To understand the relationship between vacuum and ignition timing and to identify the cause of reduced fuel economy.

For non-computer assisted and some partially computer assisted vehicles, the student will:

1. Learn to identify the ignition system's vacuum sensing device.
2. Acquire the knowledge in order to test for a faulty vacuum sensing device.
3. Calibrate, for optimum fuel economy, a vacuum sensing device (when possible).

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. Improper timing can significantly change a vehicle's mpg. Three types of timing are used for gasoline engines. They are:

1. Static timing;
2. Mechanical (centrifugal) timing; and
3. Pneumatic (vacuum) timing - mechanical.

Each has a specific purpose.

1. Static timing establishes a physical relationship between engine crankshaft position and the distributor. This is fixed and should not change. "Basic" and "initial" are terms also associated in the static timing.

2. Since fuel burns at a relatively fixed rate (i.e., time to burn is in milliseconds), then the faster the engine runs the earlier the fuel must be ignited so that the millisecond time frame will remain constant. Thus, mechanical timing advances are used. A device, usually a centrifugally activated weight control balanced by a spring, advances the timing proportionate to engine RPM's. The faster the engine runs, the more timing advances. This is true up to approximately 2,700 to 3,500 RPM's. Mechanical timing is critical to acceleration and power response in a gasoline engine. This timing is added to the static timing.

3. The third timing, pneumatic (vacuum) - mechanical, provides an extra measure of timing over and above static and any mechanical advance. The vacuum advance extends the burn cycle even longer so that all the heat energy may be absorbed by the pistons resulting in more power yield per measure of fuel and more mpg. The sensing device is a spring offset, vacuum activated diaphragm. This diaphragm receives its vacuum signal from the carburetor or intake manifold. In many vehicles this signal is modified by the transmission gear selector position, by coolant temperature sensor(s), by vehicle speed sensor, or by combinations of the above. Early computer assisted vehicles retained the vacuum advance system but employed the computer to modify the signal to the diaphragm, thereby optimizing the system performance.

Total advance is the combination of initial, centrifugal, and vacuum. See Figure 1.

Figure 1

Vacuum advances are sensitive devices. By design, vacuum advances are spring offset to give zero advance. Only when a strong enough vacuum signal is present will it advance the ignition timing. The higher the vacuum signal, the greater the timing advance. High vacuum signals are indicative of low engine loads at a closed or nearly closed throttle position. At idle, when the throttle is almost completely closed, vacuum is highest; however, during idle, no vacuum signal is present at the vacuum advance. This is accomplished by tapping the vacuum for the advance system above the throttle plate. Excessive vacuum advance results in part throttle ping and a loss in mpg. Insufficient vacuum advance results in poor part-throttle response and a loss in mpg. Notice both insufficient as well as excessive vacuum advance results in a driveability problem and a reduction in mpg. Many drivers try to correct the part-throttle ping by purchasing higher octane fuels. This may "fix" the ping problem but mpg will still be reduced (see "Fuel Octane Selection Guide".) Some technicians attempt to "fix" poor part-throttle response by advancing or retarding the static timing. This may correct the part-throttle problem, but basic, wide open, and near wide open throttle positions will have excessively advanced or retarded timing. This could result in an extremely hot combustion chamber temperature and long term, severe engine damage may result. Worn cylinders, cracked piston rings, burned valves, worn valve guides, damaged piston ring lands, or holes in pistons are some of the possible long term engine damages that may occur with excessively advanced static timing. Again, owners may compensate for the pinging noises by purchasing higher octane fuel. The result is the same, lower mpg and wasted energy. (See Fuel Octane Selection Guide.) High combustion chamber deposits, low fuel economy, or poor power output can occur with retarded engine timing.

The correction for each of these problems is a complete, systematic, diagnostic approach using the manufacturer's recommended procedures and specifications. Static timing must first be set properly. Mechanical advance may then be checked and finally the vacuum advance checked. If the distributor fails any test, it should be removed for repair, recalibration, or be replaced. Prior to disconnecting a distributor's vacuum advance system, always check hose routing, system sensors, and soundness of the hoses and connections. Ultrasonic vacuum detectors or propane detectors are good diagnostic devices for locating leaking vacuum lines and connections. Improper hose routing, by- passing system sensors, failed system sensors, and/or leaking hoses account for significant numbers of malfunctioning or poor operating vacuum advance systems.

It is important to remember that even a slightly mistuned engine will deliver poor fuel economy. Optimum fuel economy may only be achieved when timing is optimized.

Use the activity guide sheet to collect data.

1. Locate a vehicle with a vacuum advance distributor, 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 for that model and emission calibration.

4. From the manual, determine the correct timing specification; static, mechanical, and vacuum advance specification.

5. Check the vehicle to determine if the vacuum routing is correct. Check the hoses and connections for soundness.

6. Check the static timing.

7. Check the mechanical timing.

8. Check the vacuum timing.

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

10. What is the recommended repair procedure?

11. After repairing, retest and road test again.

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





Emission Calibration Number___________________________________

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

B __________________________________________________________

C __________________________________________________________

3. Road test results: 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: ____________________________________________

4. From shop manual, determine:
A. Static timing:
________°BTDC @ ______ RPM

B. Mechanical timing:
________°BTDC @ ______ RPM

________°BTDC @ ______ RPM

________°BTDC @ ______ RPM

C. Vacuum timing:
________°BTDC @ ______ RPM @ ______ in Hg.

________°BTDC @ ______ RPM @ ______ in Hg.

________°BTDC @ ______ RPM @ ______ in Hg.

D. Check the vacuum schematic. Are hoses routed properly ?
______Yes _____No
If no, correct the routing.

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.

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

1._______ Total timing advance at any given RPM is a mixture of static, vacuum, and centrifugal.

2._______ Since fuel burns at a fixed rate, timing should only change when a different fuel is used.

3._______ A change in engine RPMs is compensated for with a vacuum timing device.

4._______ At wide open throttle, vacuum timing should be the greatest.

5._______ Internal engine damage may result from excessively increased timing.

6._______ Optimum fuel economy may be achieved when all timing devices are calibrated within manufacturer's optimum specification range.

For too many years, technicians have taken liberties with engine timing to correct minor, local engine performance problems. Taking these liberties can result in reduced fuel economy. A general accepted rule of thumb has been a ½ mile per gallon loss for each degree of off-standard static timing. However, off-standard centrifugal and vacuum timing negatively impacts engine power and fuel economy.

Over-advanced timing may not only damage engine components, but may also negatively impact fuel economy. Many technicians mistakenly believe that advancing timing beyond the factory specified range can improve mpg. This is inaccurate and a misconception. Some also believe that advancing the timing and burning high octane, premium fuel improves mpg. This is also a misconception. To burn high octane fuel efficiently, engines must have sufficient compression ratios. The timing advance profiles for premium fuels are different than those for regular octane fuels. This lesson attempts to bring these points home by getting the student technician to investigate the three types of timing.

1. True.

2. False. Fuel does burn at a fixed rate: Engine timing, compression ratio, and timing profiles are inherent to each design. Altering timing profiles changes that fuel's burn time. As a result, fuel energy would be wasted due to incomplete combustion.

3. False. Centrifugal timing compensates for changes in RPM's. Vacuum timing compensates for engine load.

4. False. Wide open throttle vacuum approaches zero thus, no vacuum timing is added to basic and centrifugal.

5. True. Many engine overhaul technicians fail to associate some internal engine damage with incorrect timing. Failure to recalibrate engine timing after an engine overhaul could lead to a repeat failure.

6. True. This is what needs to be done to optimize fuel economy for a given vehicle.

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

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