Mechanics

CHEVY NO FUEL TO TBI

1993 Chevrolet Truck

Electrical problem
1993 Chevy Truck V8 Two Wheel Drive Manual 170000 miles

Truck suddenly would not start this morning. Originally had no fire past distributor. Replaced distributor cap and rotor but not working. Eventually turned distibutor and now have spark at plugs but no fuel. Eventually found no voltage from ECM going to fuel relay on green/wht wire. I applied 12 vdc and activated the relay. Measured 12 vdc on the output gray wire all the way to the fuel pump. Pulled off one of the fuel injector connectors and applied 12 vdc to the injector and it activated but still no fuel. What should the resistance of the fuel pump be? I do not hear it running. Need to know what other sensors if any can cause the ECM to not power this relay and am concerned that there is something else that the ECM is seeing to cause both the spark and fuel problems. Presently I can pour gasoline in the carburator and the engine wants to start so it's a fuel issue at this time. Any suggestions would be appreciated.
Forgot to add that there were no code faults registered.
Avatar
Bevo995
October 3, 2008.



Sounds very similar to the issues I am having with my truck.
Take a look at this post:

http://www.2carpros.com/forum/1992-chevy-truck-chevy-43-no-start-no-fuel-vt191543.html

I will be watching for resolution for your problem
Good luck

Tiny
Soonerbills
Oct 3, 2008.
Soonerbills - I will check the fuel pump today but even if it would have worked after I forced the fuel pump relay to activate, my thoughts are that the ECM would not have fired the injectors as it does not send voltage to the fuel relay coil winding and something seems to be telling the ECM to stay shut down like an emergency fuel cut-off switch. I can't find one in the chassis electrical diagram though. Wished I had a detailed schematic diagram of the ECM as it would help in figuring out what is broke.
Thanks for the response.

Tiny
Bevo995
Oct 4, 2008.
Is the service engine soon light on? Try grounding terminal "G" on the test connector, this should power the fuel pump, this is just to test it, do not drive like this! Read the codes while your at it!


http://www.2carpros.com/forum/automotive_pictures/62217_GM_DLC95_Older_4.jpg


Cross these two terminals as shown, then turn the key on. Count the flashes, a 22 will flash as 2 flashes then a short pause, then 2 flashes, if more than one code is stored, the pause will be longer. The codes will display three times and end with 12. Write them down and post here. This connector is right under the middle of the dash...

Merlin2021
Oct 5, 2008.
Merlin2021, I did not try grounding the G connector to see if the fuel pump ran. I had originally run the diagnostics and there were no error codes but failed to add that to my post. I did read code 12 for about 10 cycles with no other codes showing.
I measured the fuel pump resistance from the connector that feeds the pump on the gray wire to ground and it is opened. I measured to the ground lug to make sure and also stuck a sewing pin into the wire a few inches from the lug and measured to chassis with 0 ohms and did the same on the gray wire to the connector to rule out the connector. I am in the process of removing the bed to replace the pump. I still don'd understand why the ECM does not power the fuel relay. I will replace the fuel pump and screen, have replaced the inline filter and will continue from there. Will ground the G terminal afterwards if it still does not function. Will let you know the results. Thank you.

Tiny
Bevo995
Oct 5, 2008.
Merlin2021, I grounded the G contact at the test connector and the fuel relay would activate when the ignition switch was turned on. I removed the ground from the G contact and turned on the ignition switch and the fuel relay came on but only for about 2 seconds. Turning the ignition switch on again did nothing. I then replaced the fuel pump, grounded the G terminal for a few seconds, heard the fuel pump work, removed the ground and heard the fuel pump turn off and then started the engine.

Now that this is fixed, the ECM has to be reading a fuel pressure sensor. Can you tell me where this is or how the ECM knows there is fuel? Hopefully this will never be an issue but would be nice to know.

I appreciate the quick response you gave as I was able to fix my truck before the new work week. Thank you.

Tiny
Bevo995
Oct 5, 2008.
THEORY/OPERATION -1993 Chevrolet Pickup C1500
Page 1 of 5
FUEL SYSTEM (GASOLINE)
FUEL DELIVERY

Fuel Module (7.4L, 5.7L G Van & 5.7L Over 8500 GVWR)
Fuel module overrides the ECM 2-second timer and fuel pump runs 20 seconds before shutting off when vehicle is not started. This added circuit corrects hot restart problems which could cause vapor lock during high ambient temperatures.

