1993 DODGE TRUCK Service Repair Manual

14-86
FUEL
SYSTEM

WARNING:
USE
EXTREME CAUTION WHEN
IN­

SPECTING
FOR
HIGH
PRESSURE
FUEL
LEAKS.
IN­

SPECT
FOR
HIGH
PRESSURE
FUEL
LEAKS
WITH
A

SHEET
OF
CARDBOARD. HIGH FUEL INJECTION
PRESSURE
CAN
CAUSE PERSONAL INJURY
IF

CONTACT
IS
MADE
WITH
THE
SKIN.

MECHANICAL
LIFT
PUMP
The mechanical lift pump
is
located
on the
left side
of

the engine cylinder block.
The
lift pump
is
driven
by an
eccentric
on the
camshaft that actuates
a
spring loaded lever
in the
pump
(Fig. 25). The
lever
is
connected
to

the pump diaphragm. When
the
eccentric
on the
cam­ shaft pushes
the
lever upward,
the
lever
in
turn pulls
the diaphragm
to its
extended position. With
the
lever in
the
extended position, fuel
is
drawn into
the
pump
chamber through
the
inlet check valve. When
the
pump
gathers more fuel
in the
fuel chamber than
the
engine
can consume,
the
outlet check valve
is
held closed. Fuel
gathered
in
lift pump chamber prevents
the
diaphragm
from relaxing.
The
lever will continue
to
move
up and

down
on the
shaft connecting
it to the
diaphragm. How­ ever,
it is
prevented from operating
the
diaphragm until
the check valve again opens
and the
fuel
in the
pump chamber
is
expelled. This process continues
as
long
as

the engine
is
running. Minimum lift pump output pressure
is 21 Kpa (3

psi).
Lift pump output volume
is 0.75
liters
(0.70 U.S.
quarts)
per 30
seconds
of
engine cranking.

Fig.
25 Lift
Pump

The lift pump housing
has a
weep hole
at the
bot­
tom
of the
casing
(Fig. 25). If the
diaphragm rup­

tures,
fuel will exit
out of the
weep hole
and the

pump must
be
replaced.
The
lift pump
is not
service­ able
and
must
be
replaced
as a
complete assembly.
CAUTION:
Do not
operate
the
engine
if the
fuel
sup­

ply
line
has a
restriction
of
more than
95 MM Hg

(3.75
in Hg).
The lift pump
has a
manual lever
(Fig. 26). The le­

ver
is
located
on the
lower right side
of the
pump
housing.
The
purpose
of the
lever
is to
prime
the
fuel system
and
bleed
air
from
the
system
if the
vehicle
has
run out of
fuel. Raise
and
lower
the
lever until
resistance
is
felt
to
prime
the
system
up to the
injec­
tor pump. During vehicle operation
the
pump
le­

ver must
be in the up
position.
FUEL
PUMP
MANUAL
JKT /
LEVER
——-—•
.
LJ*y^"^^

J9114-3

Fig.
26 Lift
Pump
Manual
Lever

SYSTEM
OPERATION
IGNITION
SWITCH
ON
When
the
ignition switch
is put in the ON (run)

position,
the
following occurs: •
The PCM
receives
an
ignition sense input.
It
then
powers
up.
Battery voltage
is
supplied through
the

battery voltage input. •
The
wait-to-start lamp
and the
water-in-fuel lamp are turned
on by the PCM for 2.8
seconds
as a
bulb test.
•
The PCM
monitors
the
charge
air
temperature
sensor input.
If
manifold
air
temperature
is at or be­

low
15°C
(59°F),
the PCM
will start
the air
intake
heater preheat cycle.
It
will energize
the
intake heat­
ers
by
grounding
the air
intake heater relays.
The

PCM will turn
the
wait-to-start light
on
until
the

preheat cycle
is
over.
If PCM
receives
a
crank signal
from engine speed sensor before preheat cycle
is
over, it will abort preheat cycle.
•
The PCM
monitors engine speed. When
the
engine speed
is
above
384 rpm, the PCM
determines that
the engine
is
running.
•
The PCM
will turn
on the
water-in-fuel lamp
if a
signal
is
received from
the
water-in-fuel sensor.

ENGINE
START-UP
When
the
ignition
key is put in the
start position,
the following occurs: •
The PCM
receives
a
crank signal input from
the

engine speed sensor.
•
The air
intake heaters
are not
energized during cranking.

