[v12-engine] 6.0L V12 fuel pressure

Gentlemen:

I’d reported in the past some ugly cold start problems,
which persisted after I had done tune-up (new sparks, wires,
everything in the ‘‘vee’’ cleaned or renewed, but the
injectors).
This time, I decided to remove the rail with the injectors
and have them tested and cleaned by our own SD Faircloth.

While Dave was doing his magic, I designed a rig to test the
fuel pressure, since this was brought-up in the past, and I
never knew the answer. I will post the pictures once they
are vetted. The test-port is on the rail, just before the
FPR.

All is back together and the results from the fuel pressure
testing are in. Again, this is the 1995 V12 with the 6.0L
engine with only one FPR on the A-bank, rated at 3 bar (44-
45 psisg).

Key observations:

  1. Key in ignition, after 2 seconds pressure is near 38-40
    psig
  2. Normal start, pressure drops to about 34 psig
  3. Engine runs, warms up to N, pressure at idle steady at
    around 32 psig
  4. Removing the vacuum (simulating load), pressure boost to
    38 psig.
  5. Re-connecting the vacuum (simulating load), pressure
    drops to 31 psig.
  6. Engine shut off from normal operating temperature,
    pressure raises back to almost 40 psig
  7. 5 minutes after shut off - fuel pressure still above 35
    psig

I’ve suspected all along that my FPR is acting up and have
asked on this forum for replacement alternatives. The BOSCH
unit is NLA, only a few vendors have them and they cost
$200+

Before I splurge on one, could someone please confirm that
this is indeed the FPR (and not the fuel pump(s) or
something else I missed). Human error in the measurement is
always possible (:-)), but I don’t see how my numbers can be
that much off from 3 bar of the later 6.0L motor (31-32 psig
is even lower than the 2.5 bar rating of the 5.3L motors).

Also, for a full disclosure, I am using a calibrated gauge
from a vendor of scientific equipment, and would not suspect
the gauge itself. The engine started/ran OK, but the ‘‘blip’’
when turning the throttle to increase engine speed seems to
suggest I’ve got fueling issue(s) to solve.

Thanks much for your input.

Steve–
'95 XJS V12 6.0L, saphire/tan
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In reply to a message from sbobev sent Tue 12 Feb 2013:

I think I have a used one on a spare 6.0 that I have. I
will check, if I do, would you like it?

Also, fuel pump, that should be fuel PUMPS, there are 2. One
runs at(for lack of a better term)low speed, when you floor
it, pump #2 kicks in.

I will have to consult my manuals as to fuel pressure when at
idle, I would not think it to be 3 bar. Maybe around 2 bar,
at idle.–
Dr. Chadbourn Bolles, JaguarXJ_S@Yahoo.com
Leesville, SC, United States
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In reply to a message from Dr. Chadbourn Bolles sent Tue 12 Feb 2013:

Dear Steve,

Do remember that the fuel pressure is not absolute, but relative to
the air pressure in the manifold (’‘MAP’’), so it ought to be lower
than 3bar when the engine is running. It’d also unsurprising that
it is slightly higher once you turn the engine off, but then tails
off.

Typically, at idle or under load, the manifold pressure may be in
the order of around 0.5-0.7 bar, so your measured fuel pressure is
relative to that, not atmospheric pressure (~1bar at sea level). In
other words, expect numbers lower than 45psi on a running engine.

kind regards
Marek–
MarekH
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In reply to a message from Dr. Chadbourn Bolles sent Tue 12 Feb 2013:

Dr. Bolles:
My tests were performed in the garage, no chance to get the
second pump kicking in

I’ve looked everywhere on this forum for data. This test is
very close to mine BUT it is on a 6-cylinder and I believe
the FPR there is 2.5 bar.

http://www.jag-lovers.org/snaps/snap_view.php3?id=1357201999

I’m missing something here but do not know what it is

Steve–
The original message included these comments:

Also, fuel pump, that should be fuel PUMPS, there are 2. One
runs at(for lack of a better term)low speed, when you floor
it, pump #2 kicks in.
I will have to consult my manuals as to fuel pressure when at
idle, I would not think it to be 3 bar. Maybe around 2 bar,
at idle.


