Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 263 (ENGINE CONTROLS - SYSTEM TESTING & ENGINE COMPONENT TESTS - 3.1L)FUEL SYSTEMSALCOHOL/CONTAMINANTS-IN-FUEL DIAGNOSIS WITH SPECIAL TOOLDIAGNOSTIC PROCEDURE
1999 GMC Forward Control 5.7 R, Automatic
Diagnostic Procedure
1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedure
WARNING: This page is about a different car, the 2003 Pontiac Grand Prix, 2003 Chevrolet Malibu, and 2003 Buick Century. However, it is still accessible from the selected car via links, so may be relevant.
NOTE:
Numbers in parenthesis in the following steps correspond to numbered components in referenced figures.
- Turn ON the Fuel Composition Tester (J 44175), or equivalent.
- Verify the fuel composition tester is operational by measuring the AC frequency output using a DVOM. A frequency without a fuel sample in the test cell indicates that the tester is working correctly.
- Install Fuel Pressure Gauge (J 34730-1A).
- Close the fuel pressure gauge bleed valve.
- Place the bleed hose (1) of the fuel pressure gauge into the 100 ml beaker (2). See Fig 1.
- Command the fuel pump relay ON using a scan tool.
- Slowly open the bleed valve on the fuel pressure gauge, until an adequate fuel sample is obtained.
- If water appears in the fuel sample, clean the fuel system and replace the fuel in the vehicle. See FUEL SYSTEM CLEANING .
- Pour the fuel sample from the beaker (1) into the fuel composition tester, until the level of the fuel is at the top of each fuel test port (2). See Fig 2.
- Observe the diagnostic LEDs on the fuel composition tester. If the Red fuel diagnostic LED is illuminated, a fuel contamination condition exists. See FUEL SYSTEM CLEANING .
- Measure the output frequency of the fuel composition tester.
- Subtract 50 from the reading on the DVOM to obtain the percentage of alcohol in the fuel sample. See examples in FUEL COMPOSITION TEST EXAMPLES table.
- If the fuel sample contains more than 10 percent ethanol, replace the fuel in the vehicle.
- Check for particulates in fuel. See PARTICULATE CONTAMINANTS IN FUEL TESTING PROCEDURE .
FUEL COMPOSITION TEST EXAMPLES
| Example | Frequency - Hz | Subtract 50 | Ethanol Percent |
|---|---|---|---|
| A | 50 | -50 | 0 |
| B | 65 | -50 | 15 |
| C | 129 | -50 | 79 |
RENDER: 1.0x
NO RELATED
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.