VACUUM, TIMING AND FUEL ECONOMY:
COMPUTER ASSISTED ENGINES

GOALS:

To understand the relationship between vacuum and ignition timing and reduced fuel economy in computer assisted engines.

OBJECTIVES:

For computer assisted vehicles, the student will learn:

1. To identify the vacuum sensing device in the ignition system,

2. To perform the test used to detect a faulty vacuum sensing device,

3. To calibrate, for optimum fuel economy, a vacuum sensing device (when possible).

LESSON/INFORMATION:

Ignition timing is the single most critical adjustment for a gasoline engine. Assuming the engine is in good mechanical condition, there are no vacuum leaks, and the fuel mixture is correct, then the ignition timing can affect the net power yield from the fuel; the end result can significantly change a vehicle's mpg. Thus, for optimum fuel economy, timing must be optimized. Three types of timing are used for gasoline engines. They are:

1. Static timing;

2. Mechanical (centrifugal); and

3. Pneumatic (vacuum) - mechanical.

Static timing establishes a physical relationship between engine crankshaft position and the distributor. This is fixed and should not change.

Since fuel burns at a relatively fixed rate (i.e., time to burn is in milliseconds) then the faster the engine runs, the sooner the fuel must be ignited so that the millisecond time frame will remain constant. Pre-computer assisted vehicles use mechanical and pneumatic-mechanical devices to accomplish these timing advances.

Computer assisted vehicles use various sensors to modify the computer controlled timing program. A manifold vacuum and/or barometric pressure, throttle position, engine RPM, coolant temperature, and vehicle speed sensor all modify the timing program. The timing program changes are the equivalent of centrifugal and vacuum timing devices formerly used. Computer controlled engines change timing more rapidly than mechanical systems. Computer systems are more closely attuned to optimum timing points than the former mechanical devices. Optimum timing results in optimum fuel economy and reduced tailpipe emissions. Some manufacturers have field serviceable computer calibration chips. These chips modify the computer program and profile for that particular vehicle with specified accessories and equipment. If a vehicle has an improperly calibrated chip, reduced power, lowered mpg, and poor driveability may result. Some aftermarket "hot-rodders" replace calibration chips to optimize engine output for racing purposes. Generally, these lower mpg and increase exhaust emissions. Some are illegal for on-road usage.

Each manufacturer specifies different diagnostic techniques to test for each sensor and for the correct engine calibration chip. Some first generation and most of the latest generation systems use a self testing diagnostic system to detect a non-standard calibration chip. These types provide "memory codes" for failed sensors. All vehicles still require correctly adjusted static timing. In distributorless engines, static timing is accomplished by a crankshaft or camshaft sensor. These sensors must be correctly installed to accomplish correct static timing. Exact procedures MUST be adhered to in establishing this timing reference point. Some manufacturers do provide some checking specifications for timing advance for certain driving conditions. Some use a portable recording device to validate computer operation during specified driving cycles. Others give no "dynamic" timing tests for computer timing and fuel profiles. These require in-shop sequential testing procedures.

ACTIVITY:

Use the activity guide sheet provided to collect data.

1. Locate a vehicle with a computer controlled fuel and ignition system, preferably a vehicle with a mpg or driveability problem. Quiz the driver/owner for symptoms. Determine fuel/octane normally purchased. Road test the vehicle, preferably with the driver/owner. Note its part-throttle, near wide-open throttle, and wide-open throttle responses.

2. Locate an appropriate shop manual.

3. Determine the correct vacuum schematic and emission calibration for that model.

4. Determine the diagnostic process for the on-board computer's memory. List all codes and follow the correct diagnostic procedures.

5. From the manual, determine the correct timing specifications and procedures for checking and adjusting the timing.

6. Which specifications, if any, are out of tolerance?

7. What are the recommended repair procedures?

8. After repairing, clear codes, retest, and clear any failure codes. Road test again. Recheck for any new or reappearing trouble codes. Repair as needed.