•

FUEL
SYSTEM
14 - 87
ENGINE
WARM-UP

• The PCM may start the air intake heater post-heat
cycle depending on intake air temperature. If manifold air temperature was at or below 15°C (59°F) when the
ignition switch was turned on, the cycle is started.
• If intake manifold air temperature is below 32°C
(90°F),
the KSB solenoid will be energized through
the air temperature switch. The solenoid remains en­ ergized until the air temperature switch opens. The
switch opens when intake manifold air temperature
is above 32°C (90°F).
• If the coolant temperature is below 16°C (60° F),
the transmission will not be allowed to enter over­ drive (automatic transmission).
• The PCM will turn on the water-in-fuel lamp if a signal is received from the water-in-fuel sensor.

CRUISE
OR
IDLE

• The PCM monitors intake manifold air tempera­
ture through the charge temperature sensor input. • If intake manifold air temperature is below 32°C
(90°F),
the KSB solenoid will be energized through
the air temperature switch. The solenoid remains en­ ergized until the air temperature switch opens. The switch opens when intake manifold air temperature
is above 32°C (90°F).
• The air intake heater post-heat cycle will be com­
pleted, if it is not already over.
• The vehicle speed sensor, engine speed sensor and
throttle position sensor inputs are used to control
transmission overdrive operation.
• If the coolant temperature is below 16° C (60° F),
the transmission will not be allowed to enter over­ drive (automatic transmission).
• If the transmission thermo-switch is open, the
transmission will not be allowed to enter into over­
drive (automatic transmission). If the switch opens
when the vehicle is in overdrive, the transmission
will be downshifted. The transmission thermo-switch opens at 134°C (273°F).
• The thermo-switch will close once the transmission
fluid temperature drops to 116°C (240°F). • The PCM will turn on the water-in-fuel lamp if a signal is received from the water-in-fuel sensor.

ACCELERATION
• The vehicle speed sensor, engine speed sensor and
throttle position sensor inputs are used to control
transmission overdrive operation.
• If the coolant temperature is below 16° C (60° F),
the transmission will not be allowed to enter over­
drive (automatic transmission).
• If intake manifold air temperature is below 32°C
(90°F),
the KSB solenoid will be energized through
the air temperature switch. The solenoid remains en­ ergized until the air temperature switch opens. The
switch opens when intake manifold air temperature
is above 32°C (90°F). • If the transmission thermo-switch is open, the
transmission will not be allowed to enter into over­
drive (automatic transmission). If the switch opens
when the vehicle is in overdrive, the transmission
will be downshifted. The transmission thermo-switch opens at 134°C (273°F). The thermo-switch will close
once the transmission fluid temperature drops to 116°C (240°F).
• The PCM will turn on the water-in-fuel lamp if a
signal is received from the water-in-fuel sensor.
• If the speed control system resume/accelerate func­
tion is being used, the PCM will only allow the vehi­
cle to accelerate at a predetermined rate. If a speed
control has been set and the resume/accelerate but­
ton is momentarily pushed in, the PCM will increase
vehicle speed by two miles per hour.
• If the brakes are applied, the PCM will disable the
speed control.

DECELERATION

• The vehicle speed sensor, engine speed sensor and
throttle position sensor inputs are used to control
transmission overdrive operation.
• If the coolant temperature is below 16° C (60° F),
the transmission will not be allowed to enter over­ drive (automatic transmission).
• If the transmission thermo-switch is open, the trans­
mission will not be allowed to enter into overdrive (au­
tomatic transmission). If the switch opens when the
vehicle is in overdrive, the transmission will be down­ shifted. The transmission thermo-switch opens at 134°C (273°F). The thermo-switch will close once the transmis­
sion fluid temperature drops to 116°C (240°F).
• The PCM will turn on the water-in-fuel lamp if a
signal is received from the water-in-fuel sensor.
• If intake manifold air temperature is below 32°C
(90°F),
the KSB solenoid will be energized through
the air temperature switch. The solenoid remains en­ ergized until the air temperature switch opens. The
switch opens when intake manifold air temperature
is above 32°C (90°F).
• If the speed control system coast/set function is be­
ing used, the PCM will only allow the vehicle to de­ celerate at a predetermined rate. If the coast/set
switch is pushed while the system is operating, the
PCM will set speed control to the rate the vehicle is
traveling at when the switch is released. • If the brakes are applied, the PCM will disable the speed control.

