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FAQ: Tune-Ups
FAQ (Frequently Asked Questions) >Tune-Ups
Basic emission control systems and what they doDo engines with distributorless ignitions need tune-upsIs it better to clean or replace dirty fuel injectorsIs it better to rebuild or replace a carburetor or throttle body
The difference between throttle body and multi-point injectionWhat are the recommendations for changing filtersWhat are the rules for replacing catalytic convertersWhat causes spark knock and how do you get rid of it
What kind of fuel hose is bestWhat should a complete tune-up includeWhy are there many different types of spark plugs
Basic emission control systems and what they do

Automotive-generated pollutants come from three sources: tailpipe, crankcase blowby vapors and fuel vapors that evaporate from the fuel tank and carburetor.

Crankcase blowby vapors are eliminated as a source of pollution by recirculating the vapors into the engine for reburning through the Positive Crankcase Ventilation (PCV) system.

The PCV valve acts like a small calibrated vacuum leak, allowing manifold vacuum to siphon air through the crankcase, taking with it moisture and blowby gases that would otherwise pollute the atmosphere. As a side benefit, it extends motor oil life.

Evaporative emissions have been eliminated by sealing the fuel system and storing vapors in a charcoal canister. When the engine starts, a purge valve on the canister opens, allowing manifold vacuum to siphon vapors into the intake manifold to be burned in the engine.

There are 3 primary tail pipe pollutants:

Carbon monoxide (CO) is formed whenever there is not enough oxygen to completely burn the fuel. The richer the mixture, the greater the quantity of CO produced.

Carbon monoxide is the worst pollutant of the three because it is deadly. CO emissions are reduced by keeping the air/fuel ratio lean, by preheating incoming air and manifold to aid fuel vaporization, and by converting the remaining CO into harmless carbon dioxide in the catalytic converter.

Hydrocarbon (HC) emissions are unburned gasoline. HC is not directly harmful, but it contributes to smog formation. A fouled spark plug, a leaky exhaust valve, or a fuel mixture so lean it won't ignite (lean misfire) can all allow unburned fuel to enter the exhaust.

HC is reduced by maintaining a balanced air/fuel mixture, by making sure compression and ignition are OK, and by reburning any HC remaining in the catalytic converter.

Oxides of Nitrogen (NOX) are formed in the combustion chamber when temperatures rise above 2,500>1|F and nitrogen begins to react with oxygen. Lean air/fuel mixtures burn hotter and increase NOX.

Though not as poisonous as carbon monoxide, NOX irritates the eyes, nose and lungs, and contributes to ozone depletion and acid rain formation.

NOX is reduced by the Exhaust Gas Recirculation (EGR) system, and by three-way catalytic converters.

The EGR system allows a small amount of exhaust gas to be siphoned back into the intake manifold to slightly dilute the incoming air/fuel mixture. This lowers combustion temperatures to reduce NOX. It also helps prevent detonation.

The catalytic converter contains a ceramic honeycomb or ceramic pellets coated with a thin layer of platinum and palladium metal. In three-way converters, a third catalyst (rhodium) is included to reduce NOX.

The converter acts to reburn pollutants. To do so, it needs extra oxygen received from an air pump or an aspirator valve.

Arrows show air flow through the system. Air travels from the air filter to each cylinder where it mixes with hydrocarbons and carbon monoxide and then escapes into the atmosphere through the exhaust system.

The air pump is belt-driven and feeds air to the exhaust manifold through a diverter valve and check valve. The diverter valve dumps excess air back into the atmosphere when it's not needed (during deceleration, for example).

On some engines, a gulp valve is another part of the plumbing. The gulp valve diverts air from the pump into the intake manifold. This momentarily leans out the mixture during deceleration, preventing backfiring in the exhaust from too much fuel.

On some engines, an aspirator is used in place of an air pump. An aspirator is a one-way valve that allows air to be siphoned into the exhaust system between exhaust pulses.

When computerized engine controls and three-way catalytic converters were added, the air pump gained yet another control valve. When the engine is cold, air is routed to the exhaust manifold to help reduce the initial HC and CO emissions.

