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Tim's Auto Repair
Tim's Auto Repair, Mechanic
Category: Car
Satisfied Customers: 12912
Experience:  Have owned a repair shop for 25 yrs.
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silverado 1500 chevy: 91 chevy silverado 5.7 Truck stopped

Resolved Question:

91 chevy silverado 5.7 Truck stopped running while driving, intermittent fuel pump prime. No power at fuse. Starts with fuel added to throttle body. Truck starts once and a while and runs with idle problems. If the truck stalls out, No pump prime again.

Replaced fuel filter, ECM, Pump Relay Have not replaced oil sending yet.
Submitted: 12 months ago.
Category: Car
Expert:  Tim's Auto Repair replied 12 months ago.

Tim's Auto Repair :

hi

Tim's Auto Repair :

please explain

Tim's Auto Repair :

does the fuelpump always come on for 2 seconds when you first turn the key on?

Customer:

No, when it does the vehicle starts. stalls then no pump prime. I cannot figure out what is resetting to alow it to prime

Tim's Auto Repair :

ok

Tim's Auto Repair :

when it does prime

Tim's Auto Repair :

will it continue to run?

Tim's Auto Repair :

or just start and die?

Customer:

yes, you need to keep it running with the peddle or it will stall, and then no prime or pump

Tim's Auto Repair :

ok

Tim's Auto Repair :

from the way you explain

Tim's Auto Repair :

it sounds as if you may have a bad fuel pump

Tim's Auto Repair :

the computer only gives power to the pump for 2 seconds

Tim's Auto Repair :

unless the engine is turning

Tim's Auto Repair :

it sounds as if your pumpmauy be sticking

Tim's Auto Repair :

may be

Customer:

I dont know why it would run if the pump was bad. I had it running last night for about 20 min before it stalled. I hate to pull tanks

Tim's Auto Repair :

yes

Tim's Auto Repair :

these can just get weak and not have enough pressure

Tim's Auto Repair :

or

Tim's Auto Repair :

can just stick and not run at all

Tim's Auto Repair :

it sounds as if yours is bad

Customer:

why would a weak pump keep the computer from the prime

Tim's Auto Repair :

tell me

Tim's Auto Repair :

does the relay click when you first turn the key on?

Customer:

no and no power at the fuse. when I put 12V direct to the fuse nothing happens

Tim's Auto Repair :

does it not have power on the fuse for 2 seconds?

Customer:

nope

Tim's Auto Repair :

for the first 2 seconds when you first turn the key on

Tim's Auto Repair :

ok

Tim's Auto Repair :

i see you have already replaced the computer

Tim's Auto Repair :

was it a new one?

Customer:

I changed the ECM in the distributor

Tim's Auto Repair :

ooooooo

Tim's Auto Repair :

i see

Tim's Auto Repair :

the ignition module

Tim's Auto Repair :

if the relay isnt clicking on for the first two seconds

Tim's Auto Repair :

the problem is the ecm[computer],,behind the glove box

Tim's Auto Repair :

though

Tim's Auto Repair :

the ecm isnt sending a signal to the relay

Customer:

will that act intermitent

Tim's Auto Repair :

yes

Tim's Auto Repair :

a bad computer can do anything

Tim's Auto Repair :

i do want to give yoiu a wire diagram

Tim's Auto Repair :

so you can check the wiring if you must

Tim's Auto Repair :

will that help

Tim's Auto Repair :

?

Customer:

well yes, I will change the comp tonight. what a pain in the ass

Tim's Auto Repair :

ok

Tim's Auto Repair :

stand by

Tim's Auto Repair :

these older computers arent too high

Tim's Auto Repair :

they cost around $90+/-

Tim's Auto Repair :

but do check your wiring first

Customer:

can I ask more questions tomorrow? if this does not work or changes the symtoms

Tim's Auto Repair :

yes

Tim's Auto Repair :

just sign in and i will answer when im on line

Tim's Auto Repair :

just reply back to this question

Tim's Auto Repair :

are you ready for some diagrams?

Customer:

yes

Tim's Auto Repair :

ok

Tim's Auto Repair :

stand by

Tim's Auto Repair :

 
















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Wiring Diagram For Chart A-5 Fuel Pump Relay Circuit Diagnosis










88_gmt91cec604-029.png
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Wiring Diagram For Chart A-5 Fuel Pump Relay Circuit Diagnosis (Two Fuel Tanks)



Tim's Auto Repair :

this is the relay..
















