19.8.
Engine Tune-up
19.8.1.
Tune-up Requirements
The routine minor tune-up is usually carried out to restore engine performance after running of a number of kilometers. The minor tune-up is the type of tune-up usually done by service stations and by individuals who maintain their own automobiles using common tools and a limited amount of test equipments. When a minor tune-up does not rectify a problem, more extensive use of test equipment is required to pinpoint the cause of a problem. Most problems can be detected during a major tune-up or by specialized test made on the specific system, which is malfunctioning.
An engine tune-up is a procedure performed on an engine to make it develop its best performance while it produces the lowest possible emissions. It is usually done at periodic intervals when engine performance has deteriorated or when the operator wants to be sure of having dependable service on a trip or in winter weather. A tune-up may also be run as a troubleshooting sequence when some abnormal engine condition is indicated.
The minor tune-up consists of a visual inspection, routine mechanical service, and instrument tests. The visual inspection includes checking the engine mounting and attachments for abnormal conditions, such as inspecting the condition of all wires, hoses, belts, and carburettor linkages. The engine should be examined for coolant, fuel, oil, and exhaust leaks. Any defects that could affect engine operation or vehicle safety should be repaired. This might include mechanical service such as freeing the heat riser, retorquing belts, and adjusting belts. Mechanical service also includes servicing the spark plugs and ignition points. It usually includes cleaning or replacing air and fuel filters and making sure that the vehicle has an adequate supply of oil, coolant, and brake fluid. Instrument checks include engine basic timing, operation of timing advance mechanisms, and setting engine idle. The minor tune-up might also include the cylinder balance test and a check of the emission-control component operation.
A tune-up cannot produce satisfactory engine operation if the combustion chamber doesn’t hold compression. Most tune-up technician follow a battery check by removing the spark plugs and checking each combustion chamber for its ability to compress the gases. This can be done either by a compression test or a cylinder leakage test.
19.8.2.
Tune-up Sequence
The engine tune-up restores driveability, power, performance, emission control, and economy that have been lost through wear, corrosion, and deterioration of engine parts. A tune-up sequence for an automobile engine is presented below.
(i) First operate the cold engine (normal for 20 minutes at 1500 rpm) until it reaches operating temperature. Any operational problems during this warm-up time should be observed.
(«) Connect the oscilloscope and exhaust-gas analyzer appropriately. Check for any
abnormal condition and, if possible, the cylinder(s) in which it appears. (Hi) Remove all spark plugs, and fully open the throttle and choke valves. Disconnect the distributor lead from the terminal of the primary coil to avoid excessive secondary voltage.
(iv) Check the compression of each cylinder. If one or more cylinder read low, squirt about 15 cc of engine oil through the spark-plug hole and recheck the compression. For further diagnosis of engine mechanical problems, perform a cylinder leakage test.
(v) Clean, inspect, file, and test the spark plugs. Reject worn or defective spark plugs. Many technicians install all new plugs instead of servicing the old plugs. Gap correctly all plugs, both old and new and then install them.
(vi) Inspect and clean the battery case, terminals, cables, and hold-down brackets. Test the battery. Add water, if necessary. Incase of severe corrosion, clean the battery and cables with brushes and a solution of baking soda and water. If the battery is old, it might have worn out, and then a new battery may be required.
(vii) Test the cranking voltage. If the battery is in good condition but cranking speed is low, then check the starting system.
(viii) If the battery voltage is low or the customer complains that the battery keeps running down, check the charging system (alternator and regulator).
(ix) Check the drive belts and replace if they are in poor condition. If it is required to replace one belt of a two-belt drive system replace both belts. Check the belt tension, using a belt-tension gauge.
Ox) Inspect the distributor rotor, cap, and primary and high voltage (spark-plug) wires.
(xi) Clean or replace and adjust distributor contact point gap. Lubricate the distributor breaker cam. On distributors with round cam lubricators, turn the cam lubricator 180 degrees or replace it if required.
