NO.14 POOR FUEL ECONOMY [MZI-3.7]

id0103d2802000

14 | POOR FUEL ECONOMY
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DESCRIPTION | • Fuel economy is unsatisfactory.
POSSIBLE CAUSE | • Contaminated air cleaner element • Improper ATF level • Poor fuel quality • Improper coolant level • Brake dragging • Improper tire pressure • PCM DTC is stored. • Engine cooling system malfunction • Weak spark ― Ignition system malfunction (ignition coil, ignition coil related circuit or spark plug malfunction) • Inadequate fuel pressure ― Intermittent open or short circuit in fuel pump circuit ― Fuel pump mechanical malfunction ― Fuel line restriction or clogging ― Pressure regulator malfunction (built-in fuel pump unit) • Improper variable valve timing control system operation • Exhaust system clogging • Contaminated MAF sensor • Improper engine compression • PCV valve malfunction • Improper valve timing due to jumping out of timing chain Warning • The following troubleshooting flow chart contains the fuel system diagnosis and repair procedures. Read the following warnings before servicing the fuel system: ― Fuel vapor is hazardous. It can easily ignite, causing serious injury and damage. Always keep sparks and flames away from fuel. ― Fuel line spills and leakage are dangerous. Fuel can ignite and cause serious injuries or death and damage. Fuel can also irritate skin and eyes. To prevent this, always complete “BEFORE SERVICE PRECAUTION” and “AFTER SERVICE PRECAUTION” described in this manual. (See BEFORE SERVICE PRECAUTION [MZI-3.7].) (See AFTER SERVICE PRECAUTION [MZI-3.7].) Caution • Disconnecting/connecting quick release connector without cleaning it may cause damage to fuel pipe and quick release connector. Always clean quick release connector joint area before disconnecting/connecting, and make sure that it is free of foreign material.

Diagnostic procedure

STEP | INSPECTION | RESULTS | ACTION
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1 | Inspect for the following: • Air cleaner element for contamination • ATF level • Fuel quality • Coolant level • Brake dragging • Tire pressure Are all items normal? | Yes | Go to the next step.
No | Repair or replace the malfunctioning part according to the inspection results. Repeat Step 1.
2 | Turn the ignition switch to the ON position (Engine off). Retrieve any PCM DTCs using the M-MDS. (See ON-BOARD DIAGNOSTIC TEST [MZI-3.7].) Are there any DTCs displayed? | Yes | DTC is displayed: • Go to the appropriate DTC inspection. (See DTC TABLE [MZI-3.7].)
No | No DTC is displayed: • Go to the next step.
3 | Caution • While performing this step, always operate the vehicle in a safe and lawful manner. • When the M-MDS is used to observe monitor system status while driving, be sure to have another technician with you, or record the data in the M-MDS using the PID/DATA MONITOR AND RECORD capturing function and inspect later. Access the PCM PID ECT using the M-MDS. (See ON-BOARD DIAGNOSTIC TEST [MZI-3.7].) Drive the vehicle while monitoring the PID. Is the PID within the specification? (See PCM INSPECTION [MZI-3.7].) | Yes | Go to the next step.
No | Inspect for following: • Coolant leakage • Cooling fan and condenser fan operations • Thermostat operation Repair or replace the malfunctioning part according to the inspection results.
4 | Perform the spark test. (See ENGINE CONTROL SYSTEM OPERATION INSPECTION [MZI-3.7].) Is strong blue spark visible at each cylinder? | Yes | Go to the next step.
No | Repair or replace the malfunctioning part according to the inspection results.
5 | Install the fuel pressure gauge between fuel pipe and fuel distributor. Start the engine and run it at idle. Measure the fuel line pressure during idle. (See FUEL LINE PRESSURE INSPECTION [MZI-3.7].) Is the fuel line pressure correct during idle? | Yes | Go to the next step.
No | Zero or low: • Inspect the following: ― Fuel pump relay ― Fuel pump circuit ― Fuel line for clogging • If there is no malfunction, replace the fuel pump unit. (Fuel pump or pressure regulator malfunction) (See FUEL PUMP UNIT REMOVAL/INSTALLATION [MZI-3.7].) High: • Replace the fuel pump unit. (Pressure regulator malfunction) (See FUEL PUMP UNIT REMOVAL/INSTALLATION [MZI-3.7].)
6 | Inspect the variable valve timing control system operation. (See ENGINE CONTROL SYSTEM OPERATION INSPECTION [MZI-3.7].) Does the variable valve timing control function properly? | Yes | Go to the next step.
No | Repair or replace the malfunctioning part according to the inspection results.
7 | Visually inspect the exhaust system part. Is there any deformed exhaust system part? | Yes | Repair or replace the malfunctioning part according to the inspection results.
No | Go to the next step.
8 | Inspect for contaminated MAF sensor. Is there any contamination? | Yes | Go to the next step.
No | Inspect for cause. Repair or replace the malfunctioning part according to the inspection results.
9 | Is the engine compression correct? | Yes | Go to the next step.
No | Inspect for cause. Repair or replace the malfunctioning part according to the inspection results.
10 | Note • The PCV valve cannot be reassembled once it is removed because the tabs will have bee n broken. Inspect the PCV valve without removing it from the vehicle. Inspect the PCV valve with it installed. (See POSITIVE CRANKCASE VENTILATION (PCV) VALVE INSPECTION [MZI-3.7].) Is the PCV valve normal? | Yes | Inspect the valve timing. Repair or replace the malfunctioning part according to the inspection results.
No | Replace the PCV valve. (See POSITIVE CRANKCASE VENTILATION (PCV) VALVE REMOVAL/INSTALLATION [MZI-3.7].)
11 | • Verify test results. ― If normal, return to diagnostic index to service any additional symptoms. (See SYMPTOM DIAGNOSTIC INDEX [MZI-3.7].) ― If malfunction remains, inspect related Service Information and perform repair or diagnosis. • If vehicle is repaired, troubleshooting completed. • If vehicle is not repaired or additional diagnostic information is not available, replace the PCM. (See PCM REMOVAL/INSTALLATION [MZI-3.7].)