NOX is not a problem when the engine is cold. As the engine warms up and NOX starts to rise, the flow of air is diverted from the exhaust manifold directly to the converter where it enters a chamber between the two catalysts.

Do engines with distributorless ignitions need tune-ups

Yes, but there is not as much to "tune." Timing is not adjustable on most applications, and there is no distributor cap or rotor to wear out or replace.

Even so, the engine will need new spark plugs every 30,000 miles. When plugs are replaced, the ignition system should be checked to verify its overall performance. Plug wires deteriorate over time, coils get weak and sensor problems can arise.

Distributorless systems eliminate the need for distributor-related maintenance, but do not diminish the need for other maintenance such as replacing the PCV valve, fuel filter, air filter, etc.

Is it better to clean or replace dirty fuel injectors

Injector Operation

Injectors include a precision-ground needle valve and are controlled by an electro-magnetic solenoid that is turned on and off by an electric control unit. Fuel is injected only during the "on" time and is metered by the size of the opening, duration of "on" time, and fuel pressure.

Injector Service

Try cleaning them first. If this is not successful, they must be replaced.

Because of their construction, fuel injectors tend to "gum up" after 15,000 to 30,000 miles of driving. Fuel spraying from the injector must pass through a very small opening in the discharge nozzle. This is necessary to create a cone-shaped spray pattern that breaks the fuel up into a fine mist for proper atomization.

Some newer style injectors are more clog resistant than their predecessors, but all are vulnerable to some extent.

Every time the injector sprays fuel, a small amount remains in the nozzle. As it evaporates, it leaves behind a wax-like residue that forms hard varnish deposits.

The rate at which deposits build up depends on the quality of gasoline burned, whether or not the gas has detergent in it (and what kind), and the number of thermal cycles the engine experiences per miles driven. Short-trip driving builds up deposits more quickly than continuous driving.

As deposits build up in injectors, they restrict the discharge orifice and break up the normal cone-shaped spray pattern. The spray pattern may develop "legs" (streamers of unatomized fuel) or turn into a continuous stream of unatomized fuel like a fire hose.

Liquid fuel does not burn as efficiently as atomized fuel, so it has a "leaning effect" on the air/fuel mixture. Accumulated deposits in the discharge orifice also restrict the total amount of fuel delivered per squirt, which further compounds the leaning effect. This can result in the appearance of driveability problems such as hard starting, hesitation, poor fuel economy, loss of power, and elevated exhaust emissions.

An engine with dirty injectors will usually show a wide variation in RPM between cylinders when doing a power balance test. There will also be a lot of variation in peak firing voltages between cylinders on a scope.

For do-it-yourselfers, there are two options - use a fuel additive to clean the injectors, or buy a can of pressurized solvent that's designed for on-car injector cleaning. Fuel additives can only do so much, so badly-clogged injectors usually need to be pressure flushed with solvent.

With on-car cleaning, pressurized solvent is run through injectors to flush out deposits. To do this, the fuel pump is temporarily disconnected so solvent can be fed directly into the test valve fitting on the fuel rail.

When the engine is started, the solvent becomes the temporary "fuel supply" while injectors are cleaned.

The resulting improvement in performance is usually quite noticeable. But on-car cleaning doesn't always do the trick, especially if an injector is badly plugged.

Unless injectors are removed and tested, there is no easy way to spot marginal injectors (those with defective spray patterns) or ones that don't deliver as much fuel as the others (mismatched injectors can reduce horsepower and increase emissions).

Off-car cleaning involves more work, but results are often worth it. For one thing, injectors that don't respond to on-car cleaning can often be restored to like-new performance with off-car cleaning.

Some available cleaning equipment can reverse flush injectors, doing a thorough cleaning job. Most off-car cleaning equipment also allows the mechanic to observe and measure injector flow patterns so bad ones can be identified.

Flow rating also allows injectors to be more closely matched for improved engine performance.

Is it better to rebuild or replace a carburetor or throttle body


If all the carburetor needs is cleaning and adjusting, then a kit will usually do the trick. A kit is also an option if the carburetor needs a gasket, diaphragm, needle valve, check valve, or other component commonly included in a kit.

