O2 sensor graphs - any comments?

[omare8]

Replaced both After-Cat O2 sensors and didn't change graph.
Can't get OBD O2 Monitors set.

Any ideas?

 

[coonerboy]

Looks like the cat is bad-bank 1

 

[omare8]

Both cats replaced 1 year ago...

 

[coonerboy]

What codes were you getting prior? And what engine?

 

[omare8]

before I got P0171 and P0174.. sometimes one.. sometimes other.. sometimes both..

Sometimes right away but for last few weeks can not get O2 Monitor set.. last couple times it took me 520 miles and 330 miles to get O2 monitors set but along with one of those codes.

My car : Mercury Mountaineer 2000; 5.0L; 4WD; Auto-transm

Replaced:
both Post-Cat O2 sensors;
MAF sensor;
upper intake manifold gasket;
idle air valve;
one vacuum pipe in question;
PCV valve;
just put new synthetic oil within limits;
fuel filter;
3 injectors just to be sure..
put in SeaFoam cleaner in last tank of gas..
air filter;
cleaned throttle;
tested throttle position sensor;
all caps are in place;
gas pressure tested - 51psi;
all new plugs properly gaped;

 

[briwayjones]

How many miles are on the plug and wires?

 

[omare8]

plugs - maybe 300 miles.. wires - don't know..

 

[briwayjones]

If you have anywhere close to 100,000 miles on the wires I would try changing them. You never know. What kind of plugs are in it? Ford vehicles tend to not be happy with plugs unless they're Motorcraft or Autolite. They especially don't like Bosch plugs.

 

[2000StreetRod]

In Theory

I have no practical experience with O2 sensors but I have read some of the theory related to them. For each bank, the only thing between the first and second sensor is the catalytic converter. Therefore, the second sensor should track the first sensor except for a time delay and an amplitude scalar. Your posted graph shows a regular cycling of lambda for bank 1 with sensor 2 tracking sensor 1. The graph for bank 2 shows an irregular cycling for sensor 1 and no tracking for sensor 2.

Concentrate first on fixing sensor 2 since it is obviously faulty. Since it and the catalytic converter have been replaced I would suspect either electrical connections or contamination. Silicone will rapidly destroy an O2 sensor. Have you sprayed any into the intake system or lubricated the sensor threads with it? Have you checked the heater voltage to the sensor? How long had the engine been running for the posted chart? Was it long enough for the sensors to be heated by the exhaust even if the heaters weren't working?

Once you get sensor 2 to show some cycling you'll be able to better tell if there is a problem with sensor 1 or the PCM richness control. If sensor 2 tracks sensor 1 and the cycling is still irregular then your problem is not sensor 1.

 

[coonerboy]

I have no practical experience with O2 sensors but I have read some of the theory related to them. For each bank, the only thing between the first and second sensor is the catalytic converter. Therefore, the second sensor should track the first sensor except for a time delay and an amplitude scalar. Your posted graph shows a regular cycling of lambda for bank 1 with sensor 2 tracking sensor 1. The graph for bank 2 shows an irregular cycling for sensor 1 and no tracking for sensor 2.

Concentrate first on fixing sensor 2 since it is obviously faulty. Since it and the catalytic converter have been replaced I would suspect either electrical connections or contamination. Silicone will rapidly destroy an O2 sensor. Have you sprayed any into the intake system or lubricated the sensor threads with it? Have you checked the heater voltage to the sensor? How long had the engine been running for the posted chart? Was it long enough for the sensors to be heated by the exhaust even if the heaters weren't working?

Once you get sensor 2 to show some cycling you'll be able to better tell if there is a problem with sensor 1 or the PCM richness control. If sensor 2 tracks sensor 1 and the cycling is still irregular then your problem is not sensor 1.

Thats not correct. A post cat sensor(sensor 2) should not track the pre cat sensor(sensor 1). If it is, its showing that the catalyic converter isn't doing anything. This would be what you would get if the cat was gutted. Ideally the post cat sensor will track a very flat path near the middle of the graph(.5mv or a 14.7 stoich. ratio)

 

[Gus Gorilla]

Coonerboy,
Bingo. But it's .5V half a volt. not .5mv an O2 can make 0-1V. Bank 2 looks pretty erratic but notice that for each sensor 1 spike, sensor 2 spikes very slightly. Except at the end where it gets stupid. I would suspect that your bank 1 cat is doing nothing but bank 1 running right all else considered. Bank 2 has something wrong happening possibly not the cat but, the slightly matching peaks suggest the sensor are working, the cat is working, something else is not.
Somebody chime in and correct my "suspect" and "possibly" and get this guy some solid help. I know there's some Master driveability techs lurking around.

 

[Heavymentill]

Replaced both After-Cat O2 sensors and didn't change graph.
Can't get OBD O2 Monitors set.

Any ideas?

What scope are you using? My Fluke says maximum peak should be at least 800mV or more. Minimum peak voltage should reach at least 200mV or less.
Bank 1 looks like a normal readout bank 2 looks all jacked up. Like a bad Sensor.

 

[2000StreetRod]

Thats not correct. A post cat sensor(sensor 2) should not track the pre cat sensor(sensor 1). If it is, its showing that the catalyic converter isn't doing anything. This would be what you would get if the cat was gutted. Ideally the post cat sensor will track a very flat path near the middle of the graph(.5mv or a 14.7 stoich. ratio)

By "track" I meant that the cycle period (time from peak to peak or low to low) should be the same for pre and post cat O2 sensors. By "time delay" I meant the time it takes for exhaust gas to travel the distance from the pre cat O2 sensor to the post cat O2 sensor. By "scalar" I meant that the post cat O2 output peak to low range will be reduced by a fixed amount. There is a good example in the following thread http://www.explorerforum.com/forums/showthread.php?t=149917

A three way catalytic converter is designed to operate with lambda approximately equal to 1.0 (0.2 volts to 0.7 volts O2 output). When lambda is only slightly greater than 1.0 the NOx emissions rapidly increase. I agree with you that the graph indicates that the bank 1 cat seems to have very little effect.

I have a question about the graph. If the horizontal axis is time increasing from left to right then the output of sensor 2 seems to be leading instead of lagging the output of sensor 1. Also, if the time base is seconds then the cycle period seems to be very long even for the engine at idle.