1999 L31 Misfiring

[Stepside_fever95]

I have a 1999 K1500 Suburban with the L31 5.7 and am having some misfiring problems around 1500-2000 rpms under load. I used the dash command app and seen that the cmp was at 3 degrees so I set the distributor so that it read 0.3, which is as close to 0 as I could get it. Now it is misfiring even more than before but at around 3000-4000 rpms.
Anyone have any ideas as to why it's doing this?
New ac delco plugs, wires, dist cap, and rotor button a few years ago when I did my intake gasket. Has only started this up in the past month.
Before moving distributor:

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After showed over 30 misfires for cylinders 2, 5, and 8 and the cmp while running was anywhere from -0.1 to 0.9


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[east302]

Is there any play in the distributor when you turn it? Is it the original distributor? Possibly a worn gear.

Are the cap hold down screws in place...the plastic tabs on the distributor can eventually break, allowing the cap to shift.

Dumb question, but the hold down bolt is tight, right?

How do the plugs on those cylinders look? Are the Oxygen sensor readings ok?


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[Stepside_fever95]

Is there any play in the distributor when you turn it? Is it the original distributor? Possibly a worn gear.

Are the cap hold down screws in place...the plastic tabs on the distributor can eventually break, allowing the cap to shift.

Dumb question, but the hold down bolt is tight, right?

How do the plugs on those cylinders look? Are the Oxygen sensor readings ok?


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There's no play and it's the original distributor to my knowledge and it is tight. Cap hold down screws look fine. I haven't had the chance to check the plugs as of yet.

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[Stepside_fever95]

O2 sensors shows its a little rich

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[Stepside_fever95]

After driving it on the highway for 15 minutes at 70-80mph it's doing better. Only had misfires on cylinder 5 and 7.

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[east302]

Can you post a graph for each oxygen sensor showing it in voltage vs time? The upstreams should oscillate regularly, the downstream being relatively constant. That could help rule out a bad sensor impacting your air/fuel mixture.

Edit: Are there any DTCs and have you done a leak down test on the fuel system? Checked for spark at the plugs with a spark tester? I would start with those two tests, though a compression test wouldn't hurt, either.





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[Stepside_fever95]

Can you post a graph for each oxygen sensor showing it in voltage vs time? The upstreams should oscillate regularly, the downstream being relatively constant. That could help rule out a bad sensor impacting your air/fuel mixture.

Edit: Are there any DTCs and have you done a leak down test on the fuel system? Checked for spark at the plugs with a spark tester? I would start with those two tests, though a compression test wouldn't hurt, either.





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I'll record and post o2 sensor results tomorrow. I'm not getting any dtcs either.

I don't have a spark tester but I did change my fuel filter about 2 months ago, I'll check to make sure the lines are tight and not sucking in air.

Is there a possibility I have gotten some bad fuel or water in my tank?

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[Stepside_fever95]

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[Stepside_fever95]

Got it to misfire a lot more around 2200-2700 rpms with slight acceleration. Not throwing any codes though...
These are all screenshots I took while it was misfiring.

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[michael hurd]

Cylinders # 5 and # 7 are next to each other in the firing order. I would change the distributor cap and rotor, since you have done it a few years ago.