All:
Having now recognized that the O2 sensor is not always reliable at idle
because the exhaust temps can be too low to “fire it” or get the system into
closed loop, I learned something new about trying to set the idle mixture.
(The Fluke 98 isn’t good for everything…sometimes ‘feel’ works best!)
Here goes:
Ten minutes or so into a “Hot Soak” condition some conflicting data is
presented to the ECU upon startup = because the water jacket temp is very
high the ECU thinks the engine is very hot, yet in reality the cylinder
chambers are relatively cool. This condition can yield a brief, very lean
idle mixture, at startup, under the following parameters:
If you unknowingly get the idle mixture screw turned out too many turns (too
lean) it will best manifest itself when you get back in the car (after a 10
minute stop, assuming a Hot Soak condition exists) and startup to find an
extremely rough idle. If you race the engine a bit, the rough idle condition
will correct itself. However the real fix is to screw the AFM mixture screw
in a turn or so until this problem goes away.
You will wind up on the rich side for most other idle conditions but as we
all know by now the primitive L-Jetronic is just one compromise after
another. Better to be on the slightly rich side for all running conditions,
including “hotsoaked idle”, on the injected XK engine.
And… a slightly rich idle is a smoother idle.
You might want to keep this in mind when you are “fiddling about” trying to
pass idle emissions testing…remember, just don’t leave it set too lean
for daily driving.
All:
Having now recognized that the O2 sensor is not always reliable at idle
because the exhaust temps can be too low to “fire it” or get the system into
closed loop, I learned something new about trying to set the idle mixture.
(The Fluke 98 isn’t good for everything…sometimes ‘feel’ works best!)
Here goes:
Ten minutes or so into a “Hot Soak” condition some conflicting data is
presented to the ECU upon startup = because the water jacket temp is very
high the ECU thinks the engine is very hot, yet in reality the cylinder
chambers are relatively cool. This condition can yield a brief, very lean
idle mixture, at startup, under the following parameters:
If you unknowingly get the idle mixture screw turned out too many turns (too
lean) it will best manifest itself when you get back in the car (after a 10
minute stop, assuming a Hot Soak condition exists) and startup to find an
extremely rough idle. If you race the engine a bit, the rough idle condition
will correct itself. However the real fix is to screw the AFM mixture screw
in a turn or so until this problem goes away.
You will wind up on the rich side for most other idle conditions but as we
all know by now the primitive L-Jetronic is just one compromise after
another. Better to be on the slightly rich side for all running conditions,
including “hotsoaked idle”, on the injected XK engine.
And… a slightly rich idle is a smoother idle.
You might want to keep this in mind when you are “fiddling about” trying to
pass idle emissions testing…remember, just don’t leave it set too lean
for daily driving.
Found your post of interest and would add the following. After extensive
fooling ( not being a professional this is what it amounts to )with the
AFM its been observed that as you state the “L” system is one of
compromises that did not address some typical real world events - heat
soak probably being foremost. It is extremely important if one is to
achieve a well balanced state of tune that the individual components
fall within the designed for parameters. To add to what you stated as
regards heat soak the time needed for the temperature sensor in the AFM
to adjust to actual temperature of incoming air needs to be well
considered. Even with the sensor mounted by an insulating block within
the AFM unit the unit itself will acquire heat from the underhood
temperatures that will in turn influence the sensor. It’s possible that
setting things up with the meter in the garage cannot duplicate the
conditions of the car when it is underway. The “feel” you suggest I
believe could be this very difference between dialing in with the meter
then subsequent slight adjustment by the seat of one’s pants to account
for the slight variations in running temperature.
There is a noticeable sluggishness in the first mile or two of operation
under conditions being discussed. As things balance out all is well
including an idle that is virtually imperceptible. With the systems’
sensitivity to its’ various low impedance inputs cleanliness of
connections cannot be over stressed. Good results can be had when all is
clean, tight, and the system is adjusted only when components have had
ample opportunity to stabilize.
Happy Trails,
Paul Spitzer--------------------------------------------------------------------------
Date: Thu, 2 Mar 2000 20:36:29 EST
From: TMack409@aol.com
Subject:
All:
Having now recognized that the O2 sensor is not always reliable at idle
because the exhaust temps can be too low to “fire it” or get the system into
closed loop, I learned something new about trying to set the idle mixture.
(The Fluke 98 isn’t good for everything…sometimes ‘feel’ works best!)
Here goes:
Ten minutes or so into a “Hot Soak” condition some conflicting data is
presented to the ECU upon startup = because the water jacket temp is very
high the ECU thinks the engine is very hot, yet in reality the cylinder
chambers are relatively cool. This condition can yield a brief, very lean
idle mixture, at startup, under the following parameters:
If you unknowingly get the idle mixture screw turned out too many turns (too
lean) it will best manifest itself when you get back in the car (after a 10
minute stop, assuming a Hot Soak condition exists) and startup to find an
extremely rough idle. If you race the engine a bit, the rough idle condition
will correct itself. However the real fix is to screw the AFM mixture screw
in a turn or so until this problem goes away.
You will wind up on the rich side for most other idle conditions but as we
all know by now the primitive L-Jetronic is just one compromise after
another. Better to be on the slightly rich side for all running conditions,
including “hotsoaked idle”, on the injected XK engine.
