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2008 - MX-5 - Engine

NO.14 POOR FUEL ECONOMY [LF]



14

POOR FUEL ECONOMY

DESCRIPTION

Fuel economy is unsatisfactory.

POSSIBLE CAUSE

  • Contaminated air cleaner element

  • Variable intake air control malfunction

  • Engine cooling system malfunction

  • Improper ATF level (AT)

  • Weak spark

  • Poor fuel quality

  • Erratic or no signal from CMP sensor

  • Clutch slippage (MT)

  • Improper variable valve timing control system operation

  • Improper coolant level

  • Inadequate fuel pressure

  • Spark plug malfunction

  • PCV valve malfunction

  • Brake dragging

  • Improper valve timing due to jumping out of timing belt

  • Contaminated MAF sensor

  • Improper engine compression

  • Exhaust system clogging

WARNING:

The following troubleshooting flow chart contains fuel system diagnosis and repair procedures. Read following warnings before servicing 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 [LF].)

    (See AFTER SERVICE PRECAUTION [LF].)

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

1

Inspect for the following:

  • Air cleaner element for contamination

  • ATF level (AT)

  • Fuel quality

  • Coolant level

  • Brake dragging

  • Clutch slippage (MT)

Are all items normal?

Yes

Go to the next step.

No

Service if necessary.

Repeat Step 1.

2

Connect the M-MDS to the DLC-2.

Turn the ignition switch to the ON position (Engine off).

Retrieve any DTCs.

Are there any DTCs displayed?

Yes

DTC is displayed:

Go to the appropriate DTC inspection.

(See DTC TABLE [LF].)

No

No DTC is displayed:

Go to the next step.

3

Access ECT PID.

Drive vehicle while monitoring PID.

(See PCM INSPECTION [LF].)

Is PID within specification?

Yes

Go to the next step.

No

Inspect for coolant leakage, cooling fan and condenser fan operations or thermostat operation.

4

Perform the spark test.

(See ENGINE CONTROL SYSTEM OPERATION INSPECTION [LF].)

Is strong blue spark visible at each cylinder?

Yes

Go to the next step.

No

Repair or replace the malfunctioning part according to spark test result.

5

Install the fuel pressure gauge between the fuel pipe and the fuel distributor.

Start the engine and idle it.

Measure fuel line pressure during idle.

Is fuel line pressure correct during idle?

(See FUEL LINE PRESSURE INSPECTION [LF].)

Yes

Go to the next step.

No

Low:

Inspect the fuel pump relay and fuel pump circuit.

Inspect the fuel line for clogging.

If there is no malfunction, replace the fuel pump unit

(See FUEL PUMP UNIT REMOVAL/INSTALLATION [LF].)

High:

Replace the fuel pump unit.

(See FUEL PUMP UNIT REMOVAL/INSTALLATION [LF].)

6

Inspect for variable valve timing control system operation.

(See ENGINE CONTROL SYSTEM OPERATION INSPECTION [LF].)

Does the variable valve timing control system function properly?

Yes

Go to the next step.

No

Repair or replace the malfunctioning part.

7

Inspect for the variable intake air control operation.

(See ENGINE CONTROL SYSTEM OPERATION INSPECTION [LF].)

Does variable intake air control function properly?

Yes

Go to the next step.

No

Repair or replace the malfunctioning part.

8

Remove and shake the PCV valve.

Does the PCV valve rattle?

Yes

Go to the next step.

No

Replace the PCV valve.

9

Visually inspect the exhaust system part.

Is there any deformed exhaust system?

Yes

Replace the suspected part.

No

Go to the next step.

10

Inspect for contaminated MAF sensor.

Is there any contamination?

Yes

Go to the next step.

No

Inspect for cause.

11

Inspect the MAF sensor for contamination.

Is there any contamination?

Yes

Replace MAF sensor.

No

Go to the next step.

12

Is engine compression correct?

Yes

Inspect the valve timing.

No

Inspect for cause.

13

  • Verify test results.

    • If normal, return to diagnostic index to service any additional symptoms.

      (See SYMPTOM DIAGNOSTIC INDEX [LF].)

    • If malfunction remains, inspect related Service Bulletins and/or On-line Repair 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 [LF].)




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