I have a 99 5.3L engine in my Jeep with EGR, 2nd O2 sensors, and other emissions related components removed. I was noticing the other day that the engine felt like it was idling a little rougher than normal but nothing crazy. I got a P0300 misfire code this morning on the way to work which is basically random and multiple misfires not directly related to any one cylinder. Since the engine felt fairly normal while driving, I decided to just reset it but it came back about 10 minutes later. Since this code is more generic and not related to any particular cylinder, it makes me think this likely isn't ignition related.

I started looking at fuel trims on the TorquePro app and found some fairly disturbing information. Bank 1 was a little high but not crazy with the long term fuel trim (LTFT) being around +10% and the short term fuel trim (STFT) being around 0% to +5%. Bank 2 on the other hand was WAY off. Bank 2 had a LTFT pegged at +25% and the STFT was sitting around +51%. I recently replaced the bank 2 O2 sensor because I needed to do some exhaust work and the O2 sensor on that side had to be removed which ultimately damaged it. I also had to replace the MAF sensor a couple weeks ago because it just died and I had to unplug it to get home.

So I'm hoping to get some ideas here. I'm personally thinking that dumping an extra 75% more fuel in half of the cylinders is probably what's causing the misfires. Do you think the new O2 sensor is junk or do you think there is more to it? My plan was to get this jeep home and check fuel pressure, all ignition connections, and inspect the pipes around the bank 2 O2 sensor. I'll probably pull plugs as well and see what I can find there as well. Any other ideas on what to check?

 

Well ended up cracking the bank 2 O2 sensor when I was raising and lowering the transmission. There was enough there to register voltage but it was obviously messed up pretty bad.