Fuel Pump
An in-tank electric fuel pump delivers fuel to injector(s) through an in-line fuel filter. The pump is designed to supply fuel pressure in excess of vehicle requirements. The pressure relief valve in the fuel pump controls maximum fuel pump pressure.
On TBI fuel systems, pressure regulator is mounted on throttle body. On Port Fuel Injection (PFI) systems, pressure regulator is mounted on the fuel rail. Regulator keeps fuel available to injector(s) at a constant pressure. Excess fuel is returned to fuel tank through pressure regulator return line.
When ignition switch is turned to ON position, ECM turns on electric fuel pump by energizing fuel pump relay. ECM keeps pump on if engine is running or cranking (ECM is receiving reference pulses from ignition module). If there are no reference pulses and vehicle is not equipped with a fuel module, ECM turns pump off within 2 seconds after ignition is turned on. For additional information, see FUEL PUMP RELAY and FUEL MODULE under FUEL DELIVERY.

Fuel Pressure Regulator (CPI)
A constant fuel pressure of 54-64 psi (3.8-4.5 kg/cm2 ) is maintained by a factory preset, nonadjustable, spring loaded diaphragm contained within CPI assembly. Spring tension maintains a constant fuel pressure to injector regardless of engine load.

Fuel Pressure Regulator (TBI)
A constant fuel pressure of 9-13 psi (.6-9 kg/cm2 ) is maintained by a factory preset, nonadjustable, spring loaded diaphragm contained within throttle body. Spring tension maintains a constant fuel pressure to injector regardless of engine load.

Fuel Pressure Regulator (4.3L Turbo PFI)
Fuel pressure regulator is a diaphragm-operated relief valve with injector pressure on one side and manifold pressure (vacuum) on the other. Pressure regulator compensates for engine load by increasing fuel pressure when low manifold vacuum is experienced.
During periods of high manifold vacuum, regulator-to-fuel tank return orifice is fully open, keeping fuel

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THEORY/OPERATION -1993 Chevrolet Pickup C1500
Page 2 of 5
pressure on the low side of its regulated range. As throttle valve opens, vacuum to regulator diaphragm decreases, allowing spring tension to gradually close off return passage. At wide open throttle (when vacuum is at its lowest), return orifice is restricted, providing maximum fuel volume and maintaining constant fuel pressure to injectors.
Fuel Pump Relay
When ignition switch is turned to ON position, ECM turns electric fuel pump on by energizing fuel pump relay. ECM keeps relay energized if engine is running or cranking (ECM is receiving reference pulses from ignition module). If there are no reference pulses, ECM turns pump off within 2 seconds after key on. See FUEL MODULE under FUEL DELIVERY.
As a back-up system to fuel pump relay, the oil pressure switch also activates fuel pump. The oil pressure switch is normally open until oil pressure reaches approximately 4 psi (.28 kg/cm 2 ). If fuel pump relay fails, the oil pressure switch closes when oil pressure is obtained, and operates the fuel pump. An inoperative fuel pump relay may result in extended cranking times due to the time required to build up oil pressure. Oil pressure switch may be combined into a single unit with an oil pressure gauge sending unit or sensor.
ECM monitors fuel pump circuit between fuel pump relay/oil pressure switch and fuel pump, enabling ECM to determine if fuel pump is being energized by fuel pump relay or oil pressure switch. A failure in this monitored circuit results in the setting of a related trouble code in ECM memory.
For additional information on fuel pump activation, see F -BASIC TESTING and I - SYS/COMP TESTS article in this sections.
FUEL CONTROL
The ECM, using input signals, determines adjustments to the air/fuel mixture to provide the optimum ratio for proper combustion under all operating conditions. Fuel control systems can operate in the " open loop" or " closed loop" mode.

Open Loop
When engine is cold and engine speed is greater than 400 RPM, ECM operates in open loop mode. In open loop, ECM calculates air/fuel ratio based upon coolant temperature and MAP or MAF sensor readings. Engine remains in open loop operation until O2 sensor reaches operating temperature, coolant temperature reaches preset temperature and a specific period of time has elapsed after engine starts.

