[x300] Fuel trims and P0420 development X300 Longish

Interesting little development on my XJR. Well, maybe not all that
interesting, but anyway, here’s my rambling report…

For ages I’ve had a P0430 code and long term fuel trims seemingly fixed at
about -98 while short term fuel trims hovered just above and below “0”.
Never isolated the cause…and never tried all that hard, either, as the car
ran great.

Anyhow, during the course of a major servicing and general futzing about I
happened across loose air injection fittings where they screw into each
manifold. I tightened them up, of course, and then (a couple days later)
thought I’d scan the system to see if things had changed. They had.

Code P0430 (B Bank cat efficiency) was gone, replaced by P0420 (A Bank cat
efficiency). And…LTFT had already swung from -98 (both banks) to +50
(both banks). STFTs still seem pretty good, with B-bank running 0 to -8 and
A-bank showing 0 to +6.

But, that’s not all…

Now its two days later and the P0420 has not returned …BUT… the LTFT
is back to -98 and STFTs unchanged. !! Arrrrghhh!

All four oxy sensors vary voltage well enough, seemingly…that is, none
appear dead or even lazy.

I’m not quite sure what to make of this. The LTFT changes occurred faster
than I thought, but there you have it. And, if STFTs have been OK for 2-3
years, why do I show abnormal LTFTs at all?

Any clever thoughts, or even rude peasant curses? The archives reveal quite
a few suffering from LTFT issues but there doesn’t seem to be a magic
bullet.

Has anyone tried clearing the LTFT cells just for giggles? Could this be a
software glitch?

I’m not wringing my hands with worry here but I wouldn’t mind getting to the
bottom of it. Maybe this weekend I’ll look for any other exhaust or air
leaks.

Cheers and thanks

Doug Dwyer
Longview Washington USA
1995 XJR

In reply to a message from Doug Dwyer sent Thu 23 Apr 2009:

Doug:
Thanks for the info: Something in the fueling of X300s just ain’t
right! The only thing I can add is to point out what you already
observed- The LTFT displays separately for each bank, but is always
the same number for both banks. So that coupled with the STFT
numbers for each bank being reasonable and O2s cycling properly is
counter-intuitive for this effect being the result of an actual
leak or similar condition. Maybe the LTFT is scaled much
differently (larger number, less control) than the STFT.
As I posted before, I have two ECUs I try on my XJ6, one displays
this effect, and the other seems to operate with LTFTs around 0.

‘‘Rude peasant curses’’ … is that Goethe? In any case, that might
be the explanation or the solution.–
The original message included these comments:

Interesting little development on my XJR. Well, maybe not all that
interesting, but anyway, here’s my rambling report…
For ages I’ve had a P0430 code and long term fuel trims seemingly fixed at
about -98 while short term fuel trims hovered just above and below ‘‘0’’.
Never isolated the cause…and never tried all that hard, either, as the car
ran great.
Anyhow, during the course of a major servicing and general futzing about I
happened across loose air injection fittings where they screw into each
manifold. I tightened them up, of course, and then (a couple days later)
thought I’d scan the system to see if things had changed. They had.
Code P0430 (B Bank cat efficiency) was gone, replaced by P0420 (A Bank cat


Ross - 89 XJS, 95 XJ6, 98 XJR, 02 XJ8
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In reply to a message from Doug Dwyer sent Thu 23 Apr 2009:

Hi Doug,
I have the same problem and try as I might, have never been
able to track it down. It would be interesting to know how many of
our listers have LTFT of around -90% or more. I suspect plenty
suffer in ignorance!!!
I note that even MIKEDEFENDER has -95% on his ‘‘as new 1995 beauty’’.
Maybe XJENGINEER knows the answer???

Mike (UK)–
The original message included these comments:

For ages I’ve had a P0430 code and long term fuel trims seemingly fixed at
about -98 while short term fuel trims hovered just above and below ‘‘0’’.
Never isolated the cause…and never tried all that hard, either, as the car
ran great.


Mike Jones, 1995 X300 3.2 Litre 89000miles
Worthing, United Kingdom
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You’re right…many probably have no idea of what lurks, as the check engine
light isn’t triggered, and the engine still runs well and delivers
reasonable economy…so no outward manifestation of a problem.

Thanks for the reply. There’s always a bit of comfort in numbers :slight_smile:

Doug Dwyer
Longview Washington USA
1995 XJRFrom: “Mick83” michael.jones873@ntlworld.com

   I have the same problem and try as I might, have never been

able to track it down. It would be interesting to know how many of
our listers have LTFT of around -90% or more. I suspect plenty
suffer in ignorance!!!
I note that even MIKEDEFENDER has -95% on his ‘‘as new 1995 beauty’’.
Maybe XJENGINEER knows the answer???

Thanks, Ross.

I’m not gonna worry about it too much but if I come across an XJR being
parted out I might glom onto the ECU just for the heck of it.