(xii) Check the distributor cap and rotor. Check also the centrifugal and vacuum advances. Set the dwell and then adjust ignition timing. Ensure that the idle speed is not too high as this can produce centrifugal advance during timing adjustment.
(xiii) Recheck the ignition system with the oscilloscope. Any abnormal conditions that
appeared in step («) should now be eliminated. (xiu) Inspect the manifold heat-control valve and lubricate it with heat-valve lubricant. Free
or replace the valve if necessary.
(xv) Check the operation of fuel-pump using a fuel-pump tester. Replace the fuel filter. Check the fuel-tank cap, fuel lines, and connections for leakage and damage.
(xvi) Clean or replace the filter of the air-cleaner. In case the engine is fitted with a thermostatic air cleaner, check the operation of the vacuum motor.
(xvii) Check the functioning of the choke and the fast-idle cam. Also check the throttle valve for its full opening, and the throttle linkage for its free movement. Clean all external carburettor linkages.
(xviii) Inspect all engine vacuum fittings, hoses, and connection. Replace brittle or cracked hose if any.
(xix) If the engine is fitted with a filter-type oil-filler cap, clean the engine oil-filter cap.
(xx) Inspect in the cooling system all hoses and connections, the radiator, water pump, and fan clutch, if used. Check and maintain the strength of the antifreeze. Pressure-check the system and radiator cap. Replace defective hose, if any.
(xxi) Check and replace the PCV valve if necessary. Clean or replace the air filter of the PCV-system, if required. Inspect the PCV hoses and connections. Replace any cracked or brittle hose. Test the system for vacuum sealing.
(xxii) In case the engine is fitted with an air pump, replace the pump inlet air filter, if necessary. Check and replace the system hoses and connections, if required.
(xxiii) Replace the charcoal-canister air filter, if necessary, in the evaporative emission control system.
(xxiv) Check the transmission-controlled spark system if fitted with the vehicle.
(xxv) Inspect and clean the EGR valve, and EGR discharge port. Apply vacuum to the valve with engine at idle and check system operation. If engine runs rough, then smooth out without application of vacuum.
(xxvi) If the engine is fitted with a turbocharger, check the operation of the waste-gate as per the manufacturer’s instruction.
(xxvii) Tighten the intake-manifold and exhaust-manifold bolts to the specified torque following the proper sequence.
(xxviii) Check and adjust the engine valves if required.
(xxix) Adjust the engine idle speed. Adjust the idle-mixture screw on older cars using an exhaust-gas analyzer, and propane if required. On newer cars, as the screw is sealed, it cannot be adjusted without disassembling the carburettor. Check the amount of HC
and CO in the exhaust gas. It is preferable to check the HC and CO both before and after a tune-up to know how much the tune-up has reduced these pollutants.
(xxx) Road-test the car on a dynamometer or on the road. Check for driveability, power, and idling. Any abnormal condition should be noted for repair.
(xxxi) Check the lubrication or refer to the maintenance sticker to know whether an oil and oil-filter change is due. Also note the schedule for chassis lubrication. Recommend an oil change and chassis lubrication if they are due.
(xxxii) Whenever the speed is raised, check the exhaust system for leaks, which can allow carbon monoxide into the passenger compartment. Also check for loose bolts, rust spots, and other under-the-car damage.
19.8.3.
Tune-up Tests Procedure
Several tests are conducted before the engine is started. In addition to the battery check, the breaker point resistance, the cranking voltage, cranking vacuum, and cranking coil output may be checked. Attach the instruments, run the engine at idle and check for dwell, idle speed, and basic engine timing.
To use timing light, connect the pick-up cable to number one ignition cable and connect the light to its proper power source, either the vehicle battery or the shop 230V/110V line. It is most important on most emission-controlled engine to attach this timing light pickup to number one spark plug cable even if it is not as accessible as one of the other spark plugs. Set the parking brake and place the transmission in neutral, then start the engine. Let it run slowly at curb idle with the vacuum line to the distributor removed and plugged so that air does not enter into the carburettor. With the timing light aimed at the timing marks, and the hold-down clamp slightly loose, adjust the distributor until ignition occurs at the specified degree. Tighten the hold-down clamp and recheck the timing. Readjust the timing if necessary, and then reconnect the distributor vacuum line.