Closed Loop
When O2 sensor reaches operating temperature, coolant temperature reaches a preset temperature and a specific period of time has passed since engine start-up, ECM operates in closed loop. In closed loop, ECM controls air/fuel ratio based upon O2 sensor signals (in addition to other input parameters) to maintain as close to a 14.7: 1 air/fuel ratio as possible. If O2 sensor cools off (due to excessive idling) or a fault occurs in O2 sensor circuit, vehicle once again enters open loop mode.
On 4.3L engines, oxygen sensor is equipped with an internal heating element. This element enables

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THEORY/OPERATION -1993 Chevrolet Pickup C1500
Page 3 of 5
system to reach " closed loop" sooner and maintain closed loop even during periods of extended idle.
Central Port Injection (CPI) - CPI is one of 3 fuel systems used on 4.3L engines. The non-repairable injector assembly consists of a fuel meter body, fuel pressure regulator, fuel injector and 6 poppet nozzles with fuel tubes. CPI assembly is housed in the lower manifold assembly. Fuel pump and pressure regulator maintain fuel pressure at 54-64 psi (3.8-4.5 kg/cm2 ) under all operating modes.
When injector is energized, pressurized fuel passes down fuel distribution tubes to poppet nozzles located at rear of intake valves. Fuel pressure forces poppet valves open, spraying fuel into cylinders when intake valves are open. As fuel pressure drops (due to all poppets opening or injector de-energizing), poppet nozzle spring pressure closes poppet nozzle until pressure again builds high enough to overcome poppet nozzle spring pressure. Excess fuel is returned to the fuel tank via the fuel return line.
Throttle Body Injection
Injectors are located in throttle body unit. All models except 2.5L use 220 Series dual injector throttle body. 2.5L models use 700 Series single injector throttle body. Battery voltage is supplied to injector when ignition is on. ECM energizes injector solenoid by providing a ground path through its internal circuitry. By regulating injector ground circuit, ECM controls injector " on" time (pulse width) to provide proper amount of fuel to engine.
Pressure regulator maintains pressure to injector at 9-13 psi (.6-.9 kg/cm2 ). Excess fuel passes through pressure regulator and returns to fuel tank.
In the " run" mode, ECM uses tach (RPM) signal to determine when to pulse injector. Fuel injectors are pulsed once for each engine revolution; each spray provides 1/2 the fuel required for the combustion process. Thus, 2 injections of fuel (2 rotations of crankshaft) are mixed with incoming air to produce the fuel charge for each combustion cycle. On models equipped with dual injectors in the throttle body, injectors are pulsed alternately.

Port Fuel Injection (PFI)
Individual, electrically pulsed injectors (one per cylinder) are located in intake manifold fuel rails. These injectors are next to intake valves in cylinder head.
The 4.3L Turbo PFI system features simultaneous double-fire injection. Fuel injectors are pulsed once for each engine revolution; each spray provides 1/2 the fuel required for the combustion process. Thus, 2 injections of fuel (2 rotations of crankshaft) are mixed with incoming air to produce the fuel charge for each combustion cycle.
The 3.8L engine uses Sequential Fuel Injection (SFI). Injectors on these models are pulsed sequentially in spark plug firing order. Main differences between sequential and simultaneous systems are injectors, wiring and the ECM.
On both systems, constant fuel pressure is maintained to the injectors. Air/fuel mixture is regulated by amount of time injector stays open (pulse width). Various sensors provide information to the ECM to control pulse width. The ECM controls pulse width using information provided by various sensors.

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THEORY/OPERATION -1993 Chevrolet Pickup C1500
Page 4 of 5
Fuel System Operating Modes
Internal ECM calibration controls fuel delivery during starting, clear flood mode, deceleration and heavy acceleration.

Starting
During engine starts, ECM delivers one injector pulse for each distributor reference pulse received (synchronized mode). Injector pulse width is based upon coolant temperature and throttle position. ECM determines air/fuel ratio when throttle position is less than 80 percent open. Engine starting air/fuel ratio ranges from 1.5: 1 at -33 °F (-36 °C) to 14.7: 1 at 201 °F (94 °C). At lower coolant temperatures, injector pulse width is wider (richer air/fuel mixture ratio). When coolant temperature is high, injector pulse width becomes narrower (leaner air/fuel ratio).
Clear Flood
If engine is flooded, driver must depress accelerator pedal to Wide Open Throttle (WOT) position. At this position, ECM adjusts injector pulse width equal to an air/fuel ratio of 20: 1. This air/fuel ratio is maintained as long as throttle remains in wide open position and engine speed is less than 600 RPM. If throttle position becomes less than 80 percent open and/or engine speed exceeds 600 RPM, ECM changes injector pulse width to that used during engine starting (based upon coolant temperature and manifold vacuum).

