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94 gmc: s10..idles fine..the truck is under load it stalls

Resolved Question:

I have a 94 gmc sonoma.. or s10 .. with a 2.2 L in it... when i start it, it idles fine. but when the truck is under load it stalls, or wants to stall out.
Submitted: 6 years ago.
Category: Chevy
Expert:  Tim's Auto Repair replied 6 years ago.


you need to check the fuel pressure..these must have no less then 41psi at all times

also the ignition module will do is located under the zone and most pats stores can test these if you take it off and take it to them


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fuel pressure

Testing and Inspection


Diagnostic Chart
Diagnostic Chart
Diagnostic Chart
Diagnostic Chart

Circuit Description
When the ignition switch is turned ON, the Powertrain Control Module (PCM) will turn ON the in-tank fuel pump. The in-tank fuel pump will remain ON as long as the engine is cranking or running and the PCM is receiving reference pulses. If there are no reference pulses, the PCM will turn the in-tank fuel pump OFF 2 seconds after the ignition switch is turned ON or 2 seconds after the engine stops running. The in-tank fuel pump is an electric pump within an integral reservoir. The in-tank fuel pump supplies fuel through an in-line fuel filter to the fuel passage within the lower manifold assembly. The fuel pump is designed to provide fuel at a pressure above the pressure needed by the fuel injectors. A fuel pressure regulator, attached to the lower manifold assembly, keeps the fuel available to the fuel injectors at a regulated pressure. Unused fuel is returned to the fuel tank by a separate fuel return pipe.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.


  1. To relieve the fuel pressure, refer to Fuel Pressure Relief Procedure. When the ignition switch is ON and the fuel pump is running, the fuel pressure indicated by the fuel pressure gage should be 284-325 kPa (41-47 psi) . This pressure is controlled by the amount of pressure the spring inside the fuel pressure regulator can provide.
  2. A fuel system that drops more than 5 psi in 10 minutes has a leak in one or more of the following areas:
    • The fuel pump check valve.
    • The fuel pump flex pipe.
    • The valve or valve seat within the fuel pressure regulator.
    • The fuel injector(s).
  1. Fuel pressure that during acceleration, cruise or hard cornering may cause a lean condition. A lean condition can cause a loss of power, surging, or misfire. A lean condition can be diagnosed using a scan tool. If an extremely lean condition occurs, the oxygen sensors will stop toggling. The oxygen sensor output voltage(s) will drop below 500 mV. Also, the fuel injector pulse width will increase.

Important: Make sure the fuel system is not operating in the Fuel Cut-Off Mode. This can cause false indications by the scan tool.

  1. When the engine is at idle, the manifold pressure is low (high vacuum). This low pressure (high vacuum) is applied to the fuel pressure regulator diaphragm. The low pressure (high vacuum) will offset the pressure being applied to the fuel pressure regulator diaphragm by the spring inside fuel pressure regulator. When this happens, the result is lower fuel pressure. The fuel pressure at idle will vary slightly as the barometric pressure changes, but the fuel pressure at idle should always be less than the fuel pressure noted in Step 2 with the Engine OFF
  1. A rich condition may result from the fuel pressure being above 325 kPa (47 psi). A rich condition may cause a DTC P0132 or a DTC P0172 to set. Driveability conditions associated with rich conditions can include hard starting (followed by black smoke) and a strong sulfur smell in the exhaust.
  2. This test determines if the high fuel pressure is due to a restricted fuel return pipe or if the high fuel pressure is due to a faulty fuel pressure regulator.
  1. A lean condition may result from the fuel pressure being below 284 kPa (41 psi). A lean condition may cause a DTC P0131 or a DTC P0171 to set. Driveability conditions associated with lean conditions can include hard starting (when the engine is cold), hesitation, poor driveability, lack of power, surging, and misfiring.
  2. Restricting the fuel return pipe causes the fuel pressure to rise above the regulated fuel pressure. Using a scan tool to pressurize the system, the fuel pressure should rise above 325 kPa (47 psi) as the gage outlet hose is pinched.
  1. Check the spark plug associated with a particular fuel injector for fouling or saturation in order to determine if that particular fuel injector is leaking.
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