'95 XJS V12 6.0L, saphire/tan
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In reply to a message from MarekH sent Tue 12 Feb 2013:

Dear Marek:
I quoted my measurement in units ‘‘psig’’ (pounds per square
inch at the gauge), well aware of the fact that pressure is
relative to something.
At idle, my vacuum gauge shows 19-20 inches of Hg vacuum in
the manifolds (which equates to ca. 0.35 bar), meaning 3-
0.35=2.65 bar=40 psig.
But what I measure at idle is 1/4 less…
Also, I did not see the pressure increase when the engine
speed increased to ca. 2000 rpm and the vacuum in the
manifolds dropped. Well there was no load to the engine,
might not have been enough to measure.

40 psig (+/-1 psig) is the number I get when my rails get
primed and no vacuum is applied to the FPR. Delaware, where
I live for practical purposes can be considered at sea
level), wouldn’t one expect to see then 45 psi at the gauge?
Thanks for your response,
Steve–
The original message included these comments:

Dear Steve,
Do remember that the fuel pressure is not absolute, but relative to
the air pressure in the manifold (’‘MAP’’), so it ought to be lower
than 3bar when the engine is running. It’d also unsurprising that
it is slightly higher once you turn the engine off, but then tails
off.
Typically, at idle or under load, the manifold pressure may be in
the order of around 0.5-0.7 bar, so your measured fuel pressure is
relative to that, not atmospheric pressure (~1bar at sea level). In
other words, expect numbers lower than 45psi on a running engine.
kind regards


'95 XJS V12 6.0L, saphire/tan
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Hi Steve,

Several observations:

(1) 1 bar = 14.503 PSI, so 3 bar (nominal rail pressure) = 43.51 PSI

(2) Bryan Neish who posted the photos & test results you linked has a
91 XJ40 with 4.0L six which, like its 4.0L XJS counterpart has a 3bar
fuel injection system, NOT 2.5 bar.

(3) Your test results certainly do NOT suggest a basis for any starting
or performance problem. It would be interesting to know what the
pressure is on full acceleration (WOT) in 1st gear when the 2nd pump
should kick in IF you are experiencing lack of proper acceleration.

(4) I would suggest identifying and reporting any starting or
performance problems you have experienced with your 95 6.0L, so others
might suggest tests or things to check.

As I understood your initial post in this recent thread, you have had a
longstanding “cold start” problem. If you mean that the car starts and
runs/accelerates fine when warm, but is slow to start after sitting for
12 hours or more, that suggests either a bad checkvalve on the fuel
pump or a leaking FPR, either of which allows rail pressure to be
dissipated faster than it should.

George Balthrop, Clifton, VA USA
89 and 85 XJ-S Coupes; 89 XJ40 VDP-----Original Message-----
From: sbobev shelxtl@yahoo.com

I’ve looked everywhere on this forum for data. This test is
very close to mine BUT it is on a 6-cylinder and I believe
the FPR there is 2.5 bar.

http://www.jag-lovers.org/snaps/snap_view.php3?id=1357201999

I’m missing something here but do not know what it is

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I quoted my measurement in units ‘‘psig’’ (pounds per square
inch at the gauge), well aware of the fact that pressure is
relative to something.
At idle, my vacuum gauge shows 19-20 inches of Hg vacuum in
the manifolds (which equates to ca. 0.35 bar), meaning 3-
0.35=2.65 bar=40 psig.
But what I measure at idle is 1/4 less…

Which it should be, because there’s a lot of vacuum at idle.

Also, I did not see the pressure increase when the engine
speed increased to ca. 2000 rpm and the vacuum in the
manifolds dropped. Well there was no load to the engine,
might not have been enough to measure.

When you have no load and the RPM is raised from idle to 2000, the
vacuum in the manifolds INCREASES.

40 psig (+/-1 psig) is the number I get when my rails get
primed and no vacuum is applied to the FPR.

