DEFINITION: Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, economy is noticeably lower than it was on this vehicle at one time, as previously shown by an actual road test.

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

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Go to Step 2

Go to the Powertrain On Board Diagnostic (OBD) System Check

1. Perform a bulletin search.
2. If a bulletin that addresses the symptom is found, correct the condition as instructed in the bulletin.

Was a bulletin found that addresses the symptom?

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Go to Step 18

Go to Step 3

Was the visual/physical check performed?

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Go to Step 4

Go to Visual/Physical Checks

Check the owner's driving habits.

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Go to Step 5

Go to Step 6

Review the items in Step 4 with customer and advise as necessary.

Is action complete?

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System OK

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1. Visually/physically check vacuum hoses for splits, kinks, and improper connections and routing. Refer to Emission Hose Routing Diagram .
2. If a problem is found, repair as necessary.

Was a repair required?

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Go to Step 18

Go to Step 7

1. Remove and inspect the air filter element. Refer to Air Filter Element and Air Cleaner Assembly .
2. Replace air filter element if necessary.

Was a repair required?

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Go to Step 18

Go to Step 8

1. Remove the spark plugs.
2. Check for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits.

Important: If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs in order to avoid damage to the new spark plugs.

3. If a problem is found, repair as necessary. Refer to On-Vehicle Service in Ignition System.

Was a problem found?

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Go to Step 18

Go to Step 9

1. Check for low engine coolant level. Refer to Cooling System Diagnosis in Cooling and Radiator.
2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 18

Go to Step 10

1. Check for an incorrect or faulty engine thermostat. Refer to Cooling System Diagnosis in Cooling and Radiator.
2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 18

Go to Step 11

1. Check for low engine compression. Refer to Engine Compression Test in Engine General Information and Mechanical Diagnosis.
2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 18

Go to Step 12

1. Check TCC operation. Refer to 4L60-E Automatic Transmission Diagnosis .
2. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 18

Go to Step 13

1. Check the exhaust system for possible restriction.
2. Inspect exhaust system for damaged or collapsed pipes.
3. Inspect muffler for heat distress or possible internal failure.
4. Check for a possible plugged three-way catalytic converter by checking exhaust system back pressure. Refer to Backfire .
5. If a problem is found, repair as necessary. Refer to On-Vehicle Service in Exhaust System.

Was a problem found?

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Go to Step 18

Go to Step 14

Check for proper calibration of speedometer:

1. Install a J 33431-C Signal Generator And Instrument Panel Tester.
2. With the ignition ON, turn the tester ON and monitor the speedometer and the speed displayed on a scan tool.

Does the speed indicated on the speedometer closely match the vehicle speed displayed on the scan tool?

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Go to Step 16

Go to Step 15

Diagnose and repair inaccurate speedometer condition as necessary. Refer to Vehicle Speed Sensor in Electrical Diagnosis.

Is action complete?

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Go to Step 18

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1. Check the the following items for air or vacuum leaks:

•	Intake manifold.

•	Throttle body.

•	EGR valve and pipes.

•	Crankcase.

2. If a problem is found, repair as necessary. Refer to 3800 Engine On-Vehicle Service .

Was a problem found?

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Go to Step 18

Go to Step 17

1. Review all diagnostic procedures within this table.
2. When all procedures have been completed and no malfunctions are found, review/inspect the following:

•	Visual/physical inspection. Refer to Visual/Physical Checks .

•	Scan tool data. Refer to Engine Scan Tool Data List .

•	Freeze Frame data/Failure Records Data.

•	All connections within a suspected circuit and/or system.

3. If a problem is found, repair as necessary.

Was a problem found?

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Go to Step 18

Review technical bulletins

Operate the vehicle to duplicate conditions present when the original symptom was noted.

Is symptom still present?

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Go to Step 3

System OK