Hello, I have done numerous searches regarding this issue. I have look at many different things.

The symptoms just occurred on after the heavy rain. The vehicle started to miss and stumble at idle, but runs fine at higher RPM. Here is what I have found:

1. Code reader in history shows code DTC P0300, intermittent misfire.
2. Checked fuel pressure, which was 60 psi and stayed constant for over 10 min
3. Checked spark on all cylinders with timing light, cylinder #2 was inconsistent.
4. #2 spark plug wire shows 1200-1300 ohms in 2 ft, #4 is similar, #2 is about 800 and is about 1’ long
5. Checked plugs #2, #4, #6, look fine, light chocolate brown in color were changed from platinum to iridium 3 years ago.
6. Checked compression
• #2-150 #1-148
• #4-148 #3-150
• #6-150 #5-148
7. Checked cap and rotor, all looks fine, very little corrosion, were changed 3 years ago.

Put it back together and started it up

1. Idles fine when cold
2. Using the timing light #2 looks the same as the others good consistent spark, consistent no blackouts
3. Revs well and is smooth
4. Once up to temperature the stumble and miss returns
5. Check #2 with the light the spark again was inconsistent with #4 being normal
6. Checked codes, with none present.

Parts changed Aug 1/15

1. Changed wires, plugs, cap and rotor. All GM
2. Checked EGR, looks fine and changed gasket
3. Changed fuel filter because it needed changing.
4. After changing the wires, the idle was bad. Checked the plug boots, there was too much dielectric grease in them. Cleaned that out. Idles OK, but there still is a miss. It seems like I am at the same point I started with.
5. Checked the coil for any issues, the high side is around 5500ohms and the low side is about 0.7ohms.
6. Code DTC P0300 still showing, the freeze frame data in open loop are:
• TPS 0.0
• Fuel system 1 OL
• Fuel system 2 OL
• Calc load 6.3
• ECT (F) 79
• STFT B1 % 7.8
• LTFT B1 % 7.8
• STFT B2 % 6.3
• STFT B2 % 7.8
• Map inHG 12.1
• RPM 895
• Vehicle speed 0
• MAF lb/min 1.4

7. Here is the closed loop data
• TPS 0.0
• Fuel system 1 CL
• Fuel system 2 CL
• Calc load 0
• ECT (F) -40
• STFT B1 % -100
• LTFT B1 % -100
• STFT B2 % -100
• STFT B2 % -100
• Map inHG 0
• RPM 0
• Vehicle speed 0
• MAF lb/min 0.0

The closed loop freeze frame data does not make any sense.