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Diagnostic Testing /
Testing Engine Vacuum
Testing Fuel System Pressure
Most vehicles utilize a fuel injection system to deliver fuel to the engine.
Correct fuel system pressure is essential for proper engine operation, in fact many
engine run-ability problems can be related to weak fuel system pressure. A fuel
injection system contains a fuel pump to supply fuel from the tank into the fuel
system. The fuel filter cleans fuel before fuel enters the fuel injectors. The fuel
pressure regulator adjusts fuel system pressure depending on the load of the engine.
Typical Fuel Injection System
Vacuum that is attached to the fuel pressure regulator to control system pressure.
When the fuel pressure regulator is not functioning properly it causes the fuel
pressure to be above normal operating pressure. This condition causes excess fuel
to be injected into the engine causing a rich mixture that produces black smoke.
To test fuel system pressure and fuel pressure regulator operation (engine off)
remove the protective cap and attach the fuel pressure gauge.
Attach Fuel Pressure Gauge
Support the fuel pressure gauge for observation, then turn the key to the on
position. The fuel pressure gauge should jump up to your system pressure. If you
do not know your vehicle system fuel pressure consult a
repair manual. Actual fuel system pressure will very slightly but should pretty
close to specification.
Fuel Pressure Gauge
Then, start the engine the fuel system pressure should stay the same. While watching
the gauge snap the throttle, the fuel system pressure should jump up about 5 psi.
If the system fuel pressure does not respond the fuel pressure regulator has failed.
If the fuel pressure gauge falls the fuel filter is plugged or the fuel pump is
failing. When testing is complete, remove the fuel pressure gauge feed line and
replace the protective dust cap.
Testing for a Blown Fuel Pressure Regulator Diaphragm. A
fuel pressure regulator is used in the fuel injection system. This component regulates
fuel system pressure by using a diaphragm and spring combination. When the fuel
pressure regulator diaphragm ruptures it allows raw fuel into the vacuum feed line
and then into the intake system of the engine. This condition creates a rich black
smoke from the tail pipe on start up and while running.
Removing the Vacuum Feed Line
To check for this condition remove the vacuum line from the regulator, if fuel
is present in the line the regulator has failed. Some GM vehicles are designed with
an internal intake regulator you must remove the upper intake manifold to be inspected.
Most fuel pressure regulators are on the fuel rail on top of the engine in plain
site. If you can not locate your fuel pressure regulator consult a
repair manual.
Diagnostic Testing /
Testing Engine Vacuum
Check Manufacturer Specific Fuel Injection
System
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