The word "tune-up" actually applies only to older cars, on which you can perform the traditional work associated with the term-spark plug replacement, ignition contact point replacement, dwell adjustment, ignition timing adjustment and carburetor idle and mixture adjustment. For most of today's cars, “engine performance maintenance” is a more accurate term. All modern cars and light trucks are equipped electronic ignition (no points) and at least one on-board computer that automatically adjusts items like the ignition timing, fuel mixture and idle speed. In fact, on modern computer-controlled cars, it's impossible to adjust these yourself.
An automotive tune-up is an orderly process of inspection, diagnosis, testing, and adjustment that is periodically necessary to maintain peak engine performance or restore the engine to original operating efficiency.
On an electronic ignition system, the basic tune-up procedures are as follows:
- Remove spark plugs
- Test compression in each cylinder.
- Clean and/or replace spark plugs and gap spark plugs to manufacturer’s specifications and install in engine.
- If applicable, check the distributor cap and rotor for cracks and wear. Replace if necessary.
- Use tachometer to set idle speed to specifications (if adjustable).
- Use timing light to set initial timing (most electronic ignition systems do not require adjustments).
The list above represents only the “bare bones” facts about tune-ups. Other functions that could be performed are using an emissions analyzer to check for compliance with emissions standards and for fuel economy, and measuring vacuum and checking for leaks with a vacuum gauge.
The tune-up is also a good opportunity to perform a general preventive maintenance check on everything in the engine compartment, and look for failed or about to fail components such as loose or damaged wiring, leaking fuel lines, cracked coolant hoses, and frayed belts.
The purpose of this section of the ProCarCare Maintenance Guide is to provide the non-mechanic with some simple inspections he or she can perform without specialized tools – chiefly on spark plugs, which can indicate many kinds of engine problems. This information is intended as a guide to use in talking with your service center. If you’re a do-it-yourselfer, you’ll want to obtain a car-care manual for your year, make and model of car for the actual tune-up procedures.
Spark plug life and efficiency depend upon the condition of the engine and the combustion chamber temperatures to which the plug is exposed. These temperatures are affected by many factors, such as compression ratio of the engine, air/fuel mixtures, exhaust emission equipment, and the type of driving you do.
Factory installed plugs are, in a way, compromise plugs, since the factory has no way of knowing what sort of driving you do, but most people never have reason to change their plugs from the factory-recommended heat range.
Spark plug heat range
Spark plug heat range is the ability of the plug to dissipate heat. The longer the insulator (or the farther it extends into the engine), the hotter the plug will operate; the shorter the insulator (the closer the electrode is to the block's cooling passages) the cooler it will operate. A plug that absorbs little heat and remains too cool will quickly accumulate deposits of oil and carbon since it is not hot enough to burn them off. This leads to plug fouling and consequently to misfiring. A plug that absorbs too much heat will have no deposits but, due to the excessive heat, the electrodes will burn away quickly and might possibly lead to pre-ignition or other ignition problems. Pre-ignition takes place when plug tips get so hot that they glow sufficiently to ignite the air/fuel mixture before the actual spark occurs. This early ignition will usually cause a pinging during low speeds and heavy loads.
The general rule of thumb for choosing the correct heat range when picking a spark plug is: if most of your driving is long distance, high speed travel, use a colder plug; if most of your driving is stop and go, use a hotter plug. Original equipment plugs are generally a good compromise between the 2 styles and most people never have the need to change their plugs from the factory recommended heat range.
Reading spark plugs
Your spark plugs are the single most valuable indicator of your engine's internal condition. Study your spark plugs carefully every time you remove them. Compare them to illustrations shown to identify the most common plug conditions.
Replacing spark plugs
New technologies in spark plug and ignition system design have pushed the recommended replacement interval to 60,000 miles (96,540 km) or even 100,000 miles (160,900 km). However, this depends on car usage and driving conditions. This holds true unless internal engine wear or damage and/or improperly operating emissions controls cause plug fouling. If you suspect this, you may wish to remove and inspect the plugs before the recommended mileage. Most platinum plugs should not be cleaned or re-gapped. If you find their condition unsuitable, they should be replaced.
A set of standard spark plugs usually requires replacement after about 20,000-30,000 miles (32,180-48,270 km) on cars with electronic ignition. These figures are dependent on your particular style of driving, however. The electrode on a new spark plug has a sharp edge, but with use, this edge becomes rounded by wear, causing the plug gap to increase. In normal operation, plug gap increases about 0.001 inch (0.25mm) for every 1,000-2,500 miles (1,609-4,022 km). As the gap increases, the plug's voltage requirement also increases. It requires a greater voltage to jump the wider gap and about two to three times as much voltage to fire a plug at high speeds than at idle. The improved air/fuel ratio control of modern fuel injection combined with the higher voltage output of modern ignition systems will often allow an engine to run significantly longer on a set of standard spark plugs, but keep in mind that efficiency will drop as the gap widens (along with fuel economy and power).
Distributor cap inspection
At every tune-up visually check the distributor cap and rotor for damage, burns or corrosion. Check the spark plug towers and their terminals under the cap to be sure they are free of corrosion that would inhibit proper spark distribution. Replace any damaged or worn components. One often overlooked part of the distributor cap inspection are the terminals on the underside. Small burn spots or deposits of corrosion here can lead to engine misfiring.
Inspect the rotor for cracks, excessive burning of the contacts and mechanical damage, and replace as necessary. Slightly burned contacts should be sanded smooth.
While primary wiring is less perishable than the secondary circuit, it should be checked for cracked insulation or loose connections. Tighten connections or replace wires as necessary.
If the spark plug wires have become unserviceable due to time and wear, it is probably a good idea to replace the cap and rotor as well..
On many late model cars the ignition timing is completely controlled by the on-board computer and is not adjustable. The following is a generic procedure for a point of reference. Check the under-hood emissions sticker or refer to model-specific instruction books for specific information on your car.
Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each of the cylinders. It is measured in degrees before or after Top Dead Center (TDC) of the compression stroke.
Because it takes a fraction of a second for the spark plug to ignite the mixture in the cylinder, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not be completely ignited as the piston passes TDC and the full power of the explosion will not be used by the engine.
The timing measurement is given in degrees of crankshaft rotation before the piston reaches TDC (BTDC). If the setting for the ignition timing is 5° BTDC, the spark plug must fire 5° before each piston reaches TDC. This only holds true, however, when the engine is at idle speed.
As the engine speed increases, the pistons go faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC. To do this, distributors have various means of advancing the spark timing as the engine speed increases. On older cars, this was accomplished by centrifugal weights within the distributor along with a vacuum diaphragm mounted on the side of the distributor. Later cars are equipped with an electronic spark timing system in which no vacuum or mechanical advance is used, instead all timing changes electronically based on signals from various sensors.
If the ignition is set too far advanced (BTDC), the ignition and expansion of the fuel in the cylinder will occur too soon and tend to force the piston down while it is still traveling up. This causes engine ping. If the ignition spark is set too far retarded, after TDC (ATDC), the piston will have already passed TDC and started on its way down when the fuel is ignited. This will cause the piston to be forced down for only a portion of its travel. This will result in poor engine performance and lack of power.