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A misfire at idle with an AAV playing up will be "normal".
Sometimes adjusting the fuel "pot" via the hole in the ECU casing MAY cloak it for a while.
As you know, the AAV closing 100% in mandatory for a stable idle on the HE.
That fuel pressure is not an issue, as Doug has already said.
My HE ended up with a PreHE FPR on it, as I was fed up with the Vac units going unstable, and I set it at 32psi, and no issues at all. OK, some other little twists to permit the enrichment to come ON earlier took care of the no vac FPR operation, and teh 16CU all made it a sweet rocket.
I checked the calibration of my cheapo gauge, and it is spot on at 2-4 bar. Incredible
Pertinent bit...
now to the B bank FPR. THIS is the item that actually controls fuel pressure delivered thru the fuel injectors to the engine. Fuel pressure ( via this FPR) keeps a constant pressure between the injector pintle and the intake manifold vacuum (which varies under different engine operational conditions) at a constant 2.5 Bar...(36.25 psi).
I read that as 36.25 psi <relative to intake pressure> and the intake is normally running a partial vacuum. When you disconnect the FPR from the manifold it should vary the pressure to something other than 36.25psi.
Assuming the statement referenced IS accurate then, under load, the B FPR would need to increase fuel pressure from 36.25 psi at 18 in hg of vacuum to 41.15psi at 3 in hg vacuum. Or to 45 psi at 0 in hg vacuum. Which is 3 bar or the flow-rated pressure bosch states for the injectors.
I read that as 36.25 psi <relative to intake pressure> and the intake is normally running a partial vacuum.
The info listed may be correct, but just because something is written on the internet down not grant any validity. A cited Jaguar source for this info sure would help.
Assuming the statement referenced IS accurate then, under load, the B FPR would need to increase fuel pressure from 36.25 psi at 18 in hg of vacuum to 41.15psi at 3 in hg vacuum. Or to 45 psi at 0 in hg vacuum. Which is 3 bar or the flow-rated pressure bosch states for the injectors.
Seems to correlate with known data points.
I don't agree to your calculations.
Assuming my link info is correct, differential pressure over the injectors shall be 36,25 PSI regardless of manifold pressure.
I agree I should look for a better source for this info...
When vacuum is zero (WOT or disconnected vacuum hose), the fuel pressure shall be 36,25 PSI measured with reference to atmospheric pressure = zero vacuum.
If inlet vacuum is 2 PSI, fuel pressure must drop 2 PSI to 34,25 PSI, again measured with reference to atmospheric pressure.
When I read 30 PSI fuel pressure with reference to atmospheric pressure in idle, I would guess that my manifold vacuum is 6,25 PSI (or thereabout...)
Assuming my link info is correct, differential pressure over the injectors shall be 36,25 PSI regardless of manifold pressure.
I agree I should look for a better source for this info...
Sounds correct to me, FWIW.
I don't have it in front of me but I'm quite sure that the service manual describes the purpose of the FPR as being ".....to maintain a constant pressure across the injectors...".
....but it goes off into the weeds a bit so I've excerpted this:
To fully understand pressure, we need to examine pressure drop.
Pressure drop can be defined as the difference in pressure between two points in a fluid carrying network. The easiest way to understand pressure drop might be with an example, so let's imagine a piece of two-inch plastic pipe that's a foot long. At each end of the pipe, we will put a pressure sensor and measure the water pressure. On the left side of this pipe, let's place a fluid pump that's pushing 40 PSI of water into the pipe. On the right side, let's begin by capping off the pipe. And in the middle of the pipe, we will place a divider with a small orifice drilled in it.
Did our pressure drop? No. Without flow, the entire pipe filled up with 40 PSI of pressure. The pressure remains 40 PSI on both sides of the orifice, because no flow is taking place. No water flow, no pressure drop. Next, let's remove the cap. We now have 40 PSI on one side of the orifice, and 0 PSI on the other side. Did our pressure drop? Yes, the pressure drop across the orifice is 40 PSI (40 minus 0 = 40).
So what would happen if we were to constantly pressurize the open end of the pipe with 10 PSI of air pressure? The two pressures would fight it out, and the higher pressure would win. High pressure always wants to move towards low pressure. When we have 40 PSI going into the orifice on one end, and 10 PSI of pressure going into the orifice from the other end, the result will be a pressure drop across the orifice of 30 PSI (40 - 10 = 30).
Sounds reasonable so far, now let's get a bit tricky. Let's take the orifice end of the pipe, and hook up a vacuum hose to it. And let's say this vacuum has a suction pressure of -25 PSI (which is roughly the equivalent of 50" of mercury, or 50 InHg on a vacuum gauge). Now instead of having two pressures fighting each other, you now have two opposite types of pressure working together. One is pushing, and the other is now pulling. You end up with a pressure drop across the orifice of 65 PSI (40 + 25 = 65).
Is there 65 PSI of pressure anywhere in this pipe when you measure it? No. The most pressure you will find is 40 PSI, no matter where you place your pressure sensor, but the pressure drop across the orifice is indeed 65 PSI because of the 25 PSI of suction combined with the 40 PSI of pressure.
Horisontal line is atmospheric pressure. Top angled line is fuel pressure measured relatively to atmospheric pressure i.e. the gauge reading. Lower angled line is inlet pressure, which will be atmospheric at WOT and lower than atmospheric when the butterflies are more or less closed.
Hi inlet vacuum at for example idle will pull the fuel pressure gauge reading down to keep differential pressure over the injectors constant 36.25 PSI.
