FUEL SYSTEM DIAGNOSIS

Fuel System Review

Fuel System Tests Gasoline test Fuel pump pressure Fuel injector tests Computer sensor and actuator tests Scan tool testing

Chemistry of Combustion Gasoline Air/Fuel mixture Combustion By-products Pollution

Fuel Related Terminology Volatility Atomization Vaporization Condensation Dilution Vapor Lock Air/Fuel Ratio Detonation Pre-ignition Octane rating Ethanol Methanol MTBE

Testing Fuel Poor fuel can cause driveability problems. Alcohol content can be the problem. Test fuel for alcohol content.

The Fuel Delivery System Tank Pump Lines Filters Fuel rail Pressure regulation Return lines

Fuel System Maintenance Air Filter a. Replacement interval. b. Checking element. c. Replacement procedure. d. Cleaning the housing. Fuel Filter a. Replacement interval b. Replacement procedure Cleaning the Throttle Body a. Removal of hoses. b. Spray carb. & choke cleaner into throttle body.

Idle Air Control (IAC) It’s a stepper motor that by-passes air around the throttle blade to control idle speed. Needs to be cleaned out periodically with carburetor cleaner. It may set a DTC if defective.

Fuel Pump Test Determine the condition of the fuel pump. Connect a pressure gauge to the fuel rail. Several tests are performed to determine proper operation.

Performing the Test Connect gauge. Look up pressure specifications. Turn key on. Start engine. High idle test. Snap throttle. Pinch off return hose. Use a vacuum pump to check vacuum diaphragm of pressure regulator.

Fuel Injection System Type of Fuel Injection System. a. Throttle Body b. Port injection c. Mechanical injection Sensors Actuators PCM location and wiring

Testing Fuel Injectors Click test. Check resistance in solenoid coil. Noid light test. Spray pattern on throttle bodies. Pulse testing. Generate a pattern using the lab scope.

Click Testing Injectors Use the stethoscope to listen to each injector. Compare the sounds. They should all sound the same.

Checking the Injector with a Multi-meter Remove the wiring harness from the injector. Using an ohmmeter put the probes across the connectors. Resistance should be within specifications. Check the power feed to the injector.

Checking the Injector with a Noid Light. This test checks if the computer is pulsing the injector. Remove the wiring harness and connect the noid light to the harness connector. When cranking the engine the light should blink. Noid light installed on wiring harness.

Checking Throttle Body Spray Pattern Remove the air cleaner to see the injector(s). Connect a timing light to the engine. While running the engine shine the light on the injector. Check the spray pattern is conical with no drips or odd shapes.

Pulse Testing the Injectors This test operates each injector by pulsing it from a special tester. A pressure gauge must be used in conjunction with this tester. Each injector is pulsed and then the readings are compared.

Performing the Pulse Test Connect the pulse tester to the first injector and pressure gauge to the rail. Turn the key on and then off to pressurize the rail. Pulse the injector and measure pressure drop. Connect the pulse tester to the next injector and repeat the process until all injectors are pulsed.

Interpreting the Results If injectors have less than a 3 psi difference, the injectors are OK. If the injectors are greater than 3 psi, replace the bad injector(s) or the whole set.

Cleaning the Fuel Injectors To clean injectors the fuel pump is disabled and the cleaner tank is connected to the rail. A cleaning compound (you know, poison) is mixed with gasoline and run into the rail for about 10 minutes. This cleans the injectors.

Diagnosis Using the Scan Tool

OBDII DTC’s

Testing Computer Sensors Sensors can be tested in a variety of ways. 1. The scan tool will let you into the data stream of the PCM to look at the sensor voltages and values. (Not possible on all cars.) 2. Each sensor can be tested with a multi-meter. Either voltage or resistance. 3. Sensors can be tested with the lab scope. Most sensors operate on a 5 volt system. The O2 sensor operates on .1v to .9v.

Testing the Coolant Temperature Sensor (ECT) Use the Ohmmeter to test the thermistor. You need to know the temperature and the resistance. Use the chart to compare the readings to see if it’s OK. Use the scan tool to watch the temperature change.

Testing the TPS 1. Use an ohmmeter to check the resistance of the potentiometer. 2. Use a voltmeter to watch voltage change. Voltage should be between .5v and 4.5v. 3. Use the lab scope to check the voltage change. 4. Use the scan tool to check the voltage change.

Testing Other Sensors MAF VSS IAT KS CKP CMP

Testing the O2 Sensor(s). Use the scan tool to observe the voltage. Connect a voltmeter to the O2 wire and observe. Use the lab scope to test the O2 sensor.

Using a Lab Scope to Test the Injectors Connecting the oscilloscope to the injector wire will give a picture of the voltage change while the injector operates. Compare the pattern you get to the good pattern in the book or manuals.

Performing the Test Set the Sun 450 to lab scope. Connect one lead to the injector wire (not the 12V feed), and the other to ground and run the engine. Observe the pattern and compare.