Floats, choke housings and choke pulloffs are usually sold separately, but are relatively easy to replace if defective. Faulty or misadjusted chokes and floats probably account for more carburetor problems than anything else.

Most carburetors today have molded foam floats rather than hollow brass floats. Some import carbs have hollow plastic floats. If a hollow float develops a leak, it will fill with fuel and sink, causing the carburetor to flood.

The solid foam variety can absorb fuel over a period of time. That's why the float should always be weighed when the carburetor is rebuilt. A heavy float will make the fuel mixture rich. Replace heavy floats.

Kits are not for everyone. It takes a fair amount of know-how to properly rebuild and adjust a carburetor. It also takes a lot of time to disassemble, clean, inspect, and reassemble a carburetor. Many professional installers, as well as do-it-yourselfers, prefer to replace a troublesome carburetor rather than to try to rebuild it with a kit.

There are some carburetor problems that cannot be fixed by a kit or by replacing faulty components. Wear around throttle shafts or warpage in the carburetor body or throttle plate can create vacuum leaks that foul up fuel metering. The only option here is replacement.

Most rebuilt carburetors are sold on an exchange basis, so it's important to make sure that the exchange carburetor is complete (all external parts intact), has not been disassembled, and is correct for the application.

If a base gasket is not included with the replacement, be sure your customer gets one before he leaves. Your customer may also need new fuel hose, fuel filter, and air filter to complete the installation.

Throttle Body

Because there are many small components included in a TBI kit, it is important to inspect each kit to verify that everything is included. Installers must pay close attention to the kit when assembling the unit to be sure small parts are placed in their respective positions.

The difference between throttle body and multi-point injection

The two basic types of electronic fuel injection (EFI) in use today are Throttle Body Injection (TBI) and Multi-Point Injection (MPI).

Throttle Body Injection

A TBI system is similar to a carburetor in that one or two injectors are located in a central throttle body that supplies fuel to the engine through the intake manifold. Instead of using engine vacuum to siphon fuel through metering circuits as a carburetor does, fuel is sprayed into the manifold through the injectors.

Multi-point Injection

In a MPI system, each cylinder has its own individual injector. The injectors are mounted at each of the intake ports so fuel can be sprayed directly into the ports. A single throttle body meters the amount of air entering the intake manifold so the amount of fuel delivered can be matched to the engine's needs.

Fuel Regulation

Fuel metering in both types of EFI systems is controlled by a combination of fuel pressure and injector timing. The longer injectors are on, the greater volume of fuel delivered to the engine.

Fuel delivery is also increased when there is a greater pressure differential between intake vacuum and fuel line pressure (which is controlled by a fuel pressure regulator).

Some of the other components in both types of EFI systems with which you should be familiar include:

1midle Air Control (IAC) Valve - used on EFI applications for idle speed control. The electric motor opens and closes a valve so air can bypass the throttle plates. Failure may cause stalling.

Throttle Position Sensor (TPS) - A rheostat-like device that mounts on the throttle shaft to inform the computer about throttle opening. Failure may cause hesitation. Sensor must be carefully adjusted when installed to give an accurate voltage reading.

Airflow Sensor - Used to measure how much air is entering the engine so the appropriate amount of fuel can be delivered through the injectors. Basic types include the "flap" style used on many import and domestic Bosch systems, and the "heated filament" and "hot wire" mass airflow sensors. Expensive to replace.

Cold Start Valve - An auxiliary fuel injector that provides extra fuel enrichment when a cold engine is first started. If defective, can cause hard starting. If leaks, can cause rich fuel mixture.

Warm-up Regulators - a device that controls fuel enrichment during engine warm-up on Bosch CIS fuel injection systems.

Fuel Pressure Regulator - A spring-loaded diaphragm that is used in EFI systems to control fuel pressure. Rebuild kits are available for certain applications.

