Please help, engine runs very poorly!

[los341]

Hello,

I have a recently acquired 2005 GMC Yukon XL 1500 SLE with a 5.3L flex engine. It's been running poorly ever since I got it. I've been trying (diligently) to figure it out to no avail. I've done so many things to it and nothing has helped. I'm running out of ideas.

Symptoms include:

1. Starts quickly and runs smoothly for a minute or so, but then begins missing and runs terribly. Not driveable.
2. Code P0300
3. Bank 1 long-term fuel trim +10% to +30%
4. Bank 2 long-term fuel trim -10% to -30%
5. Bank 1 old spark plugs are light tan and look burned, as if Bank 1 is running too lean.
6. Bank 2 old spark plugs are black, as if Bank 2 is running too rich.
7. Cylinders 2, 4, 6, 8 (Bank 2) constantly misfiring, with Cylinder 6 being the worst at approximately 150 misfires per minute.
8. Cylinders 1, 3, 5, 7 virtually no misfires.

Here are the things I've done so far:

1. I noticed borderline fuel pressure issues, replaced fuel pump assembly, verified good pressure while running, no change.
2. Replaced all 4 O2 sensors with brand new ones, no change.
3. Electrically confirmed all 4 O2 sensors are wired to the correct pins on the ECU.
4. Electrically confirmed all 8 fuel injectors are wired to the correct ECU pins.
5. Replaced 8 fuel injectors with GM genuine new injectors, no change.
6. Replaced 8 spark plugs and spark plug wires with new, no change.
7. Cylinder compression check (PSI):
1: 184
2: 181
3: 176
4: 180
5: 185
6: 186
7: 179
8: 180
8. Used an oscilloscope to confirm all 8 ignition coils have a similar and good-looking ignition pulse.
9. Removed intake manifold, installed new manifold gaskets and throttle body gasket, also verified no intake blockages in the manifold or heads, no change.
10. Verified no vacuum leaks.
11. Replaced both knock sensors with new, no change.
12. Verified both pre-converter exhaust pressures at O2 sensor holes using an exhaust pressure gauge. Both sides were less than 1 PSI, well below specified maximum of 3 PSI.
13. Verified with stethoscope that all 8 fuel injectors are clicking.

I don't know what else this could possibly be, any help or advice would really be appreciated, thank you!

Carl

 

[Marky Dissod]

If you STILL have a misfire @ 6, swap that coil with a coil on the other side.
If the fault moves to that cylinder, replace that coil.

Check your electrical grounds. By check, I mean don't be too gentle.

 

[Fless]

The first clue is that it runs good until the PCM has fuel control -- but we need short term fuel trims. At hot idle, what is the MAF reporting in g/sec (please don't bother posting the value in lb/min; change your scanner settings)?

Graph the upstream O2 sensors to see how they compare and if they're moving appropriately. Age? Mfr? (that's "manufacturer," not mother-f'er...)

What ethanol % fuel is in the tank? What does the scanner say the alcohol % is? (compare)

When was the fuel filter on the rail last changed, if there is one.

While it's warmed up run a CASE relearn if nothing else to rule it out.

Could do an injector balance test.

 

[rockola1971]

OP says Fuel pressure good. Id like to know what fuel pressure is at idle and around 2k rpm. Didnt see that the spark plug wires were replaced? How about that braided ground wire running from engine to firewall? Inspect for corrosion at each end and go ahead and clean both ring terminals and connection points with wire brush. Ive seen those braided grounds rot right in half.

 

[los341]

If you STILL have a misfire @ 6, swap that coil with a coil on the other side.
If the fault moves to that cylinder, replace that coil.

Check your electrical grounds. By check, I mean don't be too gentle.

Thank you Marky, I'll try swapping coils and checking engine to chassis ground quality.

Carl