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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.

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Fuel Injection Pressure and Pressure Regulator Test

Typical Fuel Injection System
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
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
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.

Fuel Pressure Regulator
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 Questions and Answers

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