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Wiring Diagram For Chart A-5 Fuel Pump Relay Circuit Diagnosi



Tim's Auto Repair :

 
















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Fuel Delivery System




The fuel metering system consists of the following components:
1. Fuel pump electrical circuit.
2. Fuel supply components, including:
a. Fuel lines and pipes.
b. Fuel pump.
c. Fuel tank.
3. Throttle Body Injection Assembly, including:
a. Fuel injectors.
b. Fuel pressure regulator.
c. Idle air control valve.
d. Throttle position sensor.


BASIC SYSTEM OPERATION:


The fuel supply system begins with gasoline in the fuel tank. An electric fuel pump, located in the fuel tank with the fuel level gauge sending unit, pumps fuel to the throttle body assembly through an in-line filter. The pump is designed to supply fuel at a pressure above the pressure needed by the injectors. A pressure regulator located in the throttle body assembly keeps the fuel available to the injectors at a constant pressure. The unused fuel is returned to the fuel tank by a separate line.

MODES OF OPERATION:

The ECM uses voltage inputs from several sensors to determine how much fuel to give the engine. The fuel is delivered under several conditions, called "Modes." All modes are controlled by the ECM, and are described below.

Starting Mode:

When the ignition is first turned ON, the Electronic Control Module (ECM) turns ON the fuel pump relay for two seconds, allowing the fuel pump to build up system pressure. On Heavy Duty models, a fuel module will override the ECM two second timer and the fuel pump will run for twenty seconds and then shut OFF if the vehicle is not started. This circuit corrects a hot restart (vapor lock) during high ambient temperature conditions. If the engine is immediately started, the fuel pump will continue to operate. If the key is left in the ON position, but the engine is not started, the ECM will turn the pump OFF. The fuel pump will remain OFF until the ECM detects the crank signal, or in the case of a faulty fuel pump relay, the oil pressure switch detects approximately 4 psi of oil pressure which will then bypass the fuel pump relay.

Fuel delivery from the injectors (which deliver fuel in all operating modes) is controlled by changing the amount of time the injectors are turned on or "pulsed" by the ECM. The ECM checks the Coolant Temperature Sensor (CTS), Throttle Position Sensor (TPS), and the crank signal, and determines the proper air/fuel ratio for starting. This ranges from 1.5:1 at -36°C (-33° F) to 14.7:1 at 94° C (201° F).

Clear Flood Mode:

If the engine floods, it may be cleared by pushing the accelerator pedal down all the way. The ECM then pulses the injector at a 20:1 air fuel ratio. The ECM holds this injector rate as long as the throttle stays wide open, and the engine speed is below approximately 600 rpm. If the throttle position becomes less than approximately 80%, the ECM returns to the starting mode.

Run Mode:

The run mode has two conditions, called OPEN LOOP and CLOSED LOOP.

Open Loop:

When the engine is first started, and engine speed is above 400 rpm, the system goes into OPEN LOOP operation. In OPEN LOOP, the ECM ignores the signal from the Oxygen sensor, and calculates the air/fuel ratio based on inputs from the coolant temperature and Manifold Absolute Pressure (MAP) sensors.

The system will stay in OPEN LOOP until the following conditions are met:
1. The oxygen sensor has a varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature)
2. The coolant temperature is above a specified temperature.
3. A specified amount of time has elapsed after starting the engine.

Closed Loop:

The specified operating conditions vary with different engines, and are stored in the programmable read only memory (PROM). When these conditions are met, the system goes into CLOSED LOOP operation. In CLOSED LOOP, the ECM calculates the air/fuel ratio (injector on-time) based on the signal from the oxygen sensor. This allows the air/fuel ratio to stay very close to 14.7:1.

Acceleration Mode:

The ECM responds to rapid changes in throttle position and manifold pressure or air flow, and provides extra fuel.

Deceleration Mode:

The ECM responds to changes in throttle position and manifold pressure and reduces the amount of fuel. When deceleration is very fast, the ECM can cut off fuel completely for short periods.