Two instruments are used to test the overall operating system. The ignition scope shows voltage being developed in the ignition system and the hydrocarbon-carbon monoxide (HC-CO) tester shows the relative amount of these gases being produced by the engine. Once trained to recognize scope patterns and HC-CO values, a quick but detailed look at the patterns and instrument readings as the engine speeds are changed provide an overall indication of engine operation. Properly using these testers, one can analyze the functioning of the entire system and can pinpoint the cause of a problem, if exists.
Required voltage is considered to be the voltage indicated by the top of the spike on a scope pattern. The engine runs as long as the ignition system has more voltage available than that required by the engine. The ignition system available voltage can be measured on the scope by setting the scope pattern dwell line on the zero volt line and then removing one of the spark plug cables to force a voltage increase on that cable, causing the ignition system to develop full voltage. For normal operation the ignition system should be able to produce more than 20 kV available to fire the spark plug. High-energy systems produce over 30 kV.
The HC-CO tester is a useful tool for checking the overall operation of the engine. Ti amount of carbon monoxide available in the exhaust is affected by the carburettor operation. High carbon monoxide results from rich mixtures and low carbon monoxide from lean mixtures. During normal operation of the engine, low hydrocarbon emissions are produced. Incomplete combustion produces excessive hydrocarbons in the exhaust. High hydrocarbon readings result
from rich or excessively lean mixtures, and from engine misfiring. It also is high if the ignition timing is not correct. This could be the result of an inoperative emission-control unit. In general terms, the engine idle should be adjusted to operate with the lowest hydrocarbon reading possible, and this requires lean carburettor mixtures. Emissions should decrease as the engine speed is increased above idle.
The engine is brought up to a specified test speed, between 1500 and 2000 rpm or the one recommended by the manufacturer. The engine scope indicates coil polarity, required voltage, available voltage, breaker point action, ignition system leakage (shown as the tail missing on the available voltage spike in Fig. 19.18), and ignition system resistance. The HC-CO tester indicates the engine operating condition. Interpretation of these readings indicates the problems that exist in the engine systems.
Fig. 19.18. Secondary leakage indicated by the tail missing from the available voltage spike.
The engine is returned to idle to check idle speed, manifold vacuum, and HC-CO if the tester is used. Quick accelerations from idle put a load on the park plugs and the engine scope indicates if the spark plugs are firing normally. The HC-CO tester indicates momentary high readings as the acceleration pump operates.
The instrument test may precede or follow standard tune-up service operations, such as distributor service, carburation service, air and fuel filter service, and valve lash adjustment on solid lifter engines. Test may indicate the necessary additional tests or adjustments. These include battery circuit voltage drop, starter amperage draw, generator output, charging circuits voltage drop, regulator setting, ignition primary circuit voltage drop, fuel pump tests, carburettor rebuilding, and tests of the filler cap and thermostat of the cooling system.
The tune-up sequence may be used before or after routine service. Running the tune-up test sequence before performing routine service points to items that require special service, limits the servicing, insures satisfactory workmanship and customer satisfaction.
Tune-up test sequence differ for each make of test equipment. Each manufacturer provides a test sequence to be followed with his equipment, and also usually supplies specifications. Automotive shop manuals also give tune-up test sequences. These are often keyed to one type of test equipment, either commercial types or a type manufactured especially for the vehicles manufacturer.
Basically, all shop test equipment is a special application of voltmeters, oscilloscopes, ohmmeters, tachometers, dwell meters, vacuum gauges, and exhaust gas analyzers. Individual test units are packaged in cases and cabinets with interconnecting switches, shunts, rheostats, and electronic controls so that many tests can be made with few engine connections. The maximum difference in test procedures results from the various methods adopted to interconnect the basic test equipments. It is important to follow the procedures provided with the test equipment being used to get meaningful results.
