I hope someone can help me put two and two together.
Having asked this forum many times about persistent idle
woes for the 6.0L V12, documented here:
I’d done a lot of work. Just as I thought all is well and I
made peace with the idea that +/-50 rpm at idle is OK,
suddenly, I failed idle emissions (high speed test was OK).
This ordeal led to the nice discussion here:
As you can see, a member suggested I do wide-band O2 sensors
test. Well, I found a shop within 20 miles that had a dyno
and could do dual wide-band O2 sensors. The test they did
(I was present) confirmed the following:
At idle, A/F mixture bounces of 14.7 with a regularity
that matches the tach needle moving up and down. Idle speed
should be 750 rpm. Mine was between 727 and 772. Kirby is
absolutely right that the ‘‘EFI system is constantly trying
to run it too lean. It’s constantly bouncing on and off
stochiometric, and the cycling up and down of the mixture to
hold it in that range results in an idle that’s not what a
V12 should be.’’
Removing the Marelli jumper changes nothing in idle.
Same result over 2 minute of monitoring.
Putting load on the engine. Power and torque increase as
they should. Max power at the wheel was achieved at 5000
rpm and was 313 HP (Jaguar spec for my US car is 278 HP/DIN
@ 5400 rpm). Max torque at the wheel was at 4000 rpm and was
347 HP (Jaguar spec for my US car is 334 ft lbs @ 2800 rpm).
Please read this carefully – at WOT, above ca. 3000 rpm
engine speed, the A/F ratio seems to fall off of a cliff –
both banks are at near 11, which suggests SUPER rich
condition. This was confirmed by the smoke coming off the
Is this normal? I recall reading about someone driving his
XJS on the Gerrman autobahn and logging mileage around 8 MPG
for sustained speed of 120-130 MPH, which probably
translates to A/F in the neighborhood of 10 or 11.
Also, if I understand correctly, at WOT, i.e., after the
second fuel pump comes online (above 2850 rpm), the O2
sensors feedback is inhibited. The 36CU does the fueling
based on MAP and CTS alone.
If A/F = 11 is indeed super rich running, what does this
mean in the context of my recent emissions data. When I
passed he results were:
HC idle – 171 ppm
HC fast – 101 ppm (this will be engine speed ca. 2400-2500
CO idle – 0.25%
CO fast – 0.46%
- Removing the Marelli jumper and repeating the dyno-test
shows the same results – A/F drops after 3000 rpm. Power
and torque were down 10%.
Please help me understand what is going on. I don’t race
the car, and rarely see the tach going over 3-4,000 rpm.
This is not a quest for more power. Adjustments at those
conditions are not what I am after, but I feel that these
results tell a story.
I am hoping someone knowledgeable can interpret the data.
'95 XJS convertible - V12 6.0L
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