Battery Correction Mode:

When battery voltage is low, the ECM can compensate for the weak spark by:

1. Increasing the amount of fuel delivered.
2. Increasing the idle speed.
3. Increasing the ignition dwell time.

Fuel Cut-off Mode:

No fuel is delivered by the injector when the ignition is OFF. This prevents dieseling. Also, fuel is not delivered if no reference pulses are seen from the distributor, which means the engine is not running. This prevents flooding. Fuel cut-off also occurs at high engine rpm, to protect internal engine components from damage

Tim's Auto Repair :

 
















53_88TBITRKFUEL-004.png
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Fuel Pump And Sender Assembly (Typical)





The fuel pump is a turbine type, low pressure electric pump, mounted in the fuel tank. Fuel is pumped at a positive pressure (above 62 kPa or 9 psi) from the pump through an in-line filter to the pressure regulator in the TBI unit. Excess fuel is returned to the fuel tank through the fuel return line.

Tim's Auto Repair :

NOTE: A fuel system pressure test is part of several of the Diagnostic Charts and Symptom Checks. To perform this test, follow this procedure:

FUEL PRESSURE TEST
1. Turn engine OFF to relieve system pressure.
- Disconnect negative battery terminal to avoid possible fuel discharge if an accidental attempt is made to start the engine.
- Loosen fuel filler cap to relieve tank vapor pressure.
- The constant bleed feature relieves fuel pump system pressure when the engine is turned "OFF".
2. Uncouple fuel supply flexible hose in engine compartment. Install fuel pressure gauge between steel line and flexible hose.
3. Tighten gauge in line to insure no leaks occur during testing.
4. Connect negative battery terminal and START engine and observe fuel pressure reading. It should be 62-90 kPa (9-13 psi). If not, refer to DIAGNOSTIC CHARTS/FUEL SYSTEM PRESSURE TEST (CHART A-6).
5. Turn engine OFF to relieve system pressure.
- Disconnect negative battery terminal to avoid possible fuel discharge if an accidental attempt is made to start the engine.
- Loosen fuel filler cap to relieve tank vapor pressure.
- The constant bleed feature relieves fuel pump system pressure when the engine is turned "OFF".
6. Remove fuel pressure gauge.
7. Reinstall fuel line.
8. Connect negative battery terminal and START engine and check for fuel leaks.
9. Install air cleaner assembly.

Customer:

where is and how do I use the fuel pump check wire listed on the diagram

Tim's Auto Repair :

you just need to check the gray wire to the pump

Tim's Auto Repair :

 
















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Chart A-6 - Fuel System Pressure Test










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Fuel Pressure Gage Test Point For Chart A-6




CHART A-6 - FUEL SYSTEM PRESSURE TEST



CIRCUIT DESCRIPTION:

When the fuel pump is running, fuel is delivered to the injector(s) and then to the regulator where the system pressure is controlled to about 62 to 90 kPa (9 to 13 psi). Excess fuel is then returned to the fuel tank.



TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. Pressure but less than 62 kPa (9 psi) falls into two areas:

^ Regulated pressure but less than 62 kPa (9 psi) - Amount of fuel to injector OK but pressure is too low. System will be lean running and may set Code 44. Also, hard starting cold and poor overall performance.
^ Restricted flow causing pressure drop - Normally, a vehicle with a fuel pressure of less than 62 kPa (9 psi) at idle will not be driveable. However, if the pressure drop occurs only while driving, the engine will normally surge then stop as pressure begins to drop rapidly.

2. Restricting the fuel return line allows the fuel pump to develop its maximum pressure (dead head pressure). When battery voltage is applied to the pump test connector, pressure should be from 90 to 124 kPa (13 to 18 psi).


3. This test determines if the high fuel pressure is due to a restricted fuel return line or a throttle body pressure regulator problem.

DIAGNOSTIC AIDS:

^ If the vehicle is equipped with a fuel module, the module must be disconnected before performing the fuel system pressure test.
^ Fuel system is under pressure.
^ On V6 or V8 engine, the fuel pressure drops to almost zero psi after pump shuts "off".

Tim's Auto Repair :

the dlc is located under the dash on the drivers side

Customer:

OK I will rap my head around this and get back to you. Thanks alot

Tim's Auto Repair :










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Tim's Auto Repair :

you are very welcome

Tim's Auto Repair :

again..i think you need a computer

Tim's Auto Repair :




Customer:

will get one tonight.

Tim's Auto Repair :

ok

Tim's Auto Repair :





\

Tim's Auto Repair, Mechanic
Category: Car
Satisfied Customers: 12912
Experience: Have owned a repair shop for 25 yrs.
Tim's Auto Repair and 3 other Car Specialists are ready to help you

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