01vetter

I am trying to diagnose the cause of a somewhat lumpy idle. I don’t have any codes and I have ruled out vacuum leaks and ignition issues. I am now focusing on my fuel injectors and O2 sensors. I am posting some data log information and would like your opinions regarding the health of my O2 sensors.

My scanner is shows that the long-term fuel trim for bank 1 is typically running 3-5 points leaner than bank 2. Here are a few sample time points from a driving data log:

At idle:

FRAME: 0 TM: 0.0

MIL STATUS Off
ABSLT TPS(%) 4.3
ENG SPEED(RPM) 810
CALC LOAD(%) 2.4
MAF(LB/M) 0.96
MAP("HG) 9.2
COOLANT(°F) 198
IAT(°F) 81
IGN ADV(°) 17.0
SECOND AIR ATMOS
ST FTRM1(%) 0.0
ST FTRM2(%) 0.0
LT FTRM1(%) 3.9
LT FTRM2(%) 1.6
VEH SPEED(MPH) 0
FUEL SYS 1 CLSD
FUEL SYS 2 CLSD
O2S11(V) 0.835
ST FTRM11(%) 0.0
O2S12(V) 0.695
O2S21(V) 0.840
ST FTRM21(%) -0.8
O2S22(V) 0.630
OBD2 STAT CA

At light throttle:

FRAME: 28 TM: 67.5

MIL STATUS Off
ABSLT TPS(%) 12.2
ENG SPEED(RPM) 1358
CALC LOAD(%) 6.3
MAF(LB/M) 2.40
MAP("HG) 10.6
COOLANT(°F) 196
IAT(°F) 82
IGN ADV(°) 36.5
SECOND AIR ATMOS
ST FTRM1(%) 1.6
ST FTRM2(%) 4.7
LT FTRM1(%) 2.3
LT FTRM2(%) -0.8
VEH SPEED(MPH) 13
FUEL SYS 1 CLSD
FUEL SYS 2 CLSD
O2S11(V) 0.880
ST FTRM11(%) 0.0
O2S12(V) 0.815
O2S21(V) 0.835
ST FTRM21(%) 0.0
O2S22(V) 0.730
OBD2 STAT CA

At full throttle:

FRAME: 43 TM: 103.8

MIL STATUS Off
ABSLT TPS(%) 99.6
ENG SPEED(RPM) 4352
CALC LOAD(%) 100.0
MAF(LB/M) 38.69
MAP("HG) 30.7
COOLANT(°F) 192
IAT(°F) 79
IGN ADV(°) 14.0
SECOND AIR ATMOS
ST FTRM1(%) -2.3
ST FTRM2(%) 0.0
LT FTRM1(%) 1.6
LT FTRM2(%) -3.1
VEH SPEED(MPH) 62
FUEL SYS 1 CLSD
FUEL SYS 2 CLSD
O2S11(V) 0.790
ST FTRM11(%) 0.0
O2S12(V) 0.930
O2S21(V) 0.710
ST FTRM21(%) 0.0
O2S22(V) 0.910
OBD2 STAT CA

The fuel trims bounce around, but in general Bank 1 is typically running leaner than Bank 2.

The following are the O2 Monitor Tests for Bank 1 and Bank 2 sensors. In general they look OK with a slight difference in switch time (.01) between bank 1 and Bank 2. This would indicate that they are OK.

O2 Bank1 Sensor1

RICH-LN Thresh
MEAS 0.495(V)

LN-RICH Thresh
MEAS 0.495(V)

Lo V for Switch
MEAS 0.300(V)

Hi V for Switch
MEAS 0.600(V)

RICH-LN Sw Time
MAX 0.09(s)
MEAS 0.06(s)
MIN 0.00(s)

LN-RICH Sw Time
MAX 0.08(s)
MEAS 0.04(s)
MIN 0.00(s)

---------------------

O2 Bank2 Sensor1

RICH-LN Thresh
MEAS 0.495(V)

LN-RICH Thresh
MEAS 0.495(V)

Lo V for Switch
MEAS 0.300(V)

Hi V for Switch
MEAS 0.600(V)

RICH-LN Sw Time
MAX 0.09(s)
MEAS 0.05(s)
MIN 0.00(s)

LN-RICH Sw Time
MAX 0.08(s)
MEAS 0.03(s)
MIN 0.00(s)

I have attached a plot the left and right bank O2 voltages at idle below. The blue curve is Bank 1 and the purple plot is Bank 2 volts. The sample rate was .2 Seconds.



This plot is showing that Bank 1 is running somewhat leaner than Bank 2. It also shows some unusual anomalies. The red circled area shows that Bank 2 is almost skipping a beat. The blue circle area shows that Bank1 as having jagged top spikes. The green area is showing an extended time of no voltage. This may be showing the lumpy idle that I am feeling. Could these be caused by my data sample rate (.2 Sec.) not being fast enough?