9. Summarize your findings. Be sure to recommend that the driver/owner uses the correct octane fuel. Be specific!

1.

Year___________________________________________________

Make___________________________________________________

Model___________________________________________________

Emission Calibration Number________________________________

2.Driver/operator complaints related to mpg and driveability.

A ________________________________________________________

B ________________________________________________________

C ________________________________________________________

3. Road test results: Note: Adhere to local shop policy concerning road test procedures. Be sure to wear seat belt. Observe traffic laws and speed limits.

A. idle: _____ Normal _____ Poor _____ Indeterminate

B. off idle: _____ Normal _____ Poor _____ Indeterminate

C. Cruise: _____ Normal _____ Poor _____ Indeterminate

D. Part Throttle _____ Normal _____ Poor _____ Indeterminate

E. Wide Open Throttle _____ Normal _____ Poor _____ Indeterminate

Did you experience ping? _____ No _____ Yes

If yes, describe: ____________________________________________

Did you experience poor throttle response? _____ No _____Yes

If yes, describe: ____________________________________________

4. From shop manual, determine:

Static timing: ________°BTDC @ ______ RPM

5. What is the general condition of the hoses?

_____ Acceptable _____ Poor _____ If poor, correct.

6. What sensors, if any, are used? List them. How are each tested? Test each sensor for proper operation. Correct any which fail.

Sensor			Test Procedure		Results

_____________	____________________________    _____Pass _____Fail

_____________	____________________________    _____Pass _____Fail

_____________	____________________________    _____Pass _____Fail

_____________ 	____________________________    _____Pass _____Fail

7. Perform the timing tests. Adjust, calibrate or replace any component that does not meet specification.

Adjustments Made			Parts Replaced

________________________________	___________________________

________________________________	___________________________

________________________________	___________________________

________________________________	___________________________

8. Re-road test vehicle after all repairs. Re-evaluate each item.

Note: Be sure to wear seat belt. Observe traffic laws and speed limits.

A. Idle: _____Normal _____Poor _____Indeterminate

B. Off idle: _____Normal _____Poor _____Indeterminate

C. Cruise: _____Normal _____Poor _____Indeterminate

D. Part Throttle _____Normal _____Poor _____Indeterminate

E. Wide Open Throttle _____Normal _____Poor _____Indeterminate

Did you experience ping? _____No _____Yes

If yes, describe: ____________________________________________

Did you experience poor throttle response? _____No _____Yes

If yes, describe: ____________________________________________

Note any performance changes from first road test.

Performance changes: __________________________________________

______________________________________________________________

_______________________________________________________________

Recommended fuel octane: _______________________________________

9. Complete repair report to vehicle driver/owner.

INFORMATION CHECK

Directions: Indicate whether the statements below are true or false. If the statements is false, explain why it is false.

1._______ Computers perform the function of former centrifugal and vacuum timing systems.

2._______ Static timing adjustments are not possible with computer controlled vehicles.

3._______ All possible ignition malfunctions are logged in the computer's memory.

4._______ Some after market computer chips may not be street legal.

5._______ Computers are able to control engine timing more rapidly and more accurately.

TEACHER'S NOTES

Students need to know how to check and set basic timing on computer assisted vehicles. This adjustment must be set as accurately as possible. Variances of as little as ½° out of specification can adversely affect driveability, emissions, and mpg. Use of a digital timing light or magnetic pickup timing device can greatly improve the static timing accuracy. While a computer assisted vehicle's timing will not "drift," shift, or change as in older non-electronic systems, poorly trained and hobbiest/owners may attempt to gain performance or "fix" a problem by indiscriminate timing changes.

ANSWERS TO INFORMATION CHECK:

1. True

2. False, static timing must be properly adjusted for the computer to properly control the engine's system.

3. False, static timing and brief intermittent failures may not set failure codes.

4. True

5. True

REFERENCES:

Ellinger, Herbert E. Automechanics, 4th Edition. Prentice Hall, Englewood Cliffs, NJ. 1988.

Comments or questions to: TechAsmt@LA.GOV

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