Slight misfire at cruise

[2004BlueBurb]

Hello all, I'm having an issue with my suburban, driving me nuts. My question is this, I have a 2004 Suburban 1500, 5.3L LM7 engine, 180k miles (my personal truck). Last Saturday I replaced the knock sensors due to a code set by bad bank 2 knock sensor, still had the baffles under the intake and sure enough, rear knock sensor hole has water and was rusty. Replaced both sensors with oe sensors with the harness. While I was there I replaced the valley cover gasket, cam sensor and oil pressure sensor. All went as planned, drove fine afterwards. The next day I was driving it the highway, noticed a slight misfire at cruise only, with a very slight load. Just a little bump here and there. Hooked up my scanner (Autel), #6 has 2-6 misfires randomly under light load cruise, which is 1500-2000 rpm. Swapped coils with #4, no change. Did the same with the plugs and wires, no change. Swapped injector with #2, no change. Has 140-150 psi compression. WOT has a lot of power, smooth, sometimes pings above 4500rpm, and idle is fine. Fuel trims at idle are bouncing up and down right around zero, wot they are about +4.

 

I'm at a loss of what to do next. In all the classes I've been to, whenever there is a mechanical problem, it's usually constant under load or at idle. I don't remember hearing about anything like this. Any advice would be very very much appreciated. Thank you guys n

 

Tyler

[Justin101714]

You need to find out what the running compression is for those affected cylinders and compare them to known good cylinders. Could have possible weak valve springs that could cause the misfire under load but not at idle.

 

 

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[2004BlueBurb]

Hey, thanks for the reply. I checked that, half of cranking compression. Used s pressure transducer to see valve opening and timing on scope, all good. Scoped the power to bank two coils, steady charging voltage. Scoped the pcm control signal to coil, good. Ended up replacing the coil wiring harness for bank two, no more misfires. Just had a bad connection in the harness somewhere that only acted up during certain load and vibration range. Weird but so glad it isn't a valve issue. I don't want to take motor apart yet! I do want to hop it up someday, just not ready to do it yet.

[Justin101714]

Awesome! Glad you got it fixed up!

 

 

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[Supreme Pizza]

Ended up replacing the coil wiring harness for bank two, no more misfires

 

 

 

 

Nice work man. Good troubleshooting.

[2004BlueBurb]

Thank you guys! It was a tough one. Have never seen that at my shop before. I replaced bank 2 harness as well, just for PM.

[2004BlueBurb]

Now to just figure out how to squeeze some more power out of her to make up for the heavier, and slightly bigger off road tires I have on it now. Haha.