Also, I’ve assuming (heh heh heh) that my P0420 and P0430 DTCs (now gone,
but for how long?) must be related to my fuel trim problems. Maybe they
aren’t at all. I dunno. I think I’ll have the exhaust analyzed just for the
heck of it.

Cheers
Doug Dwyer
Longview Washington USA
1995 XJRFrom: “sparkenzap” rpryor@factoryautomation.com

Thanks for the info: Something in the fueling of X300s just ain’t
right!

As I posted before, I have two ECUs I try on my XJ6, one displays
this effect, and the other seems to operate with LTFTs around 0.

In reply to a message from Doug Dwyer sent Thu 23 Apr 2009:

My P0420 was triggered by the front exhaust manifold splitting
underneath after the cracks widened further. My trims hover in the
30’s with a cracked rear one.

Have you tried getting a tool (a kid and a meter) to read the MAF
voltage while you’re rolling? If that’s sending the wrong signal
to overfuel your O2’s will send the go lean asap signals to the ECU
right?–
Sunny, '95 XJR (Turquoise/Cream), '89 Supra Turbo
Agoura Hills, CA, United States
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In reply to a message from JaguarXJ6 sent Fri 24 Apr 2009:

Arrr !! another one of those jaguar curiocitys. Mine does
the same -95 on both banks, done all the usual
suspects,plugs, coils, fuel injectors, repaired the cracked
exhaust manifolds, checked for inlet leaks.
Have yet to re-scan her since but am glad to hear the LTFTs
change that quickly, I thought I had to stick a few hundred
miles on her before checking agian.–
xjr6 1995
essex, United Kingdom
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I’m not sure if they are intended to change that quickly. Maybe they
are…I’m just not sure.

I’m thinking of clearing the memory cells and seeing what happens. The
typical method is to remove both battery cables from the battery and touch
them together. Can anyone think of a reason that wouldn’t be OK on our Jags?

Cheers
Doug Dwyer
Longview Washington USA
1995 XJRFrom: “fasterstill” carl@budgetpcrepairs.co.uk

Have yet to re-scan her since but am glad to hear the LTFTs
change that quickly, I thought I had to stick a few hundred
miles on her before checking agian.

In reply to a message from Doug Dwyer sent Fri 24 Apr 2009:

I’ve cleared the LTFTs with the Jaguar handheld at my mechanics.

It won’t cause any harm nor will you notice anything different and
destory your engine!

At idle, the values will read xx% but once you begin to hold the
idle high, say 2,000rpm or accelerate and drive for a few minutes,
they will quickly change back.

The mysterious behavior we are seeing with the LTFT is that the ECU
is not treating these as averages as with OBD2 systems but
maximums, which will change as you do things to adjust the mixture
like replacing parts or checking vaccuum.

The reason why you get mixture fault codes is that the mixture goes
beyond what the ECU is able to correct and why + or - 50% is
considered normal! Very frustrating.–
Sunny, '95 XJR (Turquoise/Cream), '89 Supra Turbo
Agoura Hills, CA, United States
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In reply to a message from Doug Dwyer sent Fri 24 Apr 2009:

Hi Doug,
I would be interested to know if ‘‘touching + / ground
cables together’’ will erase ECM memory. Good if you could report
back if you try it.
Mike (UK)–
The original message included these comments:

I’m thinking of clearing the memory cells and seeing what happens. The
typical method is to remove both battery cables from the battery and touch
them together. Can anyone think of a reason that wouldn’t be OK on our Jags?


Mike Jones, 1995 X300 3.2 Litre 89000miles
Worthing, United Kingdom
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In reply to a message from Mick83 sent Sat 25 Apr 2009:

I have tried the old zero volt reset strategy and saw no indication
it reset anything on the X300 or X308. (I don’t remember trying it
on the XJS).–
The original message included these comments:

    I would be interested to know if ''touching + / ground 

cables together’’ will erase ECM memory. Good if you could report
back if you try it.


Ross - 89 XJS, 95 XJ6, 98 XJR, 02 XJ8
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In reply to a message from sparkenzap sent Sat 25 Apr 2009:

would be handy if we could find two XJR6 drivers, one with
the LTFT problems and without that live near each other so
we could swop the ECU and see if the problem follows the ECU
or the car?–
xjr6 1995
essex, United Kingdom
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In reply to a message from fasterstill sent Mon 27 Apr 2009:

fasterstill:
On my xj6, it follows the ECU.–
Ross - 89 XJS, 95 XJ6, 98 XJR, 02 XJ8
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In reply to a message from sparkenzap sent Mon 27 Apr 2009:

Interesting!!! Not much point me trying to chase down the
problem then, except maybe find another ECU.–
The original message included these comments:

On my xj6, it follows the ECU.


xjr6 1995
essex, United Kingdom
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