IGNITION SWITCH
OFF
• When the ignition switch is turned to the off posi­
tion, the PCM still receives battery voltage through
the battery input. Battery voltage is needed to keep
PCM memory alive. The PCM memory stores diag­ nostic trouble code (DTC) messages and the mini­
mum TPS value from the previous key-on.

DIESEL FUEL INJECTION—GENERAL DIAGNOSIS

INDEX

page

Air
in
Fuel System
90

Air Intake Heater
. 90
Diagnostic
Trouble Code (DTC)
. 95

DRB
II
Scan
Tool
95

Engine
Speed
Sensor
91

Fuel Heater
91

Fuel
Injection
Pump
91

Fuel Injectors
92

Fuel Supply Restrictions
92
page

Fuel/Water Separator
Filter 92

High
Pressure Fuel Line Restrictions
93

KSB
Solenoid
. 93

Mechanical
Lift
Pump
93

On-Board
Diagnostics (OBD)
94
System
Schematics—5.9L Diesel Engine
....... 95

Throttle
Position
Sensor
Test
94
Visual
Inspection
88

VISUAL
INSPECTION
A visual inspection for loose, disconnected, or incor­
rectly routed wires and hoses should be made before attempting to diagnose or service the fuel injection
system. A visual check will help find these condi­

tions.
It also saves unnecessary test and diagnostic

time.
A thorough visual inspection of the fuel injec­
tion system includes the following checks: (1) Be sure that the battery connections are tight
and not corroded (Fig. 1).
• POSITIVE

v^3t
^TERMINAL J91 14
QUICK
DISCONNECT
PCM

MOUNTING
BOLTS

PCM
CONNECTOR CONNECTOR
MOUNTING

BOLT
J9314-165
Fig.
2 Powertrain Control
Module
(PCM)
HEATER

RELAYS
Fig.
1
Battery
Connections
(2)
Be sure that the 60-way connector is fully en­
gaged with the PCM. Verify that the connector
mounting screw is tight (Fig. 2).

(3)
Be sure that the electrical connections at the
air intake heater relays (Fig. 3) are tight and not
corroded.

(4)
Inspect the starter motor and starter solenoid
connections for tightness and corrosion (Fig. 4).
(5)
Verify that the electrical connectors are con­
nected to the charge air temperature sensor and air
temperature switch. Inspect the connectors for corro­ sion or damaged wires (Fig. 5). (6) Verify that the electrical connector is connected
to the air temperature switch (Fig. 5).
WHEEL
^

WELL
J9114-67
Fig.
3 Air Intake Heater
Relays

FUEL
SYSTEM
14-89

Fig.
4 Starter Motor Connections—Typical
Fig.
5
Charge
Air
Temperature
Sensor
and Air
Temperature
Switch

(7) Verify that the Water-in-Fuel (WIF) sensor
electrical connector is connected to the sensor (Fig.
6).
Inspect the connector for corrosion or damaged

wires.
Fig.
6 Water-in-Fuel (WIF)
Sensor
(8) Place a drain pan under the drain tube (Fig. 6).
With the engine not operating, hold the drain open
until all water and contaminants have been removed and clean fuel exits the drain. Dispose of mixture in
drain pan according to applicable regulations.
(9) Verify that the electrical connector is connected
to the fuel solenoid on the injection pump (Fig. 7). In­ spect the connector for corrosion or damage.
(10) Verify that the electrical connector is con­
nected to the KSB solenoid on the injection pump (Fig. 7). Inspect the connector for corrosion or dam­

age.
Fig.
8 Air Intake Heater
(12) Inspect the throttle linkage and accelerator
linkage for binding (Fig. 9).
(13) Be sure that the throttle return spring is con­
nected. (14) Inspect all fuel lines for signs of leakage.
(15) Be sure that the ground cable connection at
the front of the cylinder head is tight and free of cor­ rosion.

14
- 90
FUEL SYSTEM
Fig.
10 Air
Filter
Element—Diesel (17) Be sure that the turbocharger output hose is
connected to the charge air cooler (intercooler) inlet

tube.
Verify that the charge air cooler output hose is
connected to the cooler and the intake manifold (Fig.
11).
(18) Be sure that the vacuum pump-to-brake
booster hose is connected and not damaged. (19) Be sure that the accessory drive belt is not
damaged or slipping.

