98 Montana -107K code P0300

[montanaguy]

Here goes, got a 98 Montana with a 3.4 throwing a P0300 code. It idles rough and no other codes are thrown. I have so far:

-Plugs are new AC Delco 41-921 gapped @ .06
-Plug wires are new
-Ohm tested all injectors (all the same @ .012)
-compression tested all 6 cylinders and got 150-153ish all cylinders
-Used a spark gap tester to check coils and got a strong blue spark from each coil tower
-I also replaced the LIM and UIM since I had the car half way apart before putting back together.
-no unusual noises
-raining today and I can smell the 'rotten egg' smell from exhaust


Here is some sample live data, all at idle parked. If I posted all 60 snapshots you would notice the spark adv stays generally around 20-21 but goes as low as 18 and jumps as high as 25

Live Data
DTC_CNT 1
FUELSYS1 CL
FUELSYS2 N/A
LOAD_PCT(%) 3.9
SHRTFT1(%) 26.6
LONGFT1(%) 21.9
MAP(inHg) 15.9
RPM(/min) 656
VSS(mph) 0
SPARKADV(°) 21
IAT(°F) 81
MAF(lb/min) 0.849
TP(%) 0.0
O2S B1S12--B2S----
O2B1S1(V) 0.715
SHRTFTB1S1(%) 26.6
O2B1S2(V) 0.910
OBD OBD2
------------------
DTC_CNT 1
FUELSYS1 CL
FUELSYS2 N/A
LOAD_PCT(%) 3.9
SHRTFT1(%) 26.6
LONGFT1(%) 21.9
MAP(inHg) 15.9
RPM(/min) 735
VSS(mph) 0
SPARKADV(°) 24
IAT(°F) 81
MAF(lb/min) 0.827
TP(%) 0.0
O2S B1S12--B2S----
O2B1S1(V) 0.235
SHRTFTB1S1(%) 26.6
O2B1S2(V) 0.910
OBD OBD2
------------------
DTC_CNT 1
FUELSYS1 CL
FUELSYS2 N/A
LOAD_PCT(%) 3.9
SHRTFT1(%) 26.6
LONGFT1(%) 21.9
MAP(inHg) 15.7
RPM(/min) 734
VSS(mph) 0
SPARKADV(°) 25
IAT(°F) 81
MAF(lb/min) 0.827
TP(%) 0.0
O2S B1S12--B2S----
O2B1S1(V) 0.230
SHRTFTB1S1(%) 26.6
O2B1S2(V) 0.910
OBD OBD2
------------------
DTC_CNT 1
FUELSYS1 CL
FUELSYS2 N/A
LOAD_PCT(%) 3.9
SHRTFT1(%) 26.6
LONGFT1(%) 21.9
MAP(inHg) 15.4
RPM(/min) 739
VSS(mph) 0
SPARKADV(°) 23
IAT(°F) 81
MAF(lb/min) 0.831
TP(%) 0.0
O2S B1S12--B2S----
O2B1S1(V) 0.520
SHRTFTB1S1(%) 26.6
O2B1S2(V) 0.910
OBD OBD2
------------------
DTC_CNT 1
FUELSYS1 CL
FUELSYS2 N/A
LOAD_PCT(%) 3.9
SHRTFT1(%) 26.6
LONGFT1(%) 21.9
MAP(inHg) 15.4
RPM(/min) 692
VSS(mph) 0
SPARKADV(°) 20
IAT(°F) 81
MAF(lb/min) 0.823
TP(%) 0.0
O2S B1S12--B2S----
O2B1S1(V) 0.210
SHRTFTB1S1(%) 26.6
O2B1S2(V) 0.910
OBD OBD2
------------------
DTC_CNT 1
FUELSYS1 CL
FUELSYS2 N/A
LOAD_PCT(%) 3.9
SHRTFT1(%) 26.6
LONGFT1(%) 21.9
MAP(inHg) 15.1
RPM(/min) 730
VSS(mph) 0
SPARKADV(°) 25
IAT(°F) 81
MAF(lb/min) 0.833
TP(%) 0.0
O2S B1S12--B2S----
O2B1S1(V) 0.285
SHRTFTB1S1(%) 26.6
O2B1S2(V) 0.910
OBD OBD2
------------------
DTC_CNT 1
FUELSYS1 CL
FUELSYS2 N/A
LOAD_PCT(%) 3.9
SHRTFT1(%) 26.6
LONGFT1(%) 21.9
MAP(inHg) 15.4
RPM(/min) 736
VSS(mph) 0
SPARKADV(°) 23
IAT(°F) 81
MAF(lb/min) 0.844
TP(%) 0.0
O2S B1S12--B2S----
O2B1S1(V) 0.310
SHRTFTB1S1(%) 26.6
O2B1S2(V) 0.910
OBD OBD2
------------------
DTC_CNT 1
FUELSYS1 CL
FUELSYS2 N/A
LOAD_PCT(%) 3.94
SHRTFT1(%) 26.6
LONGFT1(%) 21.9
MAP(inHg) 15.4
RPM(/min) 690
VSS(mph) 0
SPARKADV(°) 20
IAT(°F) 81
MAF(lb/min) 0.812
TP(%) 0.0
O2S B1S12--B2S----
O2B1S1(V) 0.240
SHRTFTB1S1(%) 26.6
O2B1S2(V) 0.910
OBD OBD2
------------------

Anyone have any suggestions or see anything wrong? Having the spark adv jump between 18-25, what does that indicate?

[pocket-rocket]

First of all, welcome to the site

-raining today and I can smell the 'rotten egg' smell from exhaust

This leads me to believe your cat is/has gone bad. The spark advance changing could be the result of the PCM adjusting spark timing due to manifold vacuum change, which could be a partially clogged cat. If you crawl under the van and bang on the cat, does it rattle any, or can you hear it rattle at all while the engine is running?

I have to compliment your diagnosis observations. Most people would have left it at "I changed the plugs and wires and I get a misfire code."