Also as it should be. When I first read through that report, I
thought, “Yeah, everything’s working fine.” Of course, I wasn’t
following the discussion too closely.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 12 Feb 2013 at 19:01, sbobev wrote:

My tests were performed in the garage, no chance to get the
second pump kicking in

Actually, someone else probably knows better, but doesn’t the second
pump kick in whenever the RPM rises past a particular value,
something like 3500 RPM? It’s not throttle-related at all, just RPM.
So, you should be able to just rev the engine up right there in the
garage and check to see if the second pump comes on.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 12 Feb 2013 at 18:31, sbobev wrote:

In reply to a message from Kirbert sent Tue 12 Feb 2013:

The pump control is described here in the 36CU section:-
http://www.jagweb.com/aj6eng/lucas_efi.php

Fuel pressure on a 6 litre should be 3 bar relative to manifold
pressure.–
The original message included these comments:

Actually, someone else probably knows better, but doesn’t the second
pump kick in whenever the RPM rises past a particular value,
something like 3500 RPM? It’s not throttle-related at all, just RPM.
So, you should be able to just rev the engine up right there in the
garage and check to see if the second pump comes on.


Roger Bywater / AJ6 Engineering
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In reply to a message from George Balthrop sent Tue 12 Feb 2013:

Hi George:
I was hoping you’d respond and remember the long discussion
from November:

http://forums.jag-lovers.org/av.php?1570882n77

I have not been able to reproduce the problem in several
tries, so, as you said, I ‘‘may never know’’.

Nonetheless, the issue with the fuel pressure bugged me and
being a scientist, I decided that I must know. Now, with
the gauge in place, after a week in the garage, I see that
the 2 seconds burst by the fuel pump primes the rail and
this should not be the root of the problem.

I have confirmed the proper working order of the TPS, CTS,
the flywheel sensor, idle switch, etc., and now the fuel
pressure. Yet, there is a small hesitation upon partial
throttle movement (aka ‘‘blip’’), which seems to suggest
fueling problem.

Also, I have not been able to answer the question why my
engine started/ran with the vacuum tube to the ECU in the
boot disconnected (a big mistake on my part).
As I am spending more and more time with the car (2.5 years
of ownership), I am beginning to get more and more
frustrated with the idle speed, which fluctuates around 750
+/- 25 rpms.

Before this goes off-topic, I’d like to to sum up:

  • both coils are providing fat blue sparks from the leads,
  • resistance for all plugs wires was checked,
  • all plugs renewed (BR7EF and gapped to 0.025’’),
  • all plugs fire,
  • Marelli cap and rotor in excellent shape
  • all injectors click (also just cleaned and tested by SD),
  • all vacuum tubing replaced in the engine bay and checked
    and re-checked for routing per the diagram,
  • vacuum gauge in the boot by the LUCAS ECU confirms strong
    vacuum at idle (ca. 20’’ Hg),
  • throttle plates adjusted with new linkage bushes,
  • correctly operating AAV
  • strong acceleration, engine revs freely

The only remaining key components I have not been able to
test/confirm are working as they should are the two ignition
amplifiers (and the 36CU, perhaps, given the 15 mile
endeavor with the vacuum tube dangling in there and no
obvious change in the starting/running condition)

Best regards,

Steve–
The original message included these comments:

(4) I would suggest identifying and reporting any starting or
performance problems you have experienced with your 95 6.0L, so others
might suggest tests or things to check.
As I understood your initial post in this recent thread, you have had a
longstanding ‘‘cold start’’ problem. If you mean that the car starts and
runs/accelerates fine when warm, but is slow to start after sitting for
12 hours or more, that suggests either a bad checkvalve on the fuel
pump or a leaking FPR, either of which allows rail pressure to be
dissipated faster than it should.


