[xj-s] 95 octane/91 octane ECU jumper incorrect - long-term effects?

XJ-S
1

Finally bothered by my poor fuel economy (around 8.5 mpg around
town) I found that the little jumper that tells the ECU to use the
95 octane setting was in place, and I use only 91. I disconnected
it, and, although it’s not been long since I did it, I’m now
getting closer to 12 mpg, which we can all agree is a Very Good
Thing.

However, given this jumper has been in place for at least 20,000
miles of driving, I’m concerned what long-term harm might have been
done. Is there anything I should check up on right away?–
'92 XJS V12 Convertible, '95 XJR, '00 Mercedes Benz S500
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2

In reply to a message from BurbankSteve sent Mon 24 Mar 2008:

BurbankSteve,

The strategy jumper is supposed to be installed for normal high
octane ops. This jumper is in series with the idle switch. If the
idle switch is stuck or misadjusted to remain in the closed
position, it is stuck in the low octane operation position. If you
get a load of pinging poor gas you can pull the jumper until you
get better gas. I actually think that the fueling goes open loop
when at idle or the link is removed.

I have a problem with my '92 also. It simply refuses to knock on 87
octane! I have slowly been nibbling away at the problem. A recent
trip shows I am getting pretty lousey mileage. So I need to re-
pursue the problem. I disconnected the inlet air temp switch
located on the back of the right air cleaner housing. It retards
the timing above temps of 75 degrees C. In Oklahoma that is an
everyday underhood temp!!. When I disconnected the IAT switch the
engine ran cooler. My problem is high coolant temp. For a while I
was restricted to days less than 85 degrees F but slowly I have
gotten it up to over 100 degree days in traffic. But I’ve still got
something haywire. I wish someone would come up with the diagnostic
code information. But it seems no one has it. The local Jag dealer
is starting up a shop and will drag out the old test equipment.
Maybe he will be able to shed some light on the diagnostics.

Noel

Keep me posted.

PS are you in the states??

PPPSSS I have the 88 and 92 training manuals. This is where I got
the above info.–
'92 XJS Conv, '88 XJS, 68 XKE, 1914 &'15 Ford Model T’s
Edmond, OK, United States
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3

Finally bothered by my poor fuel economy (around 8.5 mpg around
town) I found that the little jumper that tells the ECU to use the
95 octane setting was in place, and I use only 91. I disconnected
it, and, although it’s not been long since I did it, I’m now
getting closer to 12 mpg, which we can all agree is a Very Good
Thing.

That’s right at the preHE level (ask me how I know). You should do
much better.

Ed Sowell
1976 XJ-S
http://www.efsowell.us/ed/myJag.html

However, given this jumper has been in place for at least 20,000
miles of driving, I’m concerned what long-term harm might have been
done. Is there anything I should check up on right away?

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4

In reply to a message from Jags+30jugs sent Tue 25 Mar 2008:

So now I’m confused, which is a perfectly normal situation for me :slight_smile:

Should I install the jumper or not? I’m in Southern California, and
premium gas is 91 octane; I just read that the UK calculates octane
ratings differently, there’s approximately a 4-point difference, so
95 octane UK would be 91 US, which would imply that I should put
the jumper back in IF the 95/91 octane settings [I believe from Jaguar] is talking about UK-calculated octane values.

I guess if I do need to reconnect the jumper, I’m back to square
one with my fuel consumption. Ed seems to think it’s at pre-HE
levels, so that’s not at all good.

I did have a suspicion this morning that the car seemed a little
more sluggish than before, I guess that can be explained if my
timing’s retarded incorrectly.

Regarding the inlet air temp switch, I’m pretty sure my normal
operating temperature in LA is going to be in excess of the 75C
tripping point of the switch. You say you’ve disconnected yours and
the engine is running cooler, but I don’t get any real problems
with overheating; if it’s a sweltering day (100+) and I’m stuck in
traffic, the temp gauge does creep towards the high side (probably
3/4 to the red), but it comes down to center within a very short
time once I’m moving again.

