The fuel pressure regulator at the end of the fuel rail is subjected to heat
expansion and fuel pressures get inadequate at idle. I learned a hard
lesson to get this knowledge. My 1988 XJ6 VDP had the same dilemma when
coming to a stop and the engine dropped to idle speed. I followed the
advice of the Haynes automotive manuals suggestion to install a second fuel
pump. I piped it into the blank fitting on the front of the fuel rail. Two
weeks later an overpressurized fuel line burst while I was driving on
Interstate 95 and sprayed raw gas into the engine compartment and ignited an
engine fire which destroyed the car in minutes after the line burst. Just
as you have experienced, even the experts didn't see this one coming. I've
handbuilt custom intakes and fuel systems for carbureted, injected,
turbo-charged and superchaged engines for classic 6 cylinder Chevy's and a
couple of 70's era Kawasaki 4 cylinder superbike motors and I've never came
across a fuel delivery system as complicated as the on the 1988 Jag that
burned or my current cat, an '89 XJ6. Do you hear what sounds like a
tapping rhythm a bad valve lifter would make, somewhere near the firewall
when you come to a stop and are at idle? You can only hear it when your
inside the car. The sound is similar to "water hammer" in a piping system
when you shut off a valve in a system without a damping standpipe. Same
thing is happening to the fuel pressure regulator. When the injectors click
shut, a good regulator will absorb the back pressure created in the fuel
delivery cycle without losing synchronization to the ignition timing. When
the diaphragm in the regulator expands from heat or ages, the regulator
loses synchronization at high fuel pressures which occur at idle. Once the
engine load is off idle, the fuel back pressure drops dramatically to supply
the now hungry injector flow and the regulator functions adequately. That
explains the "experts" suggestion to find an ignition timing fault. The CPU
controls injector timing based on feedback from the ignition sensors and
fuel pressure sensors. The back pressure leak through the regulator changes
the fuel delivery volume and the now fuel delivery volume tells the CPU a
false value and it corrects with an ignition timing adjustment and the fuel
pressure at idle is inadequate to open the injectors. Sort of like a leaky
heart valve in a human. This is what I believe caused the burst fuel line in
my '88. I was at highway speed, (for my '88 that was usually between 85
and 105mph) I let off the accelerator and an instantaneous but brief
backpressure in the fuel lines from the air flow snapping shut, told the
CPU to reduce injector flow. At that instant both fuel pumps were
delivering high volumes through the regulator and the backpressure leak blew
the dual pumped fuel rail line. Have you ever seen a "top fuel" nitro car
blow a supercharger off the engine on a burnout? Same physics here. Bad
fuel delivery timing and back pressure into the fuel lines. Nitro burners
react violently to lean fuel to air ratios !!!
The solution is not so simple. A new regulator will cure most of the
problem. I suspect Jaguar Cars limited would benefit by putting drag racing
veteran experience into their CPU logic to accomadate the timing issues at
the critical idle and throttle closure timing and fuel volume sensors
evolutions. Something along the line of an intercooler or heat shielding of
the regulator would lend a positive mark to a snag that appears to be
somewhat of a legend among Jaguar owners. A tap off and return of the Air
Conditioner line before the evaporator to a small cooling coil at the fuel
rail and regulator could do the trick. This is a customised solution, but
it could result in a horsepower gain historically associated with cooler
cylinder charge temperature.
Well yes the steel shell will expand slightly when heated, however,
the fuel circulating through the system actually cools the regulator
and keeps it within operating spec. As the regulator ages, the spring
inside weakens, resulting in lower max pressures.
What? LOL! Did they spec it out for you? What page number?
Into the front of the fuel rail? It should have been the back as the
front is the return end that leads to the regulator. The regulator
controls pressure after the injectors, not before it. Now what page
was that on?
It is a Bosch system; a very common system found in most cars. From
Toyota to GM. It is rather simple, not complicated.
Hmmm. sounds as if the damper(s) have failed. There is one at the back
of the rail (inlet) , and another just after the regulator (outlet).
These dampers absorb the pulses from the pump as well as the injector
pulses. Once the dampers fail the regulator gets "Punished". Your
problems could have been fixed by replacing the inlet damper and a new
regulator as long as the pump was ok.
Sorry that is incorrect. Ignition and fuel pressure do not sync. and
there is nothing to tie these together.
NOPE. Cpu follows a programmed "map" and the Lambda gives a rich or
lean feedback the the CPU. The lambda is in the exhaust.
Uh.... nope. Fuel pressure is not monitored. And backfeed is not going
to happen as the system is pressurized. Explain how to backfeed
through a regulator when the return line has almost no pressure in
it?(Unless caused by your second pump in the return line flowing the
wrong direction.) Injectors are electronic and open from a pulse, not
fuel pressure. Is this a diesel?
I believe that your second pump was to blame, and possibly that you
used standard fuel hose instead of injection hose. Top fuel cars blow
intakes because of leanout and ignite the fuel in the intake manifold.
Airflow snaps shut, potentiometer tells cpu "Hey cut the fuel". Top
fuel is a constand feed and is not pulses; no timing here just flow.
We racers have long known about cooling the fuel by means of a "Ice
can". In the old days we would take a coffee can, tin in a fuel coil,
add water and ice and bingo, you have cooled the fuel. Now we use
igloo's and dry ice.
I think it would be a great benefit for you to purchase the Bosch Fuel
Systems Handbook. One can be found here -
(Amazon.com product link shortened)
I am not speaking down to you, although it would seem that way when
anyone corrects someone.
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