mjshort

I have a 2000 Malibu with a 3.1, about 140,000 miles on it, completely stock. It ran OK, except it would throw a misfire code (p03xx) occasionally, plus the lower intake manifold had a slight leak. I decided to replace the lower intake gasket. I have it all back together and now it's constantly throwing a multiple misfire code (P0300) after about 30 seconds of running. It idles rough and there was a strong rotten egg smell when I let it run longer. I replaced the 7x crank trigger, replaced the plugs with AC rapid fire, replaced the igniition module, checked the coils (each of the plugs looked like the spark was good, the plug wires look good, I didn't see any arcing when looking in the dark. I can feel a 'knock' if I put my hand on top of the intake manifold. It seems to be running lean according to the computer. I read somewhere that it's probably because of the misfires. Any suggestions would be very helpful! I'm stumped!
Here's some data I recorded with the code reader (I attached the whole file):

VEH SPEED(MPH) 0
2000 Chevrolet Malibu J = 3.1 With AP


I/M Monitors

Since DTCs Cleared

Misfire Monitor n/a
Fuel System Mon n/a
Comp Component n/a
Catalyst Mon inc
Htd Catalyst n/a
Evap System Mon inc
Sec Air System inc
A/C Refrig Mon n/a
Oxygen Sens Mon inc
Oxygen Sens Htr inc
EGR System Mon inc

=====================

DTCs (Codes)

P0300 MOD $10

Random - Multiple Misfire Detected

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P0300 MOD $10

Random - Multiple Misfire Detected

PENDING

=====================

State OBD Check

MIL Status ON
Codes Found 1
Monitors OK 0
Monitors Inc 6
Monitors N/A 5

=====================

Record Data

FRAME: -1 TM: -1.9

MIL STATUS On
ABSLT TPS(%) 0.4
ENG SPEED(RPM) 715
CALC LOAD(%) 3.5
MAF(LB/M) 0.75
MAP("HG) 13.3
COOLANT(°F) 149
IAT(°F) 84
IGN ADV(°) 22.5
SECOND AIR ATMOS
ST FTRM1(%) -0.8
LT FTRM1(%) -3.1
VEH SPEED(MPH) 0
FUEL SYS 1 CLSD
FUEL SYS 2 CLSD
O2S11(V) 0.135
ST FTRM11(%) -78.1
O2S12(V) 0.935
OBD2 STAT CA

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FRAME: 0 TM: 0.0

MIL STATUS On
ABSLT TPS(%) 0.0
ENG SPEED(RPM) 690
CALC LOAD(%) 3.5
MAF(LB/M) 0.76
MAP("HG) 13.3
COOLANT(°F) 149
IAT(°F) 84
IGN ADV(°) 22.5
SECOND AIR ATMOS
ST FTRM1(%) 0.8
LT FTRM1(%) -3.1
VEH SPEED(MPH) 0
FUEL SYS 1 CLSD
FUEL SYS 2 CLSD
O2S11(V) 0.195
ST FTRM11(%) -60.9
O2S12(V) 0.930
OBD2 STAT CA

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FRAME: 1 TM: 1.9

MIL STATUS On
ABSLT TPS(%) 0.4
ENG SPEED(RPM) 699
CALC LOAD(%) 3.5
MAF(LB/M) 0.77
MAP("HG) 13.0
COOLANT(°F) 149
IAT(°F) 84
IGN ADV(°) 21.0
SECOND AIR ATMOS
ST FTRM1(%) -0.8
LT FTRM1(%) -3.1
VEH SPEED(MPH) 0
FUEL SYS 1 CLSD
FUEL SYS 2 CLSD
O2S11(V) 0.155
ST FTRM11(%) -68.0
O2S12(V) 0.930
OBD2 STAT CA

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2wd-K5

did you put dielectric grease on the new plugs and wires? some new vehicles require it, im sure about yours, but it is a good thing to do

mjshort

Yep. I also put anti-seize on the plugs before installing them. I swapped the 24x crank sensor yesterday, but it still didn't help.

mjshort

Got it! The problem was the upstream O2 sensor. Strange that it caused a misfire code while still in open loop mode. But in any case that was the problem.