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O2 sensor diagnostics, duty cycle in %, long post
Some additional info on O2 sensors and
adjusting the frequency valve duty cycle.
John Karasaki said:
> Scott Mockery and I (mostly Scott) did a thorough check of all the engine
> systems on the 5kcstqw I purchased this week. When we measured the duty
> cycle, it was about 52%, removing the oil dip stick raised this up to ~ 65%,
> but the engine did not stall. Ned and others said that the dip stick test
> was a good way to check for leaks. I think your duty cycle of 50-55 is
Orin commented on Johns 5KCSTQW
>Thats degrees not %. It's too lean for my liking - especially since
>the computer will set it to 60% for 'enrichment'.
Acutally, when we took a quick look at the freguency valve duty cycle waveform on
John's car using an Oscilloscope we were reading the duty cycle % and not using
a dwell meter. I was doing a quick check on John's car to make sure the duty cycle
wasn't way off. We didn't take the time to readjust the mixture that day.............
As Scott (PDQSHIP) has commented, the factory spec for the duty cycle
is 50 +/- 8 %. The Bently manual mentions using the VW 1367 meter which allows
the user to select DWELL readout in % (The Germans are used to seeing the ignition
dwell represent in % ). PDQSHIP also mentions that a Dwell meter (degrees) can be
used. You just take the reading in degrees (4 cylinder setting) and divide by 90 to
get the actual %. ie 38 degrees dwell divided by 90 = 42% duty cycle.
The other main point to remember as PDQHSIP mentioned, is the duty cycle you are
measuring is NOT constant and is actually cycling between TWO values as it responds
to the O2 sensor voltage cycling between the slightly rich ~0.9 volts and slightly
lean ~0.1 volts. If you disconnect the O2 sensor the ECU sets the duty cycle
to a default value around 50%.
Normally the upstream CO% (before the cat converter) is supposed to be adjusted
between 0.6 and 1.2% CO% with the O2 sensor disconnnected, the crankcase breather
disconnected and intake hose plugged. The evaporative canistor hose cap should be
removed and the fuel vapor solenoid should be disconnected on the 86-88 1/2 5KtQ's
to prevent these fuel vapors from affecting the mixture adjustment/check.
On the later 89-91 10V turbos the two canistor solenoids should be disconnected.
After you adjust the mixture using the above procedure yada yada yada..........
and then reconnect all the hoses etc and vapor canistor solenoids and the connect the
O2 sensor you will see the duty cycle change back and forth as mentioned. For example
on my 89 200TQ 10V , when the upstream mixture is adjusted to 1.1% and then all the stuff
is reconnected, the duty cycle changes between 42% and 56%. With the upstream
adjusted to 1.6% the duty cycle was between 40% and 53%. On the older style analog dwell
meters you can watch the needle swing back and forth. The new digital meters may
respond quickly enough to show the duty cycle changing but it is hard to read the fast
changing numbers, some meters can smooth out the response.
Ned Ritchie and others recommend just setting the duty cycle to around 42% (38 degrees dwell)
with all O2 connected etc. I don't know if this recommended value of 42% is
the "Average" value between the two extremes or the minimum duty cycle seen.
The above example on my car would indicate that the 42% adjustment recommendation
is the minimum duty cycle seen and not the average. This gives you a mixture closer
to the desired 1.2% CO upstream reading. The above readings assume my O2 sensor is
responding correctly, others may want to comment on this.......
If you find that your freq. valve duty cycle is way off at idle you should look for a reason
(ie vacuum leak, or if it is too rich, crankcase oil diluted with fuel etc), before you readjust
Some Digital Multi-Meters have a bar graph which can be used to watch fast changing
values. You can use this bar graph to watch the O2 Voltage cycle back and forth between
around 0.1 and 0.9 volts. If your meter has a Delta or zeroing feature you can connect
the meter to the O2 sensor wire from the computer with the ign. key on, zero out
the 0.45 V reference and then watch the bar graph go above and below the 0.45 volt
reference as the O2 senses rich and lean conditions.
The post regarding the 91 200 TQ that had an O2 voltage of
1.4 volts indicates a problem as the sensor should not go above 1.0 volt or below
0.0 (ie negative) (Note: some resistive type sensors on other makes of vehicles have
a 0-5 V output normally) Contaminated or poisened sensors will respond to slowly
(voltage should transition in less than 100ms rich to lean, lean to rich) and can exceed
these voltage limits. If I remember correctly the rich to lean transition time is typically
a little slower and can be very slow to respond if the mixture has been rich for awhile.
Using a friends portable exhaust gas analyzer, the O2 sensor on my car (85K miles)
keeps the A/F ratio at 15.5 to 1 at a steady cruise, which seems a tad lean. I am going to
replace the sensor and see if the cruise mixture gets closer to the recommended 14.64
to 1 ratio for lowest emissions. I don't notice any surging at this point when the car
is warmed up. The NOx output will go ballistic when the mixture gets too lean.
Hope all this rambling helps