AIR
IN
FUEL SYSTEM
Air will enter the fuel system whenever fuel supply

lines,
separator filters, injection pump, high pressure
lines or injectors are removed or disconnected. Air
trapped in the fuel system can result in hard start­
ing, a rough running engine, engine misfire, low
power, excessive smoke and fuel knock. After service is performed, air must be bled from the system before
starting the engine. •

Fig.
11
Charge
Air
Cooler
(Intercooler) Inspect the fuel system from the lift pump to the
injectors for loose connections. Leaking fuel is an in­
dicator of loose connections or defective seals. Air can also enter the fuel system between the fuel tank and
the lift pump. Inspect the fuel tank and fuel lines for damage that might allow air into the system.

AIR INTAKE HEATER PREHEAT/POST-HEAT CYCLE
The PCM provides a ground path for the air intake
heater relays. The ground path is provided if intake
manifold temperature is below 16°C (60°F) when the
ignition key is in the ON position. When the ground is provided, the intake heaters are energized to start
the preheat cycle.
The preheat cycle can be tested with a voltmeter or
test light. If the intake manifold temperature is above 16°C (60°F), the wait-to-start light will not
come on and the air intake heaters will not be ener­ gized.

CAUTION:
Do not
energize
the air
intake
heater
re­
lays
more than once
per 15
minutes.
If the
relays
are cycled,
the key
turned
off and
then
turned
back
on,
the
engine could
be
damaged. Wait
15
minutes before turning
the key
back
to the ON
position.

(1) With the engine not running and ambient air
temperature below 16°C (60°F), turn the ignition key
to the ON position.
(2) The wait-to-start light will come on and the air
intake heater relays should click on signaling the
start of the preheat cycle.

(3) Check for battery voltage at both intake air
heater terminals. The heaters will only be ener­ gized for 10 to 20 seconds. Refer to the Preheat Cycle Time Interval Chart.

PREHEAT
CYCLE
TIME
INTERVAL
SENSOR
MOUNTING
BOLTS

Intake

Manifold

temperature
Preheat
Cycle
Time

Ignition
Key ON

Engine
Net
Running
Pestheat
Cycle

Ignition
Key ON Ingin©
Running

Above 15 °C (59
°F)
0 Seconds
No
-8°C{18°F)*o 15°C(59
°F)
10 Seconds
Yes
-17°C{1 °F) •9°C(16°F) 15 Seconds

Yes

-26°C(-15°F) fo-18°C (0
°F)
17.5 Seconds
Yes
Below
-26
°C
(15T) 20 Seconds

Yes

J9114-239
AIR INTAKE HEATER TEST (1) Disconnect negative battery cable.
(2) Note wire positions on heater terminals for as­
sembly. Disconnect wires from heater.
(3) Use an ohmmeter to test resistance from each
heater terminal to ground. The resistance should be
zero (0). If the resistance is not to specifications, in­ spect for corroded or dirty connections. Clean or re­
pair the connections and retest before replacing
heaters.

ENGINE
SPEED
SENSOR
The engine speed sensor (sometimes referred to as
the crankshaft position sensor) is located on the front
of engine (Fig. 12). To perform a complete test of the engine speed sen­
sor and its circuitry, refer to the DRB II scan tool.
Also refer to the appropriate Powertrain Diagnostics
Procedures manual.

FUEL
HEATER
A malfunctioning fuel heater can cause a wax
build-up in the fuel/water separator (Fig. 13). Wax
build-up in the fuel/water separator can cause engine starting problems and prevent the engine from rev­
ving up. It can also cause miss fire under load and
blue or white fog like exhaust. Voltage to operate the fuel heater is supplied
through the ignition switch. The heater operating
range is from 6° - 13.5°C (43° - 53°). A minimum of 7
VIBRATION

DAMPER
J9214-69

Fig.
12
Engine
Speed
Sensor
volts is required to operate the fuel heater. The re­
sistance value of the heater unit is 2.5 - 3.0 ohms.
««.
* rr ,,

J9114-84

Fig.
13
Fuel
Heater

FUEL
INJECTION
PUMP
The fuel solenoid is the only serviceable component
of the fuel injection pump. The injection pump is
not to be serviced or the warranty may be
voided. If the injection pump requires service,
the complete assembly must be replaced. Incorrect injection pump timing can cause poor per­
formance, excessive smoke and emissions and poor
fuel economy. Refer to the 5.9L Diesel Service Proce­
dures section of this manual for injection pump tim­
ing.

