[xj-s] Manifold Vacuum Question

While chasing where to connect the unconnected vacuum line
on the left side of the engine bay I stumbled on another
possible problem - low manifold vacuum. (I now know that
the unconnected vacuum line should NOT be connected as it is
the vent line from the distributor.)

What manifold vacuum should I be seeing at one of the vacuum
ports on the rear of the A bank manifold? This is a '91
with Marelli ignition.

With a warm engine, smooth 750 RPM idle, I am getting a
steady 14.5 in-Hg [49 Kpa]. On other cars I have had this
is too low and indicates late ignition timing. I recently
had a '92, that idled roughly, with the same manifold vacuum.

Since there is no timing adjustment and there should be no
advance at idle, is is possible to have late timing on this
car? It seems to run fine and I am getting 15 MPG city 17.5
MPG highway.

thanks–
David Christensen, Tucson, 13 XJ, 91 XJS Conv, 65 Etype OTS
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

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In reply to a message from davchr sent Sat 6 Aug 2016:

Dave, don’t know how you measured it, but you are correct.
14.5 in-Hg vacuum is marginal, at best.
I am getting 18-19 in-Hg at idle. See here – last two
pictures:

http://www.jag-lovers.org/v.htm?id=1362255765

If you engine has power and runs good, I would be suspicious
of the measurement itself.
If indeed less than 15-16 in-Hg, your engine health will have
to be evaluated. Have you done compression check BTW?
Steve–
The original message included these comments:

With a warm engine, smooth 750 RPM idle, I am getting a
steady 14.5 in-Hg [49 Kpa]. On other cars I have had this
is too low and indicates late ignition timing. I recently
had a '92, that idled roughly, with the same manifold vacuum.


'95 XJS convertible - V12 6.0L
–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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In reply to a message from sbobev sent Sat 6 Aug 2016:

I have not done a compression check. I did find the
following reference in the archives:

‘’…the manifold pressure will immediately drop down to
about 40-45kPa as the revs take off, before settling around
the 45-50kPa mark.’’

My manifold pressure (vacuum) is ~49Kpa so I am in line with
this poster’s [MarekH] numbers.

While I cannot imagine that the finest vacuum gauge Harbor
Freight offers could in any way be faulty, but that is a
possibility I will investigate further.

thanks–
David Christensen, Tucson, 13 XJ, 91 XJS Conv, 65 Etype OTS
–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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In reply to a message from davchr sent Sat 6 Aug 2016:

I checked the vacuum with another gauge and they have
similar readings. I am at 3000 feet elevation so my vacuum,
adjusted to sea level, would be ~ 16.5 in-Hg.

Is there any timing adjustment that I can make on a '91?

thanks,–
David Christensen, Tucson, 13 XJ, 91 XJS Conv, 65 Etype OTS
–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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In reply to a message from davchr sent Sat 6 Aug 2016:

For completeness, this is the the thread where the numbers
quoted herein come from:

However, going deeper in the archives, you can find other
posts. Specifically, I’d like to re-quote R. Bywater:

Verbatim from LUCAS EFI / AJ6 Engineering :

Over-run cut off above 1500 r.p.m. was activated by a
throttle closed signal and a simultaneous vacuum in excess
of 21’’ Hg, but was disabled until fully warm. Once activated
the cut off applied until the vacuum fell to about 10’’ Hg
or speed dropped below 1100 r.p.m. Cut off in Lambda sensing
versions was active over a much narrower range from 18’’ back
to 15’’ Hg but in both cases the vacuum effect varied with
speed. Quite why this vacuum requirement was thought
necessary is a mystery, and it was abandoned for the later
16CU which relied simply on speed and the throttle-closed
signal from the potentiometer.

Another thread which I have saved in my own archives is this
one:

Read Kirbert’s explanations.

Steve–
The original message included these comments:

I have not done a compression check. I did find the
following reference in the archives:
‘’…the manifold pressure will immediately drop down to
about 40-45kPa as the revs take off, before settling around
the 45-50kPa mark.‘’
My manifold pressure (vacuum) is ~49Kpa so I am in line with
this poster’s [MarekH] numbers.
While I cannot imagine that the finest vacuum gauge Harbor
Freight offers could in any way be faulty, but that is a
possibility I will investigate further.


'95 XJS convertible - V12 6.0L
–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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Is there any timing adjustment that I can make on a '91?

The only timing adjustment available to the owner/operator is the removal of the infamous link, which puts the engine onto an alternate, retarded timing map. Supposedly in case you get a tank of bad fuel. The link is typically under the hood on the port side near the firewall, but beyond that we’ve gotten numerous different descriptions of what it looks like. All agree, though, that you do NOT want to be operating this car with the link pulled. The performance and fuel economy are both supposedly lousy with it out.

– Kirbert

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