Corvette
and Cadillac XLR
Midyear of 2003, a new
fuel tank system was introduced on the Corvette.
This system continues with slight
differences in the 2004 Corvette and Cadillac XLR.
TIP: For 2003 Corvettes only,
this system was designated by RPO code FFS. However, this name has
been unofficially applied to all vehicles with the new system.
Advantages and Features The new fuel tank system was
designed to accommodate future LEV 2 emission requirements. This was
accomplished by moving as many components and fuel lines as possible
inside the fuel tanks, to minimize hydrocarbon emissions. A flexible
metal crossover hose assembly replaces the former rubber one,
because the permeable rubber allowed a small amount of hydrocarbon
to pass through.
The redesign also includes more isolation
and noise control for the electric fuel pump, which now has greater
flow capacity to supply higher output engines.
Component Layout TIP: Use the reference letters to
identify and locate the various components. A
Left side tank B Right side tank C
Fuel sensor reservoir D Turbine fuel pump
E Venturi pump F Fuel fill hose
G Secondary fuel pressure regulator H
Siphon jet pump J FLVV K
Crossover hose L Filter M
Fuel feed pipe to engine N Fuel feed pipe to RH
tank P Fuel return pipe to LH tank Q
Left fuel level sensor R Right fuel level sensor
S Anti-siphon hole T Fuel pump speed
control module U EVAP canister V
Primary fuel pressure regulator W Check valve
TIP: On the XLR only, a speed
control module (T) slows the fuel pump when the engine is idling, to
further control pump noise.
Two fuel tanks are used, and
they're joined by crossover plumbing.
The left side (driver side) tank (A)
is considered the primary, and the right side (passenger side) is
secondary (B).
Each tank contains a sensor module, which includes a float and
resistor card.
On the left side, the sender module includes a
reservoir (C), containing the turbine fuel pump (D).
There's also a primary fuel pressure regulator (V)
and a venturi pump (E). The left tank is also
supplied by the fuel fill hose (F), and has a
rollover vent valve.
On the right side, the sender module
contains a secondary fuel pressure regulator (G)
and a siphon jet pump (H). There is also a fill
limiting vent valve (FLVV) (J).
Operation When the fuel tanks are filled, fuel first
fills the left tank. As the fuel rises to the level of the crossover
(K), fuel flows into the right tank. As fuel
occupies the interconnected tanks, air is forced to vent from the
tanks, through the FLVV in the right tank. When both tanks are full,
the FLVV float in the right tank closes, preventing fuel from
entering the vent system. This also causes fuel to back up in the
fill hose, causing the gas pump nozzle to shut off.
When the
engine is running, the turbine fuel pump (D)
in the left tank pressurizes the fuel feed pipe (M). The entire fuel
supply system, from the pump to the injectors, is pressurized. The
turbine pump creates more pressure and more fuel flow than the
engine needs. Excess pressure and excess fuel is allowed to bleed
back into the left tank by the primary fuel pressure regulator (V)
within the tank.
TIP: The pressure regulator is in
the fuel tank, not on the fuel rail on the engine. This type of fuel
system is called returnless, or demand. This means that excess fuel
is diverted before it leaves the tank, instead of passing through
the fuel rail before being diverted. The result is that hot fuel is
not constantly returning from the engine compartment, so the fuel in
the tank stays cooler, improving evaporative emissions.
The
majority of the pressurized fuel is directed through the filter (L)
and on to the fuel feed pipe (M) to the engine,
where it is injected into the cylinders for combustion.
Some
of the pressurized fuel is directed through a feed pipe (N) inside
the crossover hose, to a siphon jet pump in the right tank. The jet
pump relies on the venturi effect to use pressurized fuel to draw
additional fuel from the tank. The combined fuel then flows from the
right tank to the left tank, through a return pipe (P)
inside the crossover hose. The jet pump is able to move enough fuel
to ensure that all of the fuel in the right tank is consumed before
the level in the left tank begins to drop.
TIP:
The return tube in the left tank has an anti-siphon hole (S), so the
fuel in the left tank does not siphon back to the right side when
the vehicle is shut down.
Some of the pressurized fuel is
directed to a venturi pump in the left tank. This pump uses fuel
flow to siphon fuel from the main tank into the reservoir, to keep
the turbine pump supplied with fuel at all times. As the left tank
fuel level drops, the venturi pump scavenges all of the remaining
fuel into the reservoir, regardless of the vehicle's attitude.
