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HOW TO: Check regulated fuel pressure, pump max output, & leak-down‏ according to the GM shop manual

If your vehicle has a driveability issue, the fuel delivery system should be the first thing on your list of things to check. Correct pressure, and the ability to hold the pressure, is critical on this engine. Testing fuel pressure and leak-down will tell you if there’s a problem with the fuel delivery system. Incomplete or incorrect testing will usually lead to a misdiagnosis which can be time consuming, frustrating, and expensive. The following is based on information from the GM “dealer issue” shop manual. It covers both the procedures and the specifications for correctly testing the fuel delivery system. It applies to all 1996 through 2005 (and a few late production CSFI equipped 1995's) 4.3L Central Sequential Fuel Injection, (CSFI) engines controlled by fully compliant On Board Diagnostics second generation, (OBDII). The 8th character in the VIN must be “W” or “X”.

The “W’ 4.3L uses 6 fuel injectors centrally located inside the plenum. There is a small fuel line that runs from each injector to each intake port. There is a nozzle and a poppet valve in the end of each line.

The “X” 4.3L uses 6 mini injectors located at the intake ports. There are no poppet valves used on this fuel delivery system. The “W” engines can be easily upgraded to this system when/if there’s a problem with an injector or a poppet valve, or you just want better performance

If the fuel pressure or volume is insufficient, or leak-down is excessive, you WILL experience driveability issues. Symptoms of fuel delivery problems include, but are not limited to: Excessive cranking hot and/or cold, fuel odor, cranks but will not start, starts and stalls, hesitation or stalling on acceleration, running rich, running lean, misfire, induction, (backfire through the intake), lack of power, and harsh transmission shift points. Fuel pressure, volume, and leak-down are not directly monitored by OBDII so it can’t set a Diagnostic Trouble Code, (DTC) in memory and illuminate the Service Engine Soon, (SES) light. However, malfunctions that occur in the fuel delivery system can affect other systems that CAN set a DTC. For example: If the fuel pressure or volume is low, the oxygen sensors could detect a lean fuel mixture and simultaneously set P0171 & P0174. Keep in mind that if the engine exhibits any of the above symptoms, there may or may not be any DTC’s in memory.

Fuel Pump Activation
When the ignition switch is initially turned to the RUN position, the Powertrain Control Module, (PCM) activates the fuel pump relay for approximately 2 to 3 seconds. This time frame is referred to as “pump prime”. If the PCM does not receive a signal from the crankshaft position sensor during this time, it deactivates the relay. Fuel pressure must be tested while the pump is running, and the engine is off. 1998 through 2005 have a pump prime terminal in the under hood fuse panel that can be used to activate the fuel pump. Applying battery voltage to the terminal bypasses the relay and activates the pump continuously.

1) Confirm fuel pump activation. (If the pump runs when the ignition is initially turned to the RUN position, or bypassing the relay, move on to step 2.) At the fuel pump module electrical connector: The gray wire must show battery voltage when the fuel pump relay is activated. The black ground wire must show less than 5 ohms resistance to chassis ground. Checking these may require dropping the fuel tank to gain access to the connector on the fuel pump module.

2) Make sure the fuel filter is not clogged or restricted.

3) Check for and repair any kinks, restrictions, or external fuel leaks in the 3/8” fuel pressure line, and the 5/16” fuel return line between the fuel pump module and the connections at the fuel metering block.

4) Connect the fuel pressure tester to the service port in the 3/8” steel fuel pressure line located at the rear of the plenum, near the distributor. Activate the fuel pump and check for leaks at the service port connection, repair if/as necessary.

Several things influence fuel pressure while the engine is running such as the fuel pressure regulator setting, ambient temperature, coolant temperature, percentage of throttle opening, altitude above sea level, overall engine condition and engine load. All of which have a direct affect on fuel injector pulse width, (the time the injectors are open), which directly affects fuel pressure. Due to these variables, GM does not publish any “engine running” fuel pressure specifications for this system. Fuel pressure and leak-down must be tested with the engine off. NOTE: Checking fuel pressure and leak-down at the service port is only a“quick check”. There are things that the “quick check” does NOT test for.