A question, as I have no detailed idea how the FRR is meant to work. Is the vacuum signal to the FPR:
(a) intended to enrich the mixture - ie increase the pressure in the rail - when the manifold vacuum drops? or
(b) intended to keep the pressure difference between the rail pressure and the manifold pressure the same across all throttle openings (as illustrated in Leo's diagram in post 33) ?
The pressure in the rail from the LH FPR is a constant to a degree.
The opening time of the Injectors at whatever is determined by the ECU from ALL its sensors, shielded wire, etc determines how long they are/are not open.
A V12 at idle needs very little fuel. An unloaded V12 at 3000rpm also needs little fuel.
A V12 on the highway at cruise also needs little fuel, but more than idle.
A V12 at acceleration, climbing a hill, etc needs lots of fuel.
The pump is capable of 110-130psi, and a large GPM flow capacity.
As long as the flow through the rail at pressure of around 36psi can be maintained, so pump integrity, filter etc are all required to be OK, the opening of the injectors that causes an extremely small pressure drop for a millisecond, is minimal due to the back up of the pumps capacity.
The return fuel to the tank is generally approx 60%, but that is not scientific, just my observation with the fuel return hose in an external container for reasons I now forget, but it filled that container much quicker than I thought.
Then the electrical part comes into the equation, and has NO bearing on teh mechanical FPR etc at all.
The fuel enrichment switch, lengthens the ON time of the Injectors, richer mixture.
The CTS has a play at the injector ON time also.
The Vac signal from the engine also alters that fueling constantly.
The ATS is a trimmer, but gets into the ECU also.
TPS position also in there.
The idle pot in the ECU also plays a small amount, too lean here, and an off idle stumble is felt.
As I said, mine has a PreHE FPR, no vac signal, and the engine is happy. The engine "sweet spot" can be tuned, just like a mixture screw on a carby (for those that are old enough), by simply turning the adjusting bolts ans listening to the engine, and I have set mine that way. The pressure is somewhere around 36ish, I THINK.
Like Ronbos, NO AAV, NO balance pipe, NO emissions junk, AJ6 throttle discs, cold air intakes, 16CU ECU, NO overrun valves, etc etc. Just a bare bones basic engine.
Last edited by Grant Francis; 06-24-2017 at 09:47 AM.
A question, as I have no detailed idea how the FRR is meant to work. Is the vacuum signal to the FPR:
(a) intended to enrich the mixture - ie increase the pressure in the rail - when the manifold vacuum drops? or
(b) intended to keep the pressure difference between the rail pressure and the manifold pressure the same across all throttle openings (as illustrated in Leo's diagram in post 33) ?
I'm going with "B".
Richer/leaner mixture is controlled by injector pulse width....as commanded by the ECU. The ability of the ECU to do its job is predicated on(among other things) maintaining a constant pressure drop across the injectors. The FPR's job is to maintain that constant pressure drop.
If we were to personify the ECU it would say to us "I'm going to control fuel mixture by adjusting the injector pulse width, based on calculations that are valid only if there is a constant pressure across the injectors".
^^^^ That's how I see it. I am not asserting that I'm correct.
OK here is how i do it,my V12 modded! and Grants explaining is very good also!
custom adjustable ECU, with GM MAP sensor for it. i have wide overlap race cams, so low vac at idle,like 10/11 in HG.
big fuel pump, big lines , way back, thought ahead, just in case i left reality, and was gonna TURBO charge V12!
looking at this topic, some very good info for a standard Jag Bosch FI system.
ECU sees manifold pressure signal from Map sensor.
ecu pulses injector to a predetermined map time on, fuel volume is squirted. so far so good(i think).
all is good , now here things get foggy, manifold pressure is in inches of mercury,Vacuum.
what do you think happens when the ecu sees ZERO VAC, BUT we are turbo/supercharged, all of a sudden most factory ecu go dead shut down, because a pressure reading is not programmed into the MAP, its in PSI, not inHG.
thats where a custom ECU is nesassary,so you can adjust injector time accordingly, WHY?
because you only needed, rough guess, 45 psi when in vac mode, but now you got 10 psig positive pressure in manifold, we/I use a rising rate FPR, that ups the gage reading 10 psi, showing gage , as 45+10=55 psi,,, 20 psi manifold would give 65psi gage reading.
i dont have to explain if you lose FP when in boost conditions,20 psi loss ,and you will be looking for a better engine.
my experience on a NA engine loss of pressure dont mean much,except loss of power, and most people let off throttle.
and tuning the custom ECU for NA is way much simpler than tuning for a positive pressure in the inlet tract!
I also USE an Air /Fuel/ratio gage in both NA and boost conditions. you know the 14.7 type thing,etc. NA lean is good, pressurized manifold rich.
anyway i dont think that what ever the " across the injector" thing is has little if anything that would help us much in everyday use.
because if you just change a set of injectors you would alter the fuel settings! NO guarentee that they are the same as you had, but close enough .
anyway i dont think that what ever the " across the injector" thing is has little if anything that would help us much in everyday use.
I agree.
There are countless aspects of engineering and physics that I don't understand ...or am even aware of. But such understanding and awareness isn't strictly required to successfully own, operate, repair, and to some extent, modify an automobile.
OTOH.....
The rudiments of XJS repair and ownership are pretty well covered and understood...making natural that we sometimes go off into the deeper weeds from time to time. A lot of it is "beyond my dimensions of comprehension" .
ALL this "stuff" to my HE was done PRE computer days, Forum days, even "The Book" days.
Basically seat of the pants stuff. I simply played, and when I got it wrong (many times) and it stopped running, back up a few steps and change directions.
Like Doug, science and physics leave me cold (even used spell check just now to spell it right), but its an engine, just a tad more fancy than carbies, so simple still.
The current S Type V6 I have scare the crap out of me.
06-22-2017, 04:05 AM