When the engine is shut off and the turbine pump stops, a reverse
flow check valve (W) maintains pressure in the
system to ensure rapid pressure buildup during the next startup
cycle.
Operation of Fuel Level Gauge When
the fuel system is operating as designed, starting with both tanks
full, the left tank will remain full until the right tank is
depleted. Then the left tank will be emptied.
Each fuel tank
has its own sensor (Q and R in the
illustrations). Modules are shown
(LH module) and (RH module). The
PCM supplies a reference of 5 volts to the two sensors. Each sensor
operates across a range from full (2.5 volts) to empty (0.7 volts).
The PCM monitors the fuel level sensor voltages and calculates how
much fuel is in the two tanks. The readout of the IP fuel level
gauge is a result of this calculation.
TIP: The fuel level sensors can
be monitored with a Tech 2.
Several "zones" are used to
describe various combinations of fuel levels in the two tanks.
Zone 1 -- The LH voltage is above its full threshold
(typically calibrated to 2.4 volts) and the RH voltage is above its
empty threshold (typically calibrated to 0.8 volts). Fuel volume =
capacity of LH tank + volume in RH tank. This is the normal
condition, before the fuel in the RH tank is completely consumed.
Zone 2 -- The LH voltage is above full threshold and RH is
below empty threshold. This is also known as the deadband zone. Fuel
volume = volume in LH tank + deadband volume - fuel used since
entering zone. This is also a normal condition. It occurs because
the actual amount of fuel in the tanks cannot be precisely indicated
by the positions of the floats. That is, when the float reaches the
top of its travel, it's possible that the tank will hold additional
fuel, which does not cause the float to move higher. Similarly, when
the float reaches the bottom of its range, there may still be some
fuel in the tank, and the float does not move any lower as the
remaining fuel is consumed. So, the fuel in the bottom of the RH
tank and the top of the LH tank will be consumed without either
float moving. This is the "deadband."
Zone 4 --
The LH voltage is below full threshold value. Fuel volume = volume
in LH tank. This condition occurs just after the zone 2 "deadband"
is passed. All of the fuel is gone from the RH tank, and the LH
sensor has begun to move down. The amount of fuel in the LH tank is
all that remains.
Zone 5 -- The LH voltage
is below its full threshold and the RH voltage is above its empty
threshold. Fuel volume = volume in LH tank = volume in RH tank. This
is a condition that should not occur in normal operation, because
the RH tank should be consumed before the float in the LH tank
begins to drop. If the fuel system is in zone 5 for a certain amount
of time, a DTC (1431, 2066 or 2636) will set, and the fuel volume
will be reported as zero by the fuel gauge.
A zone 5
condition could occur if the jet pump in the RH tank becomes
clogged, preventing fuel from being siphoned from the RH tank. In
this case, only fuel in the LH tank is available.
TIP: Service kits are available
for each sensor, including: - float - wire arm - wiper -
card - card holder
Diagnostic Situations
The following situations may apply to the Corvette, the Cadillac (XLR) or both, as
noted.
Jet Pump Clog (C5, XLR) -- If the jet
pump in the RH tank becomes clogged, fuel will not transfer to the
LH tank. When this occurs, the vehicle eventually runs out of fuel,
even though there is actually some in the RH tank. When the DTC
(1431, 2066 or 2636) sets, the fuel gauge drops to empty and the
customer perceives an "erratic gauge." One cause of this condition
was a piece of plastic left in the jet pump during the manufacturing
process. This has been remedied.
Fill Quality (C5) --
A customer may comment that the fuel nozzle shuts off prematurely,
before the fuel tanks are full. The maximum flow rate for gas
station pumps is supposed to be 10 gallons (37.9 L) per minute, and
the fuel system is designed to accommodate this rate.
TIP: In reality, pump nozzles
vary considerably in configuration, flow and shutoff sensitivity.
Ask the customer if the condition occurs at all stations, all the
time, or at just one pump. This could point to a pump nozzle
problem, not a vehicle problem.
Be sure the rubber hose to
the canister vent solenoid (next to the transmission) is not
restricted and that the vent solenoid is not stuck closed. Both can
cause fuel to back up in the tanks and filler hose, causing the pump
nozzle to shut off.
TIP:A replacement filler hose
between the fill pipe and the LH fuel tank is available with a
smaller inside diameter for C5. Although this sounds like it would
make the condition worse, the smaller diameter allows less swirling,
creating smoother fuel flow. A bulletin is pending.
