closed loop now. I can't say enough good things about Carl. He was excellent to do business with.
Kudos to 2XLS1 for coming so close right off the bat. The physical wiring was all correct but the way the ECM was reading the data was the problem.
10-17-2014, 10:47 AM
Closed loop misfire. P0300 
