Oxygen sensor's:
Good Information posted about them - glad to see it.
The Basic type is the standard "rich/lean" mixture
that just tells you if you are running rich or lean.
Those fancy bar graph meters make it look like they
read out a wide range, but they are just little dumb
blinking lights (LM39xx is the bargraph IC if you want
to build your own -- I forget the exact one, but there
were three LED display bargraphs, LM3914, LM3915, LM3916
as they were either linear or log scale. You want linear
scale -- its been over 10 years since I built that thing).
As noted there are two flavors of those Basic
Sensors: One Wire & Three Wire (which can have
three or four wires to provide separate ground
paths for the heating element & sensor). The
closer the oxygen sensor is to the exhaust ports,
the faster it heats up.
The wide band sensors are newer developments, but
are to measure the degree of running lean. Honda
is a big user of those sensors, so the price has
dropped quite a bit. Not sure of current pricing,
but they were down around $200 for replacement
parts, not bad for a sensor that once cost about
$800 to replace! These sensors measure the lean
region from 14:1 to 20:1.
There are some "research use" Wide band Oxygen sensors
that run about $10K (1996 price) to provide a full
range of rich to lean (8:1 to something like 16:1)
FOR THE ROADSTER:
The Oxygen sensors do NOT like soot! When you start with
the choke every morning, and have a puff of black smoke,
it will coat the sensor in a short amount of time. So
you really can not leave the sensor in the exhaust full
time if you are running carbs.
The Datsun really loves to be kept in the rich region
at all time. Just they way they are, and where they have
the best performance response. Carbs do vary on the A/F
metering quite a bit, so best to keep them in the rich
region.
ROAD(STER) Testing:
My old '67 2000 would hesitate on uphill sections and
full throttle. This still occurred after a full tune
up wit new plugs, rotor, cap, ignition wires and valve
adjustment. Didn't really make sense as it would be
around 3500 to 4000 rpm.
Made up my own oxygen sensor tester. I had a straight
tailpipe on the car, not the original megaphone, so it
was easy to find a piece of pipe to snugly slip over
the tail pipe.
Since I was measuring a location so far back in the
exhaust path, I needed to use a heated (three wire)
sensor. I used a four wire sender off a ('90?) Nissan
Truck as it had a small 14mm threaded boss, rather
than the 18mm used by Bosch.
Started the roadster up, let it warm up. Then shut it
off. Slipped the pipe in place, with a wire to help
keep it from falling off the back of the car, and I
was off! Use caution as the tail pipe can get hot
rather quickly, best to have heavy work gloves on.
With the sensor wired up for proper heating, and
monitored the sensors output voltage with a trusty
FLUKE voltmeter I was off to try some runs.
It showed it was running rich all the time, which
is what I was aiming for.
Got to my trusty uphill section, full load and full
throttle... and discovered the hesitation was when the
sensor was reading lean!
A few more test runs all confirmed the engine was going
lean under full load conditions. This would have also
added the the engine running hotter on those long uphill
sections when heading into Mt. Shasta that previous July.
One the last run, with no traffic around me, did my full
throttle up hill run and once I was running lean, did my
quick stop of killing the engine, putting in the clutch,
and coasting into a perfectly placed rest area. Removing
the tops of the float covers showed both floats to have
fuel in them, ruling out any fuel supply problems.
DISCOVERY:
Since the engine was running lean, under WOT (Wide Open
Throttle) conditions, it pretty much ruled out any throttle
shaft leakage or manifold air leaks. Both of those conditions
are more of a problem at idle, with higher manifold vacuum.
Checking the carbs, the float levels were correct. Jet hoses
clean and the Jets looked good. I had the correct Roadster
needles in the SU's, so this left me rather stumped.
Only thing I could see was a "milky" film on the needles.
Opaque, not sure what it was but my finger nail couldn't
feel it, but it was a slight discoloration of the brass.
Quick call to Ross & Sports Imports got me new jets and
needles by the next weekend. Installed the new items,
and went out for a follow up run. I was convinced that
was not the problem, but my trouble shooting left either
that or a weak full pump at full load.
First thing I noticed upon heading up the hill was it
was faster! Quick glance at the meter showed I was still
running rich under full load. Eureka!!!
RECAP:
Handy to have the three wire (heated) Oxygen Sensor,
but I do not recommend permanently installing it in
a roadster.
Like your timing light, just install it when you wish
to confirm the engine is running under full load
conditions.
Tom Walter '68 2000
Austin, TX
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