If the inlet air temp switch is there precisely for the reason to
retard the timing if the underhood temp exceeds 75C, would that
imply that if I’m sucking air that would normally be between 70F
and 110F, the switch is tripping too early? It seems odd that if
it’s there to check the underhood temperature, it’s measuring the
air temperature inside the intake which can be skewed by
temperature variations from outside. I’d guess that I’m almost
always running with the timing retarded if this switch is
functioning as designed. If my temp gauge is pretty well rock-
steady ‘‘N’’ under most circumstances, then I don’t need this switch
messing around with the timing, so I guess I should go ahead and
disconnect it?

I’ve got some other issues to deal with (I’ve got a surging idle
problem which I think I now have the information to track down and
fix) which may be contributing to the poor fuel economy depending
on what the eventual cause turns out to be. I’ll keep you posted.
Thanks for the help so far.–
The original message included these comments:

The strategy jumper is supposed to be installed for normal high
octane ops. This jumper is in series with the idle switch. If the

–
'92 XJS V12 Convertible, '95 XJR, '00 Mercedes Benz S500
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

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5

In reply to a message from BurbankSteve sent Tue 25 Mar 2008:

First of all, the IATS isn’t a switch… it is a variable thermal
resistor (thermistor)?? If you look in the service manual (I can’t
as I am in Dallas, TX instead of Atlanta) you will see that there
is a range of temps that that sensor adjusts the timing in relation
to the temp. It is not an on/off situation. Secondly, there will be
only anecdocatal relationships between the IATS and the temp gauge
on the dash. I live in Atlanta, and in the dead summer when it is
100 degrees F, and 100 percent (or more) humidity, there is no and
I mean not even a trace of knocking, or pinging, or pinking even on
cheap regular fuel. There is however a MAJOR lack of power
(tourque) on the low end. But I have not unplugged the IATS, I have
however replaced the IATS with a resistor that was equel to an
ambient air temp of about 45 degrees F. with no significant, or at
least noticible increase in performance. (One would expect that in
a high ambient temp. situation with the jumper in, and the IATS in
a cold temp setting for the engine to knock) Mine doesn’t at all…
that is why I have wanted a way to hack the Marrelli box to set
different curve maps.
Dave
Atlanta, temporarily in Dallas–
Penfold99
Atlanta, GA, United States
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6

In reply to a message from Penfold99 sent Thu 27 Mar 2008:

Uhhhh! Penfold,

My ''92 and all Marelli’s, I would guess, have a SWITCH set for 75
C. When I tested mine it tripped at 56C. Really a low temp.

Repeat---- This is a switch, disconnect it. It will keep the engine
advanced all the time with respect to IAT and the ignition. This
simple fix caused my engine to run cooler due to the advanced
timing. As I stated, in OKC 75C is an everyday underhood temp. The
switch then retards and the coolant gets a new load of heat. The
Marelli does use a thermister coolant temp sensor, CTS, and the MAP
to determine the air density or pressure altitude.

Penrod,
there is a inlet air temp thermister, IAT, in the inlet of the B
bank horn for fuel control in a Marelli, my '92 has one. Marelli
does not use any Fuel sensors. It has it’s own independent sensors.

Penrod, I urge you to hack the Marelli. I bought a digital Scope to
see if I could figure out the diagnostic serial code. I haven’t had
time to do it. If I can help you here in any way, please call me. I
have the 1992 and 1988 Tech material. 405-255-0895
Noel–
The original message included these comments:

First of all, the IATS isn’t a switch… it is a variable thermal
resistor (thermistor)?? If you look in the service manual (I can’t
is a range of temps that that sensor adjusts the timing in relation
to the temp. It is not an on/off situation. Secondly, there will be
only anecdocatal relationships between the IATS and the temp gauge
on the dash. I live in Atlanta, and in the dead summer when it is
100 degrees F, and 100 percent (or more) humidity, there is no and
I mean not even a trace of knocking, or pinging, or pinking even on
cheap regular fuel. There is however a MAJOR lack of power

–
'92 XJS Conv, '88 XJS, 68 XKE, 1914 &'15 Ford Model T’s
Edmond, OK, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –

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