[xj-s] Disconnecting Fuel Pressure Regulator

XJ-S
1

Does anyone know what the expected result should be when
disconnecting the vacuum line to the fuel pressure regulator?

At idle, with a hot engine, the oxygen sensors did not change at
all. (Mind you, I am running rich, sensors only fluctuate between
0.15 and 1.25 while driving, at idle they wander around the 0.25
range.)

Should the oxygen sensors report a change? Maybe when I’m
driving? I haven’t driven with the vacuum line plugged.

No error codes, and no squirting gas from the original regulator.
Just trying to figure out why my O2 sensors don’t wander to the
upper limits of the sensor range (4.50)

Thanks.–
1996 XJS 2+2 Convertible with 4.0L AJ16
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2

In reply to a message from Veekay sent Mon 2 Feb 2015:

If you disconnect vacuum from the FPR, there should be
slight fuel pressure increase (2-3 psi) to the fuel rail,
which will increase fueling thru the injectors. However, if
you did not plug the port where the FPR vacuum hose was
connected (from the intake manifold), then you are now
allowing excess air (a vacuum leak) into the intake and very
very shortly therefter (5 seconds or so), the idle speed
will increase and stay that way.

FYI, and this was very recently posted by me, with your
newly fitted titania O2 sensors, your voltage readings
should fluctuate between 0.2 and 4.5 volts. Your readings
are constantly low. LOW voltage = LEAN. LOW = LEAN.

With a low voltage reading being transmitted from your O2
sensors to the ECU, the ECU ‘‘thinks’’ your engine is running
lean, and will increase injector open time to add more fuel
to the engine. I think the upper limit is around 10%
enrichment.

Over this past weekend, I went back and searched all your
postings for the past 3 months, and it seems this all
started when you had a spark plug ‘‘blow up’’ which cracked
the intake. Sine then, you have replaced multiple parts, but
the problem is still there. A 32 second consistent miss, and
now constant low O2 sensor voltage.

It may be time to haul your vehicle to a shop, and pay for
some diagnostic testing. You can ask for diagnosis work
only, and then you can do the repairs yourself.

SD Faircloth–
The original message included these comments:

Does anyone know what the expected result should be when
disconnecting the vacuum line to the fuel pressure regulator?
At idle, with a hot engine, the oxygen sensors did not change at
all. (Mind you, I am running rich, sensors only fluctuate between
0.15 and 1.25 while driving, at idle they wander around the 0.25
range.)
Should the oxygen sensors report a change? Maybe when I’m
driving? I haven’t driven with the vacuum line plugged.
No error codes, and no squirting gas from the original regulator.
Just trying to figure out why my O2 sensors don’t wander to the
upper limits of the sensor range (4.50)

–
www.jaguarfuelinjectorservice.com
Jacksonville, Florida, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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3

In reply to a message from Veekay sent Mon 2 Feb 2015:

It should raise the fuel pressure in the rail, which in turn
would increase the fuel delivered per pulse. How the ECU
handles this increase depends on the closed loop trim range.
The ECU may have enough range to bring the fuel pulse down to
the correct ratio, so you would see no change in the O2 sensor
reading at all.

But, it the ECU runs out of trim range, the O2 sensor will read
high and a rich code will be triggered.

For open loop, ie. before operating temp is reached or during
heavy acceleration, you would likely see nothing but a slightly
rough idle when the engine is cold. Nothing would change for
full throttle accel.–
John. '95 XJS 6.0L convertible. Southlake, TX
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4

In reply to a message from CJ95 sent Mon 2 Feb 2015:

Thank you, both.

I have sent my car to a shop previously, this past August. It was
a Jag Specialist, with a working PDU. Basically, they just seemed
to throw parts at it.

At their direction, I gave them six new coils, a new EGR, a new
IACV and four new 02 sensors. It resolved the problem for a few
weeks and the 32 second idle dip came back over time. They also
recommended new spark plugs, which I had just recently replaced.
They wanted to install some kind of iridium plugs…I declined.

I don’t think I can take my car back to them, as I don’t believe
they really ever found the problem.

I suppose I can take it to a local mechanic, that I trust. At
least he’ll return the car if he can’t find the problem.

I appreciate the advice, help and support.–
1996 XJS 2+2 Convertible with 4.0L AJ16
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5

In reply to a message from Veekay sent Mon 2 Feb 2015:

Veekay,

I agree with your report on throwing parts at your vehicle,
by a ‘‘Jag Specialist’’. Rarely will a engine problem be
caused by more than one or two sensors or parts
faulty…unless the vehicle has sat unattended…for months
on end. Sounds like your specialist…specialized in
selling parts and labor.

Suggest you check with your local mechanic. Ask if he has an
o’scope, and cost for a diagnosis. You should write out a
very detailed report on parts replaced, when, and any other
pertinent info to hand to him…such as what is currently
going on with your engine.

Where are you located ?

SD Faircloth–
www.jaguarfuelinjectorservice.com
Jacksonville, Florida, United States
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6

In reply to a message from SD Faircloth sent Mon 2 Feb 2015:

I ended up providing most of the parts. So it was mostly labor.

He assured me I had no vacuum leaks, but I’m not confident that
they ever really tested it.

I’m in Arlington, VA, just outside of Washington DC.

Thanks again.–
1996 XJS 2+2 Convertible with 4.0L AJ16
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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7

In reply to a message from Veekay sent Mon 2 Feb 2015:

Final update:

The problem reappeared, slowly, but definitely. I took it
to another Jag shop (how lucky that the DC area happens to
have two independent shops specializing in older Jags) and
it took them some time, but finally found a leaking intake
manifold gasket!

I had it fixed a few months ago and I can definitively say
that the case of the 32 second idle dip has been cured.
I’m going to pin this one on the intake manifold gasket.

If anyone ever experiences this problem, make sure to
order the hose connecting the water rail to the pump, part
NBC2273AB. It’s tucked right under the manifold and is a
quick replacement job. It’s a $10 part if ordered in
advance. It would be a shame to have that manifold off
and not take advantage of replacing this difficult to
access hose.–
1996 XJS 2+2 Convertible with 4.0L AJ16
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
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