Occasional leaded fuel and O2

[Garfield Willis]

On Mon, 17 Jul 2000 13:09:48 -0700 (PDT), Clay B <clayb at sonic.net>
wrote:

>If I use 100LL Avgas on occasion, interspersed with tanks of 92 octane
>pump fuel, how much will the lifespan on my (heated Bosch) O2 sensor be
>reduced? What is the failure mode? Will I be able to see it coming?
>
>Anyone else have experience using leaded fuel with their O2 sensor?

Unhappily, this is a subject fraught with great controversy. While it it
known that lead contamination is a very real issue with O2 sensors,
beyond that, little consensus seems to have formed.

(1) There have been EXTREMELY wide ranges of longevity
reported/experienced by both switch & wideband O2 sensor users. Some of
this has been explained away by the conjecture that the lead deposition
rate has alot to do with the actual placement of the sensor in the
exhaust stream. By creating a wee pocket (or better yet, putting it in a
crossover pipe) for the sensor, some have surmised that's why theirs are
lasting many hundreds of hours with 100LL. IOW, keeping it out of the
direct blast of the dragon's belch may help extend the life
significantly, or the temps of the exhaust gases when they hit the
sensor may be important.

(2) BUT even users who've run NTK systems with leaded racing fuel, have
reported seeing little if any sensor degradation; Frank Parker onlist is
one of those, and I've heard the same from others lately. In these
setups, I really question the sensor actually being hidden much from the
exhaust stream; most racing headers are by their nature pretty sleek
flowing. So again, it's not clear why the range of results.

(3) I'm running a couple with 100LL in XAviation applications as much as
I'm able, but also haven't been able to detect any degradation in either
of these "guinea pig" sensors set aside for just that purpose. However,
they each only have around a dozen flight hours on them.

(4) Since I have no first-hand examples of lead contamination, all I can
pass along is what I've read and heard so far; the assumed mode of
contamination is apparently not electrochemical, but simple occlusion of
the porosity/diffusion of the gases thru the surface of the active
elements, which tends to choke the sensor slowly, reducing it's output
swing (in either switch or WB types), and in process therefore, would
tend to lower the slope of the nonlinearity at stoich, which is
considered the cause of the "lazy/slow sensor" phenom observed in
switch-type sensor, which also occurs simply from aging or carbon
contamination if burning too rich a mix or excessive oil burning.

Indeed, the manufacturing variation in diffusion rates from sensor to
sensor is mostly what is calibrated or "trimmed" out even when you do a
free-air calibration with a new WBO2 sensor/setup. In such a WBO2 setup,
the aging/contamination severity shows up as "how much" from the
original calibration, you have had to adjust, over time, to bring the
sensor back into calibration. Some very high end devices costing 10s of
thousands of $$ (one built by E.C.M. for example) even keep track
internally of how far from the original calibration you are, and even do
an "estimated age" of the sensor from the amount of calibration "offset"
ifyawill, that you've had to dial in to keep the sensor working within
range. Once you run out of recalibration range, the sensor has to be
replaced.

So the question you have about "will I see it coming" is alot easier to
answer for a WBO2 sensor, than for the switch type you're dealing with.
Yes, apparently you'll easily see it coming in a WB once it starts to
take effect, just due to the sensitive nature of the system, but with a
switch type, it may be that monitoring cross-count frequency would give
you the best indication. Alas, that frequency is also highly dependent
on a bunch of other parameters like time of year, as well as the state
of tune of your engine management system, so if you're making changes
there as well, it may very well be hard to see anything but very
long-term trends.

Sorry it's all so wishy-washy. Some of you guys with FelPro/SpeedPro
systems with the WBO2 option might tell us what that vendor has to say
about the issue, as well as telling us your own experiences, if you've
run a significant amount of leaded fuel thru your system. As far as I
can tell, the answer is YMMV.

Gar


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