Below are two plots showing 2K RPM snap throttle tests for Bank 1 and 2 O2 voltages. These were done at 0 vehicle speed and 0 engine load to show O2 sensor response to rich to lean conditions. The sample rate was .1 Sec.


and


At 2K RPM with no engine load I do feel an occasional skip. I have circled some anomalies in red that I think might correspond to the skipping that I am feeling. Note that I don’t feel any skipping when the engine is under load.

I think my O2 sensors are OK, but my injectors might be dirty. What are your opinions?

Thanks!

MVP'S ZO6

O2 sensors appear to be switching normally

FixedRoof

Normally a BAD O2 will just be dead or stuck. Very very little fluctuation. No switching at all. I had one do that and a buddy of mine did as well. Car ran like total crap. They can get a little bit lazy over time. If they are very old or you are really worried about them, change them.

There will be some variation in the LTFT and STFT from bank 1 and 2.

How are your plugs and plug wires? Have you logged or checked the MAF? A dirty MAF can cause some odd stumbles or rough running.

Idle

That info is not the greatest but your WOT run shows low voltage on the passenger side. I'd be looking at passenger side stuff.

01vetter

I have thoroughly checked my plugs and wires. I replaced the plugs, checked the resistance of my wires, looked for arcing and ran an in-line spark tester. I even swapped wires from Bank 1 to Bank 2.

I cleaned the MAF. It didn't do anything. I also cleaned the throttle body, which also didn't change anything.

Other than cleaning a MAF, how do you check one? I have a data log of MAF(LB/M), which I can plot, but how do I interpret the data? Any other checks I should look at?

01vetter

Yes I am looking at Bank 2. I am thinking that it is running rich. If this is the case, wouldn't that account for the low voltage?

Gordy M

How old are your O2 sensors, with unleaded gas the original type sensors were prone to aging by the ethanol. Delco revised the sensors to make them more resistant. A friend who is very knowledgeable about PCM tuning on aC5 was amazed at what he found when he updated his O2 sensors. With his idle starting a slight surge he thought the injectors were starting to clog and pulled them to clean. No difference, so he changes the O2's and was amazed at the smooth idle and his scans showed improved performance.

tblu92

1st remember that the 02's are only used during P/T. operation
When you step on the gas pedal and the thotttle goes past approx. 65% two things happen. The 02's are now NOT being used to fuel the eng and the ECM goes into "power enrichment mode" where the fuel is commaned by whatever AFR you are asking for in your tune
2nd. A properly working 02 during p/t should oscilate very rapidly from about .100 millivolts to about .950
Yours seem to be fine. A lazy 02 will oscilate slow
When an 02 completly fails or fouls the voltage will flatline around .450 millivolts
When your LTFT's show a larger difference between left and right is is usuallly only 2 thins. One 02may be lazy, however the MOST COMMON reason is an exhaust leak.
The richer side is the side with the leak . It doesn't take much of a leak either. On the side with the leak. That 02 has less exhaust to read and does not read hot as the other side
so that 02 reads richer and the LTFT's will have a HIGHER negative number than the other side
A small split between right and left is OK
This all can be very confusing !!!!!!
If you would like to discuss it further you can call my cell anytime and we can try ro sort it out
Tom. 209-603-2983. (Calif. time)

BOUT X

I was going to start my own thread when I came across this one here is my issue any help would be great.

The Car

2000 C5 Coupe LS-6 6SPD fast intake LG Longtubes with no cats just running front 02's vararam ported ls2 T.B. RED 32 lbs injectors

The Problem

car idles ok but when I give it gas it seems to bogg down and seems to be running real rich. Then as I get the RPM'S up it seems to run better. I put the car on a scanner (snap-on) my fuel trims look good but my 02's read way differant drivers side is sharp and crisp on the meter. The one on the Passanger side is real slow large arc's not crisp.
The MAF is clean and I drove 70 miles and used about 2/3 of a tank of gas. The thing is when I am doing 50-80 mph car feels fine but on take off it wont get out of it's own way.

If you have any idea's please post up TIA to all and tblu92 I see you are in the 209 I am also in Modesto can I call you to talk about this problem.

Regards,

DanSavage

I'm not trying to hijack your thread, but like yours, my car has a slightly lumpy idle and it accelerates without issue under load. (no codes)

First, I cleaned the MAF & throttle body. That helped a little.

Then, I changed the plugs and wires. That helped a little, but the lumpy idle keeps coming back.

I haven't checked for a vacuum leak yet, but I do have the gauge to check it.

So, if I may ask you a couple of questions.

1) Where did you tap into the intake to check the vacuum?
2) What software/scanner are you using to produce the plots?

TIA,

Dan

directnosfogger

for ***** and giggles make sure that the 02 rich/lean vs. airflow tables are the same. if the voltages are different, that might cause issue. this can also be adjusted if necessary to meet proper voltage (ie adding headers may change the characteristics). i'm no expert, just the research i have done on hptuners forum.

hptuners>engine>fuel>open & closed loop> 02 Rich/Lean vs. Airflow > Bank 1 / Bank 2