misfire at idle, getting worse, Help
 

The vehicle: '06 Yukon XL 5.3 Flex Fuel (VIN 'z' I believe that is?)
Just bought it a few months ago, now 114k miles on it.

The issue: misfie starting about 3 weeks ago... was getting P0300 code most days, but not all the time, has been increasing, idle especially rough now, and has been low, ~550. When misfire seems especially pronounced, the lights dim, voltage and RPM's drop on gauges. Been getting worse over 3 tanks of gas, different stations (each 100+ miles apart) and different brands, so don't think it's just a bad tank of gas.

I have a cheap scan tool, so I've been watching what I know to look for, always P0300 (so generic/multiple misfire, not single cylinder).
I (or mechanic) have done the following so far: techron in tank, new spark plugs, new evap vent (had gas cap code as well previously, gone now), checked engine-off fuel pressure (ok at 55, held for 5 minutes without leakdwn), new wires, 'professional' fuel system cleaner, ohmed out coil packs ok, new coil pack on cylinder 1 (had a single P0301 before now ~20 P0300 codes that I keep clearing).

Other observations:
Both O2 sensors before cat usually ~.7V, but dip down to 0.1 or so intermittently (frequently, like every 5th reading or so,with refreshing 1 or 2 times a second)... I think that implies lots of oxygen, and thus either lean or no firing, right? I've not gotten a 'lean burn' code like a P0171 or anything like that at this point.
This seems to happen at all engine speeds I've done with parking-lot and parked revving tests, but only 'feel' it at idle. Again, both banks, so don't think this is just a single cylinder issue...

Fuel system (injector) cleaning helped for brief period, then went back to about te same.
Ended up replacing intake manifold, gasket was leaking... that helped for a while, now getting mix of more p0301 and p0300 codes, so still pointing towards cylinder 1...

code is back, getting misfire code almost every time I start the car, sometimes random, sometimes cyl1.

sorry, extra post...
but any thoughts? anyone?

new work done, still having issues:

The latest, both knock sensors and camshaft position sensor (based on the fact that #1 was misfirng most, and #8 about 20% as often).
Still misfiring.

At this point, I've changed most of the sensors, intake manifold...
Still getting misfie that you feel at idle, but o2 sensor still fluctuating throughout the rev range, so i's still misfiring.
Could it be the computer? Given 1 and 8 are the misfires, doesn't 'seem' mechanical, nothin that would indicate mechanical at this point.

Please, I'm really needing some help, for an '06 with 'only' 117k miles, this thing is giving me a big headache.

You need new lifters...at least that's what they told me.

HoleInOne,

Thanks or finally giving me a response. After over 300 views, I was hoping someone would have some ideas :)

The first mechanic I took it to suggested lifters as well, but what I don't understand, is why would lifters affect the long-term misfire? I understand (in theory) the lifter noise at startup, but I still have the misfire after driving the car for Hours, so it's not a 'cold engine' issue, but a constant running issue.
Anyone have any more insights on how the lifters could/would cause this issue?

My 05 5.3 Burb was doing the same thing when I returned from overseas. I like to keep my truck in top condition so I did a complete tune up with all OEM. Since I had the money I went ahead and replaced my MAP and Throttle Body. It helped but I still could not get the idle to smooth out. Today I took it to the Chevy dealer and talked with a Mechanic. He told me to look for arches in the plug wires and coil packs. I found all kind of sparks going on in the dark when the engine was running. So now I am trying to figure out if it is ok to see sparks in the coil packs. But I am sure that arching from the boot to the manifold is bad. This is a very tricky situation for us DIYers.

Same problem here
 

I have a 96 gmc with the 5.7 vortec. I've replaced the following:
Fuel filter, fuel pump, wires and plugs, distributor, cap, rotor, crank shaft and cam position sensor... Think that sums it up. I'm not about to bust out a voltmeter and pretend I know what it's telling me. But after talking with several people and researching online for a couple months, I believe it is the spider fuel injection that is causing the issue. Not wanting to throw another $300-$400 at it to find out but it will be cheaper than a mechanic doing it. Any other thoughts on what it may be?

Get a compression test (rent the tool). I am having the same in my 99 hoe 5.7 . Did the test and had 1 lower compression cylinder. This suggest worn piston rings or stuck valves . people always jump on the spider injection only to realize 400$ later it was not the issue....

I made my peace with the misfire. I still get 17 mpg though highway
so Ill just let her go slowly and maybe put in a new engine at some point.

Yeah, I still drive my truck daily to work and back and pull a trailer from time to time. It still works good. Just can't be good on the engine plus it needs to be inspected next month.