Heavy Acceleration
ECM provides fuel enrichment during heavy acceleration. Sudden opening of throttle valve causes rapid increase in MAP signal. Pulse width is directly related to MAP, throttle position and coolant temperature. Higher MAP and wider throttle angles give wider injector pulse width (richer mixture). During enrichment, injector pulses are not in proportion to distributor reference signals (non- synchronized). Any reduction in throttle angle cancels fuel enrichment.

Deceleration
During normal deceleration, fuel output is reduced. This reduction in available fuel serves to remove residual fuel from intake manifold. During sudden deceleration, when MAP, throttle position and engine speed are reduced to preset levels, fuel flow is cut off completely. This deceleration fuel cut-off overrides normal deceleration mode. During either deceleration mode, injector pulses are not in proportion to distributor reference signals.

Battery Voltage Correction
ECM compensates for low battery voltage by increasing injector pulse width and increasing idle RPM. ECM is able to perform these commands because of a built-in memory/learning function.

Fuel Cut-Off

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When ignition is turned off, injectors are de-energized to prevent dieseling. Injectors are not energized if RPM reference pulses are not received by the ECM, even with ignition on. This prevents flooding before starting. Fuel cut-off also occurs at high engine RPM to prevent internal damage to engine. Some models may also cut off fuel injector signals during periods of sudden, closed throttle deceleration (when fuel is not needed).
IDLE SPEED
ECM controls engine idle speed depending upon engine operating conditions. ECM senses engine operating conditions and determines best idle speed.

Idle Air Control (IAC) Valve
The IAC valve controls engine idle speed to prevent stalling during engine load changes. The IAC valve is mounted on throttle body and controls the amount of air by-passed around the throttle plate. The IAC valve controls engine idle speed by moving its pintle in and out in steps referred to as " counts" (0 counts, fully seated; 255 counts, fully retracted). Counts can be measured by plugging a Scan tester into the Assembly Line Data Link (ALDL).
If engine RPM is too low, pintle is retracted and more air is by-passed around the throttle plate to increase engine RPM. If engine RPM is too high, pintle is extended and less air is by-passed around the throttle plate to decrease engine RPM. Normal counts on an idling engine should be 4-60. When engine is idling, ECM determines proper positioning of IAC valve based on battery voltage, coolant temperature, engine load and engine RPM.
If IAC valve is disconnected or reconnected with engine running, IAC loses its reference point and must be reset. On some models, IAC is reset by turning ignition on and off. Other models require driving vehicle at normal operating temperature over 35 MPH with circuit properly connected. Problems in IAC circuit should set a related code.
The IAC valve affects only the idle system. If valve is stuck fully open, excessive airflow into the manifold creates a high idle speed. Valve stuck closed allows insufficient airflow, resulting in low idle speed. For calibration purposes, several different IAC valves are used. Ensure replacement valve is proper design.

10/6/2008

Merlin2021
Oct 6, 2008.
Im having the same problem, sometimes it doesnt start, but starting fluid gets it started for 2 seconds. Checked for all good grounds, and changed the starter wire. Im hearing the fuel pump come on, checked the lines- pumps good psi, and im hearing the relay " click" on. This morning it starts but only with one injector running twice as hard. After driving for 30 minutes the other kicks on and it runs perfect the rest of the day, so long as I dont turn it off for too long. A friend also mentioned cold weather having an impact, but not with fuel, right? Im going to get a noid light, any help would be great.

Tiny
1993Z-71
Dec 16, 2008.
I'm going to have to try that " g" ground process on terminal. IF I can't fix my *no start* problem today, I am going to post what my findings are in my application. Some of the different posts on here are almost identical, but they never get resolved or the owner never respnds AFTER they get it foxed!

Tiny
SFBDBURBAN
May 27, 2009.
Alright I had a question for you guys, I got a 1993 Chevrolet C3500. It had a 5.7 but I turned it into a 383 stroker carbureted with a HEI Vacuum advanced MSD Distributor. It will start and run for about 20-30 seconds before the fuel pump kicks off, I assume it's because the ECM is not receiving signal from the Ignition control module or somewhere along those lines. Is there a way to bypass this? Or do I just have to run a positive lead from the fuel pump to a toggle? I have a pressure regulator with a return running from the fuel lines so I'm getting correct pressure I just can't keep the fuel pump running.

Tiny
Keero816
Jan 18, 2014.