Thursday, June 29, 2017 Detailed Auto Topics
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The truck cranks over but will not start.  On General Motors vehicles a bad fuel pump is common and often the first guess.  Sometime it is the cause, other times not.  It’s an expensive mistake to guess wrong.  These tips will greatly increase the odds of being right.


Fuel pressure should be checked to confirm a bad fuel pump


To run, an engine must have four things, ignition-spark, compression, the proper timing and fuel in the cylinders.  The fuel pump is only one component in the fuel delivery system.  A lack of fuel, does NOT mean the pump is bad.  Tests are necessary to learn the problem.  Time spent testing components, will save unnecessary part replacement.  


The GM models covered


The information below, relates to the very popular, 1988 through 2013, Chevrolet and GMC.  The Silverado, Sierra, Suburban, Yukon, Tahoe and similar vehicles account for the large number of fuel pump failures.  GM fuel systems continue to change and get more expensive.  An expected failure seems to be one constant feature.

A lack of fuel pressure is determined with a gauge.  Methods, such as listening for the pump, near the fuel tank, can cause many mistakes.  Running does not show the necessary pressure, and perfectly good pumps often make no discernable sound.  Other things needed are, a method of connection and the fuel pressure specifications.


Guidelines for checking fuel pressure


Switching the ignition from off to run will cause the fuel pump to run for about two seconds.  This charges the fuel lines with pressure and enables starting.  The pump will not continue to run, until the engine starts or until cycling the ignition.  Connecting the fuel-pressure gauge, before turning the key, is necessary.

 

Testing fuel pressue to see if the fuel pump is bad


With the key on, engine off (KOEO) we take a reading.  The engine should be cold when testing, below 150 degrees Fahrenheit.  Higher engine temperature causes alcohol in fuel to boil and greatly increases pressure.  False readings can make a bad pump seem okay.


Getting a consistent reading may take several tests.  Readings above or below the specified range shows a problem.  No fuel pressure means we need additional tests, as described below.


Finding the fuel-pressure test-port


Fortunately, GM provides a standard test port on later models.  The location varies, depending on the engine type and the type of injection system used.  Early models and throttle body injection require an adaptor to connect the fuel-pressure gauge.  GM employs many systems, and each has different pressure requirements.  These specifications and locations will often apply.  Consult the service-data to verify.


Throttle body injection


A fuel pressue test adapter


This system is widely used from before 1988 until 1994.  A fuel-pressure gauge connects with an adapter, in the fuel line.  The fuel pump is inexpensive, by more modern comparisons.  This system operates at only nine to thirteen pounds per square inch (PSI) of pressure.  Pressure should rise slightly when we accelerate the engine.  Higher or lower pressure is a cause for concern.


Central port injection

 

The test port is hard to find, at the rear of the engine


Around 1995, they introduced the central port injection system.  The eight-cylinder  engines use this system until around 1999, when the new generation 4.8L and 5.3L replaced the small block.  Depending on the model, the six-cylinder, 4.3L continues the system until at least 2002.  With central-port injection, the KOEO pressure should be 60 to 66 PSI and rise slightly, on acceleration.


Multec2 injection

 

V8 test port on right side of engine


Around 2003, the system changed, and they specify fuel pressure as 55 to 62 PSI on the V-8 and six-cylinder.  Tests are with a cold engine, key on, engine off.  Acceleration will cause a slight increase in pressure.  This system is before the body style change, around 2007.

Later module engine with electronic fuel pressure control


The fuel pressure specification for the non-Classic style, in 2007 is 50 to 60 PSI.  This again changes with the introduction of the fuel-pressure control-module, around the 2009 model.  Until 2013, the specification is 50 to 100 PSI.

 

A lack of fuel pressure still does not necessarily mean a bad pump


Well-meaning folks often replace fuel pumps because of no pressure.  Current flow and ground should be confirmed before condemning the pump.  A lack of power to the pump results in no fuel pressure.


typical GM fuel pump connector wiring


Fuel pump wiring will vary, but gray is widely used as the power wire to the pump.  The ground and fuel-level sensor return both use a black wire.  A wiring diagram is necessary for the various connectors.  Current and ground along with no fuel pressure suggest a bad fuel pump.  Finally check that there is fuel in the tank.  Inoperative fuel gauges cause many good fuel-pump replacements.


A blown fuse, broken wires, bad relay and theft deterrent systems have caused a lack of power.  For more on basic electrical and relay testing, see our Detailed Topic, How to Check Fuses and Relays.


Fuel pump replacement is sometimes necessary.  Checking the other alternative first can prevent an expensive mistake.





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