'95 XJS V12 6.0L, saphire/tan
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In reply to a message from Kirbert sent Tue 12 Feb 2013:

Kirby:

I am measuring both fuel pressure and manifold vacuum
simultaneously.
At idle on a warm engine, my vacuum gauge shows between 19-
20 inches Hg (very small dial, hard to determine it more
accurately)
Therefore, I calculated I should be seeing ca. 40 psig, yet
the measured value was closer to 30 psig

Also, my vacuum gauge does not show increase in vacuum as
the throttle plates are opening and the engine speed
increase (even with no load)

Would you like to elaborate on your comments, I am afraid I
did not understand them

Steve–
The original message included these comments:

At idle, my vacuum gauge shows 19-20 inches of Hg vacuum in
the manifolds (which equates to ca. 0.35 bar), meaning 3-
0.35=2.65 bar=40 psig.
But what I measure at idle is 1/4 less…
Which it should be, because there’s a lot of vacuum at idle.
Also, I did not see the pressure increase when the engine
speed increased to ca. 2000 rpm and the vacuum in the
manifolds dropped. Well there was no load to the engine,
might not have been enough to measure.
When you have no load and the RPM is raised from idle to 2000, the
vacuum in the manifolds INCREASES.


'95 XJS V12 6.0L, saphire/tan
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In reply to a message from RogerBywater sent Wed 13 Feb 2013:

Mr. Bywater:
I’ve read all the information on you page, thank you very
much for responding.
Is it safe the rev the engine in Park to up to 3000 rpm to
test the second pump? I have no issues with the
acceleration and would not expect any problems but would be
nice to test and confirm.

Also, perhaps you are the best person to answer the question
‘‘is it possible for the 36CU to allow engine
starting/running with the vacuum tube to the ECU
disconnected’’?
I was testing the vacuum and forgot to attach it back – a
silly mistake on my part – but the car started and I drove
it to work and back when I realized that the idle speed was
hire and there was hissing in the boot…
There were no DIC displayed.

Best regards,
Steve–
The original message included these comments:

The pump control is described here in the 36CU section:-
http://www.jagweb.com/aj6eng/lucas_efi.php
Fuel pressure on a 6 litre should be 3 bar relative to manifold
pressure.


'95 XJS V12 6.0L, saphire/tan
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At idle on a warm engine, my vacuum gauge shows between 19-
20 inches Hg (very small dial, hard to determine it more
accurately)
Therefore, I calculated I should be seeing ca. 40 psig, yet
the measured value was closer to 30 psig

Lessee, one bar is about 14.7 psig, right? So 3 bar is 44 psi. 20
inches of vacuum is roughly 10 psig, subtract and get roughly 34 psi.
Sounds close enough, especially since the rail pressure seems to be
changing correctly with startup and with disconnecting the vacuum
line. With the numbers you posted, I would have concluded that fuel
supply was not a problem.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 13 Feb 2013 at 6:53, sbobev wrote:

Mr. Bywater:
Is it safe the rev the engine in Park to up to 3000 rpm to
test the second pump? I have no issues with the
acceleration and would not expect any problems but would be
nice to test and confirm.

I know this wasn’t addressed to me, but of course it’s safe to rev it
to 3000 in Park – as long as the engine is fully warmed up.

Also, perhaps you are the best person to answer the question
‘‘is it possible for the 36CU to allow engine
starting/running with the vacuum tube to the ECU
disconnected’’?
I was testing the vacuum and forgot to attach it back – a
silly mistake on my part – but the car started and I drove
it to work and back when I realized that the idle speed was
hire and there was hissing in the boot…
There were no DIC displayed.

Basically, the ECU should think you’re at full throttle at all times
and run on the enrichment map. Probably not good for fuel economy or
engine durability, but otherwise it sounds like you got away with it.
I am kinda surprised there wasn’t a fault flagged.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 13 Feb 2013 at 7:06, sbobev wrote:

In reply to a message from Kirbert sent Wed 13 Feb 2013:

I would respectfully disagree. Perhaps the 1st and only time I
would dare to do so

20 inches of Hg vacuum equates to ca. 0.35 bar pressure.
(1 bar is ca. 30’’, pumped 2/3-rds down. You are left with 1/3
of it)

I still believe my calculation at idle speed:

3-0.35=2.65 bar=39-40 psig

Steve–
The original message included these comments:

Lessee, one bar is about 14.7 psig, right? So 3 bar is 44 psi. 20
inches of vacuum is roughly 10 psig, subtract and get roughly 34 psi.