14-92
FUEL
SYSTEM

• A broken injection pump timing mechanism spring
will cause the timing to be fully advanced resulting
in torque loss, a fuel knock and possible engine over­
heating.
An improperly operating KSB (cold start) solenoid
will cause white smoke during engine warm-up. The
KSB solenoid is not serviceable.
A defective or non-adjustable fuel injection pump
can cause starting problems or prevent the engine
from revving up. It can also cause:
• Engine surge at idle • Rough idle (warm engine)
• Engine miss under load
• Low power
• Excessive fuel consumption
• Poor performance
• Low power
• Black smoke from the exhaust • Blue or white fog like exhaust
• Incorrect idle or maximum speed A worn fuel injection pump plunger can effect fuel
pressure and the amount of fuel injected. This results in reduced engine power. In most cases, if the injec­
tion pump is delivering fuel from one outlet, it will deliver fuel from all outlets. If the internal plunger is defective, the fuel injection pump must be re­
placed.
Engine power is also effected by the governor set­
ting and performance. Do not attempt to adjust the governor. If the governor seals on the external adjustment screw are broken, the fuel rate may
be out of adjustment. The warranty of the injec­ tion pump and the engine may be void if the
seals have been tampered with or removed.

FUEL
INJECTORS
A leaking fuel injector can cause fuel knock, poor
performance, black smoke, poor fuel economy and
rough engine idle. If the fuel injector needle valve
does not operate properly, the engine may misfire and produce low power. A leak in the injection pump-to-injector high pres­
sure fuel line can cause many of the same symptoms as a malfunctioning injector. Inspect for a leak in the
high pressure lines before checking for a malfunc­
tioning fuel injector.

WARNING:
THE
INJECTION PUMP SUPPLIES HIGH
PRESSURE
FUEL
OF
APPROXIMATELY
59,000
KPA

(8,000
PSI) TO
EACH INDIVIDUAL INJECTOR
THROUGH THE HIGH
PRESSURE
LINES. FUEL UN­
DER
THIS AMOUNT
OF
PRESSURE
CAN
PENE­
TRATE THE SKIN AND CAUSE PERSONAL INJURY,
WEAR
SAFETY GOGGLES
AND
ADEQUATE PRO­
TECTIVE CLOTHING. AVOID CONTACT
WITH
FUEL
SPRAY
WHEN BLEEDING HIGH
PRESSURE
FUEL
LINES.
WARNING:
DO NOT
BLEED
AIR
FROM
THE
FUEL

SYSTEM
OF A
HOT ENGINE.
DO
NOT ALLOW FUEL
TO SPRAY ONTO THE EXHAUST MANIFOLD WHEN
BLEEDING
AIR
FROM THE FUEL SYSTEM.

To determine which fuel injector is malfunctioning,
run the engine and loosen the high pressure fuel line nut at the injector. Listen for a change in engine
speed (Fig. 14). Tighten the line nut. If engine speed

drops,
the injector was operating normally. If engine
speed remains the same, the injector is malfunction­
ing. Test all injectors in the same manner one at a

time.

Fig. 14
Inspecting
Injector Operation
Once an injector has been found to be malfunction­
ing, remove it from the engine and replace it. Refer
to Diesel Engine Service Procedures for injector re­ moval and installation.

FUEL
SUPPLY
RESTRICTIONS

CAUTION:
Do not
operate
the
engine
if the
fuel
sup­ ply
line
has
a
restriction
of
more
than
12.7
kPa
(3.75
in
Hg).
Refer
to
Lift
Pump
Test.

Fuel supply line restrictions can cause starting
problems and prevent the engine from revving up.
The starting problems include; engine miss under load, low power and blue or white fog like exhaust.
Test all fuel supply lines for restrictions or blockage.
Flush or replace as necessary. Bleed the fuel system
of air once a fuel supply line has been replaced.