Regulator Not Seated, Clip Loose (C5, XLR) -- If the clip
retaining either regulator is not fully engaged, the fuel system
will lose pressure and the vehicle may stall. This condition was
remedied in production.
If the secondary regulator (right
tank) is not seated, it could result in fuel not transferring from
the right tank. It could also cause long crank times, because
residual pressure is lost in the fuel feed pipe to the engine when
the engine is shut down.
TIP: With the engine off, a
pressure gauge connected at the fuel rail should indicate 52 psi
(359 kPa). If pressure drops rapidly, the regulator may be unseated.
Open Fuel Level Sensor Circuit (C5, XLR) -- If an open
occurs in a fuel level sensor circuit, the fuel gauge will drop to
empty and a DTC will set. This could be caused by an unseated
electrical terminal. Check the wiring circuits before replacing fuel
system components. Another cause, which has been corrected, was the
sensor wiper fingers not in contact with the resistor card, over the
full sweep of the sensor.
TIP: The DTC specifies which
sensor circuit has the issue. Only that sensor should be replaced.
LH Module Opening Too Small (C5, XLR) -- In some LH tanks,
the module opening was undersized, making it difficult or impossible
to remove the module. This has been corrected in production.
TIP: In any case, be careful of
the sensor float wire when removing the module, to avoid damage.
Service Procedure Notes TIP:
Refer to IDL course 10260.22D, Technology Close-Up from October
2002, for additional information.
TIP: Always consult SI before
performing a service procedure. The following are highlights and
tips only.
Removing Fuel Tanks -- The
crossover hose must be disconnected from a fuel tank which is being
removed. The crossover is located above the driveline and exhaust
system, making removal appear difficult. Both SI and the Labor Time
Guide allow lowering the driveline and exhaust for access. See
Driveline Support Assembly Replacement in SI. Once you have
performed this procedure, you will gain enough knowledge of the
components that you may be able to do the procedure in the future
without lowering the driveline.
Crossover Hose --
The crossover hose is made of corrugated flexible stainless steel.
It is retained to each fuel tank by a collar and a CPA (Connector
Position Assurance). With the CPA aside, the collar can be turned by
hand. It may be necessary to wiggle the crossover while pulling it
straight out -- DO NOT TWIST.
The fuel feed
and fuel return pipes for the transfer pump are inside the
crossover, and are sealed with O-rings. The crossover is sealed to
each tank with two O-rings. When installing the crossover, lube the
O-rings and O-ring sealing surfaces with 1051717 rubber lubricant.
Then align the pipes and push the crossover into place. DO
NOT FORCE.
TIP:There is a T-shaped alignment
feature between the feed pipes
which can be assembled only one way.
With the crossover in
place, turn the collar. If the crossover and pipes are properly
aligned and assembled, the collar can be turned with two fingers.
Then snap the CPA into place.
Fuel Tank Module
Replacement -- It is necessary to remove the fuel tank from
the vehicle before removing the module. Procedures are different for
LH and RH tanks.
TIP: Fuel Sensor Lock Ring Tool
J-39765A is required.
When the lock ring is loosened, the
module will spring upward, because it is spring-loaded to ensure it
is bottom-referenced and to resist noise.
Follow the SI
procedure exactly. It is necessary to disconnect and reconnect
numerous fuel lines. The only access is through the module and
crossover openings.
TIP: When the module is
installed, be sure to check the empty and full readings of the fuel
level sensor. Your DMM should read 40 ohms with the tank in vehicle
orientation (simulating empty) and 250 ohms with the tank upside
down (simulating full).
Fuel Line Quick Connectors
Fuel lines use quick connectors, described in the February 2004
TechLink. See page 6 for details.
TIP:
To release, push on the retainer using hand pressure only. Do not
attempt to remove it.
If the retainer becomes broken, it can
be replaced using the following part number.
5/16-inch (0.3125) |
3/8- inch (0.375) |
5/8-inch (0.625) |
External vapor and internal liquid lines |
Internal liquid lines |
FLVV connector to the evap canister |
214992748 |
22717568 |
21992746 |
Bulletin
02-06-04-010A -- This bulletin applies only to vehicles
with the previous fuel system, which could be affected by fuel with
an aggressive sulfur content. It does not apply to any XLR or
2003-04 Corvette with the FFS fuel system. DO NOT use this bulletin
to justify reprogramming the PCM or replacing fuel sensors/modules
in vehicles with the FFS fuel system.
TIP: In the new system, DO NOT
reprogram the PCM unless specifically told to do so by a diagnostic
procedure.
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