In reply to a message from mike waldron sent Mon 25 Jul 2016:
I do not have the wide-band sensor and really doubt it can
do what you say it does. There only a handful (perhaps even
less) people that have successfully messed with the 36CU.
I’ve got two of them, one moded by Roger Bywater, and idle
is never perfect.
In any case, this does not explain the idle speed failure.
Something else is going on. The next paragraph confirms it.
So, last night I disconnected the battery, cleared the
memory and took the car for a re-test today. It passed.
Two years ago, the results were:
HC idle – 183 ppm
HC fast – 92 ppm
CO idle – 0.33%
CO fast – 0.49%
Today, the results are:
HC idle – 171 ppm
HC fast – 101 ppm
CO idle – 0.25%
CO fast – 0.46%
Noticed the higher idle numbers. The CO at idle when It
failed two weeks ago was over 1.2%, now is more than 4 times
I/WE WILL NEVER KNOW. If someone has an idea why the idle
test results are always worse than the 2500 rpm ones, please
Possible explanations for the success today:
- The test equipment was contaminated two weeks ago, now
was in proper order (unlikely, but…)
- I drove the car 850 miles between then and now. Must have
cleaned some gunk. Very likely old/bad fuel contributed
- There was an ignition system fault somewhere – plugs,
rotor, wires, cap. (the misfire must have been gross and I
would have noticed btw). The fault was corrected when I
pulled/replaced the plugs and reseated the wires.
- Clearing the memory from the ECU did the trick. Perhaps
there is a bad capacitor as suggested earlier, perhaps the
idle trim does things to the mixture nobody can understand.
In my view, this is the most likely reason for the pass
Many thanks to all that contributed. We will see again in
two years what happens.
PS I found a tuner shop near me, will take the car for a
test. They can measure O2 under load–
The original message included these comments:
It will let you know what your mixture truly is and even
let you ‘‘trim’’ where lambda is, so the ECU thinks it’s
operating at 14.7 but it’s really 13.5 or whatever you’d
'95 XJS convertible - V12 6.0L
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