FUEL/WATER
SEPARATOR
FILTER
A blocked or clogged fuel/water separator filter can
cause starting problems and prevent the engine from
revving up. It can also cause engine miss under load,
low power and blue or white fog like exhaust. The maximum allowable fuel pressure drop across
the fuel/water filter separator is 21 kPa (3 psi).

•

FUEL SYSTEM
14-93 There is a drain at the bottom of the separator fil­
ter (Fig. 13). Place a drain pan under the drain tube. With the engine not running, hold the drain open
until all water and contaminants have been removed and clean fuel exits the drain. Dispose of mixture in
drain pan according to applicable regulations.
If excess water gathers in separator/filter in a short

time,
the fuel tank must be removed, drained and cleaned.

HIGH
PRESSURE FUEL LINE RESTRICTIONS
High pressure fuel line leaks can cause starting
problems, poor engine performance and engine miss under load. Restricted high pressure lines can cause starting problems, poor engine performance, engine
miss under load and black smoke from exhaust.

WARNING:
USE
EXTREME
CAUTION
WHEN
IN­
SPECTING
FOR HIGH
PRESSURE
FUEL
LEAKS.
IN­

SPECT
FOR HIGH
PRESSURE
FUEL
LEAKS
WITH
A

SHEET
OF
CARDBOARD.
HIGH
FUEL
INJECTION
PRESSURE
CAN
CAUSE
PERSONAL
INJURY
IF

CONTACT
IS
MADE
WITH
THE
SKIN.

Start the engine. Move the cardboard over the high
pressure fuel lines and check for fuel spray onto the cardboard. If a high pressure line connection is leak­
ing, bleed the system and tighten the connection. Re­
fer to the 5.9L Diesel Service Procedures section for air bleeding procedures. Replace damaged, restricted
or leaking high pressure fuel lines with the correct replacement line.

CAUTION:
The
high pressure
fuel
lines must
be

clamped securely
in
place
in the
holders.
The
lines cannot contact each other
or
other components.
Do
not
attempt
to
weld high pressure
fuel
lines
or to

repair lines
that
are
damaged. Only
use the
recom­
mended lines when replacement
of
high pressure

fuel
line
is
necessary.

KSB
SOLENOID
The KSB solenoid will click when it is energized
and de-energized. If the solenoid does not click when 12 volts is sup­
plied to it, replace the solenoid.

MECHANICAL
LIFT
PUMP
An improperly operating lift pump can cause low
engine power. Inspect the fuel supply line to the
pump for restrictions, kinks or leaks. Fuel leaking
from the weep hole (Fig. 15) in the pump casing in­
dicates a leaking pump diaphragm. The lift pump is not repairable and must be replaced as an assembly.
CAUTION:
Do not
operate
the
fuel
system
with
a

suction restriction
of
more than
95 MM Hg (3.75
inch Hg). Refer
to
Lift
Pump Test.
Fig.
15
Lift
Pump
Low lift pump output can be caused by a worn ec­
centric on the engine camshaft. A properly operating lift pump will pump a minimum volume of 0.75 liter (0.70 U.S. quart) of fuel in 30 seconds at engine
cranking speed.
The maximum allowable pressure drop across the
fuel filter is 21 Kpa (3 psi). As the filter removes contaminants in the fuel, the pressure drop across
the fuel separator filter will increase. Frequent re­
placement of the separator filter can indicate a worn lift pump. Refer to the Maintenance Schedule at the
front of this manual for recommended replacement intervals.

LIFT
PUMP
TEST
(1) Disconnect fuel inlet tube at the lift pump. (2) Install T-fitting between disconnected fitting
and lift pump inlet. Connect vacuum gauge to T-fit­
ting.

WARNING:
USE
EXTREME
CAUTION
WHEN
EN­
GINE
IS
OPERATING.
DO NOT
STAND
IN
DIRECT

LINE
WITH FAN.
DO NOT PUT
HANDS
NEAR
PUL­

LEYS,
BELTS
OR FAN. DO NOT
WEAR
LOOSE

CLOTHING.

The vacuum gauge will not indicate a vacuum un­
til the lift pump begins to operate at full capacity.
(3) Operate engine at approximately 1500 rpm for
30 seconds. Vacuum should not exceed 12.7 kPa (3.75
in. Hg).
(4) If vacuum exceeds 12.7 kPa (3.75 in. Hg) check
fuel tube for restriction. A partially clogged in-tank fuel filter can also cause excess vacuum.