Pressure Readings At The Service Port
Activate the fuel pump to pressurize the system. The pressure indicated on the tester is either regulated fuel pressure, or maximum fuel pump output pressure, whichever is LOWER. Regulated fuel pressure must be 60 psi to 66 psi while the pump is running. If the pump can only build let’s say, 40 psi , you won’t know what regulated pressure actually is. Going by this pressure reading alone, a fuel pressure regulator could easily be misdiagnosed as the problem. To avoid this, maximum fuel pump output pressure must be checked.

Leak-down Test
With the exception of the return lines, this will test the entire systems ability to hold fuel pressure, (fuel pump, lines, regulator, and injectors). Activate the fuel pump to pressurize the system, and then deactivate the pump. Fuel pressure must remain above 55 psi for at least 10 minutes. If this test fails, you will need to perform the fuel pump maximum output pressure and leak-down test to determine if the cause is in the plenum, or in the fuel tank.

Fuel Pump Maximum Output Pressure And Leak down Test
The pump must be capable of supplying sufficient pressure, and be able to hold it after the pump shuts off. This is most easily tested at the fuel filter outlet, (forward connection on the filter). It requires an adapter between the filter and the pressure tester. All fuel pressure and flow must end at the tester, with no fuel allowed to the engine. Disconnect the fuel filter outlet line from the filter. Connect the adapter to the filter outlet and connect the pressure tester to the adapter. Activate the fuel pump. Pressure must be 73 psi to 108 psi. Deactivate the fuel pump. Pressure must remain above 55psi for at least 10 minutes. If this leak down test fails, there is a leak inside the fuel tank. If leak-down is within specs on this test, and leak down at the service port is excessive, there is a leak in the plenum.

Fuel Pressure Regulator
When the fuel pump is activated, it delivers pressurized fuel from the pump to the regulator inlet port, (arrow on left). The injector and service port are teed into the line before the fuel reaches the regulator. Spring pressure on the bottom side of the diaphragm keeps the valve plate closed which allows the system to build pressure. The amount of spring pressure on the valve plate determines regulated fuel pressure. The higher the spring pressure, the higher the fuel pressure is. When fuel pressure on the top side of the diaphragm overcomes the spring pressure on the bottom side of the diaphragm, the valve plate opens and directs the excess pressurized fuel back to the fuel tank through the fuel return line, (arrow on top). When the valve plate is open, fuel pressure decreases until the spring pressure overcomes the fuel pressure on the top side of the diaphragm, and the cycle repeats. The port labeled “vacuum connection” does not have any hoses connected to it. It is exposed to manifold absolute pressure inside the plenum. When idling or decelerating, (low fuel demand) the port is exposed to high vacuum which pulls on the bottom side of the diaphragm decreasing spring tension on the valve plate. This causes fuel pressure to decrease. When cruising or accelerating, (high fuel demand) the port is exposed to low or zero vacuum which increases spring tension on the valve plate which causes fuel pressure to increase. The highest regulated pressure is obtained when the engine is off, (zero vacuum in the plenum) and the fuel pump is running. Some, not all, fuel pressure regulators have an adjustment screw, however; adjustment is done at the factory and tampering with it is not recommended. If it’s not regulating the fuel pressure within specs, replace it. The fuel pressure regulator is quite simple in both design and operation; as a result, it rarely fails. It is however, a mechanical part and can develop leaks both internally and externally.

1995 & older CMFI engines:
Regulated pressure must be 55 to 61 psi.
It must remain above 50 psi for at least 10 minutes after the fuel pump shuts off.

At the fuel filter outlet with the ignition ON, engine OFF, fuel pump running pressure must be 70 to 100 psi.
It must remain above 50 psi for at least 10 minutes after the fuel pump shuts off.

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Excellent write up!
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