Poor Fuel Economy
Checks
| Action
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DEFINITION: Fuel economy, as measured
by an actual road test, is noticeably lower than expected. Also, the economy
is noticeably lower than it was on this vehicle at one time, as previously
shown by an actual road test.
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Preliminary Checks
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| • | Check the air cleaner element (filter) for dirt or restrictions. |
| • | Visually/physically check the vacuum hoses for splits, kinks,
and proper connections as shown on Vehicle Emission Control Information label. |
| • | Check the owner's driving habits for any of the following: |
| - | The A/C system (defroster mode) is ON all the time. |
| - | The tires are kept properly inflated. |
| - | Heavy loads are frequently being carried. |
| - | Heavy acceleration and high speeds are common. |
| - | Prolonged periods of idling. |
| - | Driving on unpaved, dirt, or rough roads. |
| • | Suggest that the owner refill the fuel tank and recheck the fuel
economy. |
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Fuel System Checks
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| • | Check the type, quality and alcohol content of the fuel. Oxygenated
fuels have lower energy and may deliver reduced fuel economy. Refer to Fuel System Specifications
and
Alcohol/Contaminants-in-Fuel Diagnosis
. |
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Ignition System Checks
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| • | Check the spark plugs for any of the following conditions: |
| - | Wet (fuel fouled) plugs. |
| - | A terminal screw that is bent or loose (rotate and pull). |
| - | A ceramic insulator that has cracks, carbon tracking or deposits
(red or black dust) |
| - | A tip insulator that has cracks, carbon tracking, deposits (oil,
carbon, glazing, fuel additive), or is loose (rattles). |
| - | A center electrode that is loose (wiggles side to side) |
| - | Incorrect spark plug gap |
| - | Platinum pads -- missing, wear, gap size (too large or too
small) |
| • | Check the ignition wires for cracking, chafing, and loose connections. |
| • | Observe the Spark parameter on the scan tool at idle, while operating
the rear defogger, A/C system, or headlights. If the scan tool display remains
fixed at 10 degrees check for a short to ground in the Diagnostic
Request circuit from PCM terminal C2-15 to the underhood diagnostic link
connector. |
| • | Check the knock sensor and the sensor's circuit shielding. The
knock sensor is not diagnosed for excessive activity. A faulty knock sensor
or signal interference could cause excessive retard of the ignition timing
advance. Observe the Spark parameter on a scan tool and compare with a known
good vehicle. A typical value in OD at 65 mph is 26-32 degrees. |
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Cooling System Checks
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| • | Check the engine coolant level. |
| • | Check the engine thermostat for always being open or for the wrong
heat range. |
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Additional Checks
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| • | Check the clutch adjustment in M/T equipped vehicles. |
| • | Check the transmission shift pattern and the TCC operation in
A/T equipped vehicles. |
| • | Check for dragging brakes. |
| • | Check for uneven or premature tire wear. A suspension misalignment
can cause a loss of fuel economy. |
| • | Check that the tires and wheels are similar to OEM. Wide performance
tires create more drag and taller tires can cause odometer discrepancies. |
| • | Check for the speedometer for proper calibration. |
| • | Check for a green engine. Fuel economy may not be near expected
levels until the engine has had 3 to 5 thousand miles to break in. |
| • | Check for add-on equipment like luggage racks and carriers. Wind
resistance can lower fuel economy. |
| • | Check the Service Bulletins for any updates or information. |
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