Poor Fuel Economy

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.

PRELIMINARY

^ Refer to Symptoms, Important Preliminary Checks Before Starting. Testing and Inspection

^ Search for bulletins.

^ Check PCM grounds for being clean, tight and in the proper locations.

^ Check owners driving habits.

^ Is A/C ON full time (Defroster mode ON)?

- Are tires at correct pressure?

- Are excessively heavy loads being carried?

- Is acceleration too much, too often?

^ Remove and check air filter element for dirt, or for being restricted. Refer to Air Filter Element/Air Cleaner Assembly Replace as necessary.

FUEL SYSTEM

^ Perform injector balance test. Fuel Injector Balance Test

^ Check for low fuel pressure. System Diagnosis - Fuel System

^ Check for restricted fuel filter. System Diagnosis - Fuel System

^ Check for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Testing Procedure. Fuel

^ Check for incorrect minimum idle speed. Using a scan tool, monitor IAC position. IAC should be between 5 and 50 counts.

^ Check each injector harness is connected to the correct injector/cylinder according to the firing order (firing order: 1-8-4-3-6-5-7-2). Relocate injector harnesses as necessary.

^ Check for injectors that are shorted internally. Compare injector resistance. Injector resistance should be within one ohm of each other. Fuel Injector Coil Test - ECT Temp Between 10-35 Deg C

^ Check for fuel in pressure regulator vacuum hose. If fuel is present, replace regulator assembly.

^ Check for foreign material accumulation in the throttle bore, coking on the throttle valve or on the throttle shaft. Also inspect for throttle body tampering.

^ Check items which can cause an engine to run rich (long term fuel trim near -14%). For a Rich condition, refer to DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1 and DTC P0152 HO2S Circuit High Voltage Bank 2 Sensor 1.

^ Check items that can cause an engine to run lean (long term fuel trim near 22%). For a Lean condition, refer to DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1 and DTC P0151 HO2S Circuit Low Voltage Bank 2 Sensor 1.

SENSOR

^ Check Air intake system and crankcase for air leaks.

^ Check Crankcase ventilation valve for proper operation. Place finger over inlet hole in valve end several times. Valve should snap back. If not, replace valve.

^ Check for proper calibration of speedometer. Connect J 334316 (Signal Generator) to Vehicle Speed Sensor (VSS) electrical connector. Ignition ON, tester ON and monitor the speedometer. The speedometer should indicate 86 km/h (54 mph).

^ Using a Scan Tool, monitor TP sensor voltage. The TP sensor voltage should be less then 0.9 volt. If the TP sensor voltage is greater than 0.9 volt, inspect the ASR/TCS adjuster assembly (If so equipped). Also inspect throttle body and accelerator cables for binding. Refer to Adjuster Assembly Throttle And Cruise Control Cables.

^ Using a scan tool, monitor KS system for excessive spark retard activity. Refer to Knock Sensor.

IGNITION SYSTEM

^ Check for proper ignition voltage output with spark tester J 26792. System Check - Distributor Ignition (DI)


^ Remove spark plugs and check for the following:

- Wet plugs

- Cracks

- Wear

- Improper gap

- Burned electrodes

- Heavy deposits

^ An improper spark plug gap will cause a driveability problem. The spark plug gapping should be done using a wire gauge gap tool (J 41319). Refer to Spark Plug Replacement If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs. Refer to DTC P0172 Fuel Trim System Rich Bank 1 and DTC P0175 Fuel Trim System Rich Bank 2 to determine the Rich condition or Engine for the oil fouling condition.

^ Visually/Physically inspect Secondary ignition for the following:

- Ignition wires for cross firing.

- Ignition wires arcing to ground.

- Ignition wires for proper routing.

- Distributor cap for cracks or carbon tracking.

^ Wetting down the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look/listen for arcing or misfiring as water is applied.

^ Check for loose ignition coil ground. System Check - Distributor Ignition (DI)

^ Check distributor vent system for being restricted or open. System Check - Distributor Vent

ENGINE COOLING SYSTEM

^ Check engine coolant level for being low. Refer to Cooling System.

^ Check engine thermostat for having a faulty part or for wrong heat range. Refer to Cooling System.

ENGINE MECHANICAL

^ Check engine mechanical for the following:

- Check compression.

- Sticking or leaking valves.

- Worn camshaft lobe(s).

- Valve timing.

- Bent push rods.

- Worn rocker arms.

- Broken valve springs.

- Excessive oil in combustion chamber-Leaking valve seals.

- Low cylinder compression.

^ For incorrect basic engine parts. Inspect for the following:

- Camshaft

- Cylinder heads

- Pistons, etc.

Refer to Engine

ADDITIONAL CHECKS

^ Visually (physically) check: Vacuum hoses for splits, kinks, and proper connections and routing as shown on Vehicle Emission Control Information label.

^ Check TCC operation. Using a scan tool should indicate an RPM drop, when the TCC is commanded ON. Refer to Transmission Control Systems.

^ Check exhaust system for possible restriction. Check for the following:

- Inspect exhaust system for damaged or collapsed pipes.

- Inspect mufflers for heat distress or possible internal failure.

- For possible plugged Three-Way catalytic converters by comparing exhaust system back pressure on each side of engine. Check back pressure by removing AIR check valves near exhaust manifolds. System Check - Restricted Exhaust

^ An engine miss condition can be caused by Electromagnetic Interference (EMI) on the reference circuit. EMI can usually be detected by monitoring engine RPM with a scan tool. A sudden increase in RPM with little change in actual engine RPM change, indicates EMI is present. If a problem exists, check routing of secondary ignition wires, high voltage components (Near Ignition Control circuits). Check distributor ground circuit for being open or loose.

^ Check the Traction Control Adjuster Assembly for being out of adjustment? Refer to Adjuster Assembly-Throttle And Cruise Control Cables.

^ Check PNP switch circuit. Refer to PNP Switch Check, or use the Tech 1 and be sure the scan tool indicates vehicle is in drive with gear selector in drive or overdrive. PNP Switch Check

^ Check for faulty motor mounts. Refer to Engine for inspection of mounts.

^ Check for intake and exhaust manifold passages for casting flash.