I just replaced the lambda on my 98 Mazda 626 GF 2L because it was showing a signal that was constantly pegged lean and I suspected it was defective.
After replacing the sensor I took the car for a 15 minute spin which I logged. One thing of interest to me was the relation between O2 voltage and STFT ( I logged this at the end of the drive ):
I was reading a page about diagnosing lambda operation and a few questions came up in my mind.
The first question was due to this sentence:
6) The O2 sensor must cycle at least once per second, which would show 3 cross counts on the scan tool PID.
It also says:
The O2 sensor not only has to cycle, it also has to cycle fast enough (proper frequency) and wide enough (proper amplitude). At least one cycle per second ( 1 Hz ) must be seen at the signal wire in order for the O2 to be considered good (not lazy). A one cycle per second will make the scope trace go across the 0.450 volts mark approximately 3 times, which the ECM recognizes as 3 cross counts.
As you can see on this zoomed in graph the sensor is only crossing 0.45 volts about once every three seconds instead of once per second.
The STFT is also spending about two seconds in negative territory for every one second in positive territory.
Now according to the same site:
10) Contrary to what many people think, an O2 sensor WILL NOT cycle by itself. The O2 sensor cycle is a direct result of the ECM response to the changes in the mixture.
So the question arises, why is my O2 signal cycling so much slower than what this site says is normal, and why is the STFT spending twice the time negative as positive? This is in a car suffering from a rough idle, with occasional stumbles during idle, and an initial lack of power on acceleration.
I can think of a few possibilities:
- This is just the normal algorithm for this car's ECU
- A defective Lambda
- A defective ECU not responding as quickly as it should
- Air / Fuel mixture not getting completely burnt due to insufficient air
I'm kind of leaning towards the last possibility since I noticed that the lambda is ranging between .18 to .89 volts with it staying mostly above .20 volts and the fact that the STFT is going negative twice as much as it's going positive.
I can think of a few possible causes:
- Injector's sticking open or excess fuel pressure
- An intake valve not opening sufficiently
- MAF biased slightly high making ECU think there is more air than there actually is
I measured cam to lifter clearances. Spec is 0.23mm to 0.30mm
+---------+-----+-----+-----+-----+ | Cyl # | 1 | 2 | 3 | 4 | +---------+-----+-----+-----+-----+ | Intake | .24 | .29 | .24 | .24 | +---------+-----+-----+-----+-----+ | Exhaust | .29 | .24 | .29 | .24 | +---------+-----+-----+-----+-----+
They're all within spec, but maybe the wear on I2 relative to the other intake valve lifter shims is causing the problem?
Anyways, I'd like to hear what all y'all think.