'95 XJS V12 6.0L, saphire/tan
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In reply to a message from Kirbert sent Wed 13 Feb 2013:

True, but I did not see any clouds behind me (or smelled
anything – it is a convertible)
Also, there lots of posts in the archives about cars with
blocked tube, which do not start or run poorly.
In my case, I did NOT notice any such conditions, engine run
fine, started, warmed-up normal…
Again, no DTC were displayed, which is worrisome – the MAP
sensor should flash FF13 since it is looking for fluctuation
of the vacuum during starting
Steve–
The original message included these comments:

I was testing the vacuum and forgot to attach it back – a
silly mistake on my part – but the car started and I drove
it to work and back when I realized that the idle speed was
hire and there was hissing in the boot…
There were no DIC displayed.
Basically, the ECU should think you’re at full throttle at all times
and run on the enrichment map. Probably not good for fuel economy or
engine durability, but otherwise it sounds like you got away with it.
I am kinda surprised there wasn’t a fault flagged.


'95 XJS V12 6.0L, saphire/tan
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No Kirby has it right. By subtracting from 30 you are converting to absolute pressure which is not the way the regulator is rated.

If the manifold pressure is 20 in hg below atmospheric pressure, the fuel pressure is regulated down an equal amount. The whole point is to keep a constant 3 bar pressure drop across the injector nozzle regardless of engine operating condition. Thus a pressure reduction of 20 in hg (.67) bar in the manifold equals a 20 in hg (.67 bar) reduction of pressure in the fuel rail. Simple as that. The calculation is 3 - 0.68 = 2.32 bar -= 33.65 psi

-GaryOn Feb 13, 2013, at 9:45 AM, sbobev shelxtl@yahoo.com wrote:

In reply to a message from Kirbert sent Wed 13 Feb 2013:

I would respectfully disagree. Perhaps the 1st and only time I
would dare to do so

20 inches of Hg vacuum equates to ca. 0.35 bar pressure.
(1 bar is ca. 30’’, pumped 2/3-rds down. You are left with 1/3
of it)

I still believe my calculation at idle speed:

3-0.35=2.65 bar=39-40 psig

Steve

The original message included these comments:

Lessee, one bar is about 14.7 psig, right? So 3 bar is 44 psi. 20
inches of vacuum is roughly 10 psig, subtract and get roughly 34 psi.


'95 XJS V12 6.0L, saphire/tan
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In reply to a message from Gary Evans sent Wed 13 Feb 2013:

Thanks Gary, I do stand corrected. My ‘‘calculation’’ is a non-
sense, not even a conversion to absolute pressure (in that
case I should’ve added the numbers)
Apologies to all reading my post above, especially Kirby.
Steve–
The original message included these comments:

No Kirby has it right. By subtracting from 30 you are converting to absolute pressure which is not the way the regulator is rated.
If the manifold pressure is 20 in hg below atmospheric pressure, the fuel pressure is regulated down an equal amount. The whole point is to keep a constant 3 bar pressure drop across the injector nozzle regardless of engine operating condition. Thus a pressure reduction of 20 in hg (.67) bar in the manifold equals a 20 in hg (.67 bar) reduction of pressure in the fuel rail. Simple as that. The calculation is 3 - 0.68 = 2.32 bar -= 33.65 psi


'95 XJS V12 6.0L, saphire/tan
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Please pardon the revival of this old thread, but it’s the top Google result when searching for “jaguar XJS fuel pressure bar”.

Throughout this older post 3 bar is consistently suggested as the spec fuel pressure for the 1995 6L.

I note that item 7 on the Jaguar Technical Service Bulletin on Terry’s Jag page Service Bulletin cites the expected file pressure on a 1992 XJ-S V12 is… "2.5bar+0.1bar."

Is the FP different on the pre-facelifts, facelift 5.3s, and also for the 6L?

This doesn’t answer my initial reason for the search ( looking for authoritative spec for an '86 V-12 ) but does suggest that it’s lower than 2.5 bar. Does anyone know an authoritative source ( manual, SB etc ) for the pre-facelifts fuel pressure?

Correct. 1992 will be 5.3L and even though the fuel rail was revised for that year, the pressure is still 2.5 bar.

3 bar is only for the 6.0L