And… a slightly rich idle is a smoother idle.
You might want to keep this in mind when you are “fiddling about” trying to
pass idle emissions testing…remember, just don’t leave it set too lean
for daily driving.
In a message dated 03/04/2000 12:25:01 AM Eastern Standard Time, CGilmoresr
writes:
<< Thanks for that piece of advive Ted. That’s part of the problem I’m
dealing with now.
Charles
Charles: You are most welcome!
I think what I have stumbled onto here is the reason why Brown’s Lane added
the heated O2 Sensor to the later Series III XJ6s…it would keep the fuel
management system in closed loop under all conditions and therefore more
consistent air/fuel mixes could be obtained.
I wonder if this can be added to earlier Series III cars??? Anyone know???
All:
Having now recognized that the O2 sensor is not always reliable at idle
because the exhaust temps can be too low to “fire it” or get the system
into
closed loop, I learned something new about trying to set the idle mixture.
(The Fluke 98 isn’t good for everything…sometimes ‘feel’ works best!)
Here goes:
Ten minutes or so into a “Hot Soak” condition some conflicting data is
presented to the ECU upon startup = because the water jacket temp is very
high the ECU thinks the engine is very hot, yet in reality the cylinder
chambers are relatively cool. This condition can yield a brief, very lean
idle mixture, at startup, under the following parameters:
If you unknowingly get the idle mixture screw turned out too many turns
(too
lean) it will best manifest itself when you get back in the car (after a 10
minute stop, assuming a Hot Soak condition exists) and startup to find an
extremely rough idle. If you race the engine a bit, the rough idle
condition
will correct itself. However the real fix is to screw the AFM mixture screw
in a turn or so until this problem goes away.
You will wind up on the rich side for most other idle conditions but as we
all know by now the primitive L-Jetronic is just one compromise after
another. Better to be on the slightly rich side for all running conditions,
including “hotsoaked idle”, on the injected XK engine.
And… a slightly rich idle is a smoother idle.
You might want to keep this in mind when you are “fiddling about” trying to
pass idle emissions testing…remember, just don’t leave it set too lean
for daily driving. >>Date: 03/02/2000 8:06:24 PM Central Standard Time
From: @TMack
To: xj@jag-lovers.org
Just to add, this am I tried to see what would happen by closing the AFM
bypass screw completely (was 1.5 turns open). The start was fine, but a
little smoky due to richness. However, idle and throttle response was better
than the day before (same temp), indicating it was a bit lean before (plugs
had been light tan). I had noticed the post-soak start behavior that was
mentioned, which is why I did this. Opening the AFM bypass 1/2 turn seems to
have made a better compromise, so will see tomorrow am and check plugs after
running. By the way, our O2 sensor is well grounded.
Alex
79xj6L SII
Spitzer wrote:
Ted -
Found your post of interest and would add the following. After extensive
fooling ( not being a professional this is what it amounts to )with the
AFM its been observed that as you state the “L” system is one of
compromises that did not address some typical real world events - heat
soak probably being foremost. It is extremely important if one is to
achieve a well balanced state of tune that the individual components
fall within the designed for parameters. To add to what you stated as
regards heat soak the time needed for the temperature sensor in the AFM
to adjust to actual temperature of incoming air needs to be well
considered. Even with the sensor mounted by an insulating block within
the AFM unit the unit itself will acquire heat from the underhood
temperatures that will in turn influence the sensor. It’s possible that
setting things up with the meter in the garage cannot duplicate the
conditions of the car when it is underway. The “feel” you suggest I
believe could be this very difference between dialing in with the meter
then subsequent slight adjustment by the seat of one’s pants to account
for the slight variations in running temperature.
There is a noticeable sluggishness in the first mile or two of operation
under conditions being discussed. As things balance out all is well
including an idle that is virtually imperceptible. With the systems’
sensitivity to its’ various low impedance inputs cleanliness of
connections cannot be over stressed. Good results can be had when all is
clean, tight, and the system is adjusted only when components have had
ample opportunity to stabilize.
Happy Trails,
Paul Spitzer
Date: Thu, 2 Mar 2000 20:36:29 EST
From: TMack409@aol.com
Subject:
All:
Having now recognized that the O2 sensor is not always reliable at idle
because the exhaust temps can be too low to “fire it” or get the system into
closed loop, I learned something new about trying to set the idle mixture.
(The Fluke 98 isn’t good for everything…sometimes ‘feel’ works best!)
Here goes:
Ten minutes or so into a “Hot Soak” condition some conflicting data is
presented to the ECU upon startup = because the water jacket temp is very
high the ECU thinks the engine is very hot, yet in reality the cylinder
chambers are relatively cool. This condition can yield a brief, very lean
idle mixture, at startup, under the following parameters:
If you unknowingly get the idle mixture screw turned out too many turns (too
lean) it will best manifest itself when you get back in the car (after a 10
minute stop, assuming a Hot Soak condition exists) and startup to find an
extremely rough idle. If you race the engine a bit, the rough idle condition
will correct itself. However the real fix is to screw the AFM mixture screw
in a turn or so until this problem goes away.
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