P0300/P0306 misfire issues

[cyoder4400]

So a little background to the issue. Just bought the truck and the day after it starts misfiring. Pulling codes, it was showing a P0300. Checking misfire history, I could see cyl 6 was the biggest offender followed by cyl 8. All other cylinders seemed good. I can also see that it is pulling negative fuel trim in the range of -20% up to -35% on bank 2. Having reset the codes, it now shows a P0306, but cylinder 8 is still misfiring more than it should. The misfire does appear to worsen as the truck is warming up to temp.

 

Things I have done:

Replaced all spark plugs, wires, and coil packs. 

Verified spark is good.

Swapped fuel injectors around to good cylinders with no change. 

Compression test was showing about 160 psi for those 2 cylinders. 

Visual inspection for any damaged wires in the fuel harness. 

 

Looking to see where I need to check next. This thing has got me stumped. Any help is appreciated. 

[riverbanks]

Think bad cat on bank 2?

[cyoder4400]

18 minutes ago, riverbanks said:

Think bad cat on bank 2?

Just curious, how would a bad cat cause the engine to misfire on some of the bank 2 cylinders but not all, or at least of bad performance on that bank?

[riverbanks]

The weak link or links show up when flow is restricted,what about swapping front 02's,see if problem stays or switches with swap?

[riverbanks]

Maybe a problem on bank 1,how were those plugs?or watch that 02,how high is it reading?

[cyoder4400]

Plugs looked like normal wear. I didn't capture the exact o2 sensor reading, but it's reading in a way that it leaning out the fuel, which I would expect from a cylinder not firing. 

[riverbanks]

My thought was bank 1,rich,both sides get a fuel correction,bank 2,now lean, just trying ideas,thinking codes were cleared when purchased,bummer, good chance all it had was 306,then over time 300 follows from lack of repair

[davester]

check that all the exhaust manifold bolts are present, as exhaust leaks can cause faulty O2 sensor readings..

Edited June 4, 2020 by davester

[cyoder4400]

16 minutes ago, riverbanks said:

My thought was bank 1,rich,both sides get a fuel correction,bank 2,now lean, just trying ideas,thinking codes were cleared when purchased,bummer, good chance all it had was 306,then over time 300 follows from lack of repair

I cleared them after getting it to start with fresh data when I started doing repairs. It was just a 300 originally, narrowed to 306 after first clear. Bank 1 runs just fine. Only bank 2 is getting fuel trimmed, I see bank 1 STFT and LTFT are in a good range. Bank 2 goes negative fast, making me believe the misfiring cylinder is getting fuel but not burning it, causing the bank to attempt to lean thinking it's too rich. I have verified spark is good though, so it just makes no sense to me. 

 

 

15 minutes ago, davester said:

check that all the exhaust manifold bolts are present, as exhaust leaks can cause faulty O2 sensor readings..

All the manifold bolts are present, I haven't noticed a leak up front, at least before any of the o2 sensors. 

[Formulabruce]

A dirty Throttle body can also cause miss fire codes. Needs to be cleaned thoroughly with Throttle body cleaner ONLY. Make sure both side of the throttle plate are done as well. Has to be spotless. Good luck.