Shipkiller
Lifer
I was going to wait a few days before I posted this, but what the hell.
I have been dismayed at the low MPG that my car gets compared to all the other cars I have read about on the forums. About 24MPH on the highway for a 2.5L running about 65-70MPH
I have done all the normal maintenance that the car requires.
Last Nov, I threw a CEL (Check Engine) light and it turned out to be one of the O2 sensors, but it cleared after a day or two. Then I forgot about it until recently.
Then it kinda dawned on me that I have not done any monitoring of the emission system with my laptop. Sorta focused on the AirBag light.
So over the last two weeks I have been on and off hooking up the laptop ODB-II scanner to the car and watching how the upstream O2 sensors (B1S1 and B2S1) were performing.
When reading voltage from a normal O2 sensor, it is supposed to read between 0V (nominally .2V) for lean and 1V for rich when the engine in 'closed loop' (at operating temp), and when reading this voltage on a graph on the laptop, the trigger pulses (more like a sign wave) should increase in frequency with the rpm's.
An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen in the exhaust. When the mixture is lean, the sensor output voltage will drop down to about 0.2 volts or less. When the air/fuel mixture is balanced or at the equilibrium point of about 14.7 to 1, the sensor will read around .45 volts.
I had noticed that sometimes the wave form would 'lose' a pulse or more than one, like the sensor just paused and it would not ramp up in the frequency as fast as the rpms.. Additionally, the max voltage that both upstream sensors was between .35 and .45 volts which put the midpoint around .2 volts instead of .4 or .5 volts.
That tells me one of two things:
1. The car is running way lean, no matter what it is doing. That's not happening. The engine is not overly hot or lacks any power. Incoming fuel DOES cool the cylinders....
2. The upstream O2 sensors are not doing what they are supposed to be doing, but are NOT faulty enough for the computer to throw a CEL. The computer is increasing the fuel injector pulse width (dumping more fuel in the cylinders) trying to get the oxygen levels in the exhaust, as read by the upstream O2 sensors, where they are supposed to be. Since they are most likely bad, this will never happen.
When the computer receives a rich signal (high voltage) from the O2 sensor, it leans the fuel mixture to reduce the sensor's reedback voltage. When the O2 sensor reading goes lean (low voltage), the computer reverses again making the fuel mixture go rich. This constant flip-flopping back and forth of the fuel mixture occurs with different speeds depending on the fuel system. The transition rate is slowest on engines with feedback carburetors, typically once per second at 2500 rpm. Engines with throttle body injection are somewhat faster (2 to 3 times per second at 2500 rpm), while engines with multiport injection are the fastest (5 to 7 times per second at 2500 rpm).
Stole this pic from website. Did not take any screen captures.
So I ordered two new O2 sensors yesterday from AdvanceAuto. There are different part numbers for each upstream sensor.
One number is for the FRONT bank and another number for the REAR bank. The front bank (cylinders 1-3) has black cable insulation and the rear bank (cylinders 4-6) has grey cable insulation. The cable length is the same for both sensors.
The front sensor was $85.00 and the rear sensor was $110.00. Why the price diff? Who knows.... :idunno:
When you pick up the new sensors, make SURE, you mark the box's as to which one's are FRONT and which one is REAR.... BEFORE you leave the store.... The box's DO NOT say.....
The OEM sensors were Bosch and the new ones were Bosch also....
Monitoring the new sensors, I get a low of .2V and a high of .9V and I am not 'missing' any crosscounts or pulses...
Time will tell..
I have been dismayed at the low MPG that my car gets compared to all the other cars I have read about on the forums. About 24MPH on the highway for a 2.5L running about 65-70MPH
I have done all the normal maintenance that the car requires.
Last Nov, I threw a CEL (Check Engine) light and it turned out to be one of the O2 sensors, but it cleared after a day or two. Then I forgot about it until recently.
Then it kinda dawned on me that I have not done any monitoring of the emission system with my laptop. Sorta focused on the AirBag light.
So over the last two weeks I have been on and off hooking up the laptop ODB-II scanner to the car and watching how the upstream O2 sensors (B1S1 and B2S1) were performing.
When reading voltage from a normal O2 sensor, it is supposed to read between 0V (nominally .2V) for lean and 1V for rich when the engine in 'closed loop' (at operating temp), and when reading this voltage on a graph on the laptop, the trigger pulses (more like a sign wave) should increase in frequency with the rpm's.
An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen in the exhaust. When the mixture is lean, the sensor output voltage will drop down to about 0.2 volts or less. When the air/fuel mixture is balanced or at the equilibrium point of about 14.7 to 1, the sensor will read around .45 volts.
I had noticed that sometimes the wave form would 'lose' a pulse or more than one, like the sensor just paused and it would not ramp up in the frequency as fast as the rpms.. Additionally, the max voltage that both upstream sensors was between .35 and .45 volts which put the midpoint around .2 volts instead of .4 or .5 volts.
That tells me one of two things:
1. The car is running way lean, no matter what it is doing. That's not happening. The engine is not overly hot or lacks any power. Incoming fuel DOES cool the cylinders....
2. The upstream O2 sensors are not doing what they are supposed to be doing, but are NOT faulty enough for the computer to throw a CEL. The computer is increasing the fuel injector pulse width (dumping more fuel in the cylinders) trying to get the oxygen levels in the exhaust, as read by the upstream O2 sensors, where they are supposed to be. Since they are most likely bad, this will never happen.
When the computer receives a rich signal (high voltage) from the O2 sensor, it leans the fuel mixture to reduce the sensor's reedback voltage. When the O2 sensor reading goes lean (low voltage), the computer reverses again making the fuel mixture go rich. This constant flip-flopping back and forth of the fuel mixture occurs with different speeds depending on the fuel system. The transition rate is slowest on engines with feedback carburetors, typically once per second at 2500 rpm. Engines with throttle body injection are somewhat faster (2 to 3 times per second at 2500 rpm), while engines with multiport injection are the fastest (5 to 7 times per second at 2500 rpm).
Stole this pic from website. Did not take any screen captures.
So I ordered two new O2 sensors yesterday from AdvanceAuto. There are different part numbers for each upstream sensor.
One number is for the FRONT bank and another number for the REAR bank. The front bank (cylinders 1-3) has black cable insulation and the rear bank (cylinders 4-6) has grey cable insulation. The cable length is the same for both sensors.
The front sensor was $85.00 and the rear sensor was $110.00. Why the price diff? Who knows.... :idunno:
When you pick up the new sensors, make SURE, you mark the box's as to which one's are FRONT and which one is REAR.... BEFORE you leave the store.... The box's DO NOT say.....
The OEM sensors were Bosch and the new ones were Bosch also....
Monitoring the new sensors, I get a low of .2V and a high of .9V and I am not 'missing' any crosscounts or pulses...
Time will tell..
Think of all the gas you used and possible damage by running too lean. 