Fuel Injector - Two types: mechanical and electronic. Mechanical injectors are used in Bosch CIS/K-Jetronic import applications, while electronic injectors are used on all domestic EFI applications. The electronic variety contain a solenoid that lifts a pintle valve open so fuel can spray out of the injector. The mechanical variety is spring loaded and calibrated to open when a certain minimum pressure is achieved. Both are susceptible to clogging from dirt and fuel residue. One new type of replacement injector has a disc valve design that resists clogging.

What are the recommendations for changing filters

It is best to follow the Severe Service maintenance schedules found in most new car owner's manuals, with a few exceptions:


  • Air filters need to be inspected regularly and replaced as often as needed, regardless of mileage or time. Dirty air filters can increase fuel consumption and exhaust emissions.

  • Fuel filters should be replaced yearly and/or at every tune-up, especially on fuel injected cars. The fuel filter in a vehicle with electronic fuel injection passes a much larger volume of fuel than its counterpart in a carbureted application. If the tank is dirty or rusty, constant fuel recirculation can pick up a lot of debris that ends up in the filter. If the filter plugs, the engine is starved for fuel or unfiltered fuel is allowed to bypass the filter. The latter can damage injectors.

  • Oil filters need to be replaced at every oil change (every six months or 3,000 miles in most cases) despite the advice in many owner's manuals to only change the filter at every other oil change. A new filter is cheap insurance against major engine damage, so why take unnecessary risks?

  • Few owner's manuals have a suggested change interval for the automatic transmission fluid (ATF) or fluid filter unless the vehicle is used for towing. Most transmission specialists say the best preventative maintenance for prolonging automatic transmission life is to change fluid and filter every two years or 30,000 miles.

  • Follow the manufacturer's recommendations on the specific type of ATF to use. The type of ATF should match the specs required for the application.

  • All GMs, most late model Chryslers and many imports use Dexron II. All 1988 and later Fords require Mercon ATF. Most universal ATF fluids are acceptable for either of these. Older Fords or imports require Type F fluid.

What are the rules for replacing catalytic converters

In a closed loop emissions operation, pump air is injected downstream between reduction and oxidation catalysts when the engine is warm.

Three-way catalytic converters contain both catalysts in a single housing, with an air inlet between the two converters.

Original equipment converters on new cars and light trucks are currently covered by an eight year/80,000 mile emissions warranty. Motorists can return to the new car dealer for free replacement as long as the converter is covered.

The customer can choose to have an independent repair garage replace the converter at his own expense if it is still under warranty. Once the vehicle is out of warranty, he pays to have it fixed no matter where he takes it.

The converter should go at least 100,000 miles on most late model vehicles. Trouble is rare unless the converter has been lead fouled (by using leaded gasoline), damaged by overheating (often due to unburned fuel in the exhaust from a misfiring spark plug or leaky exhaust valve), or removed.

Removing the converter and replacing it with a straight pipe is not permissible. The new Clean Air Acts make anyone (including the motorist himself) liable for a $2,500 fine if they remove, disconnect or render inoperative any emission control device.

If the vehicle has flunked an emissions test and the cause is determined to be a bad converter, or if the converter is clogged, damaged, lead-fouled, rusted out, physically damaged or missing, it is okay to replace it. Federal law prohibits aftermarket garages from replacing converters as long as they're under the five/50 emissions warranty, unless any of the previously-mentioned reasons exist for replacement.

The shop must first document the reasons, along with the vehicle's odometer reading, and have the customer sign it before the converter is replaced. The shop must keep the old converter for 15 days and the paperwork for six months. The replacement converter must be the same type as the original (two-way, three-way or three-way plus oxygen), be EPA-certified, and be installed in the same location as the original.

Aftermarket replacement converters meeting EPA requirements must have a minimum lifespan of 25,000 miles, and include a five year/50,000 mile warranty covering exterior shell and welded pipes against defects in materials and workmanship.

Used converters are no longer allowed unless the supplier can certify the converter is still capable of cleaning up 50% of the unburned hydrocarbon (HC) and carbon monoxide (CO) emissions within two minutes of start-up, and 75% of the HC and CO emissions within 200 seconds.

All approved replacement converters are required to carry a permanent label that identifies the type of converter (N for new, U for used), a code number issued to the manufacturer by the EPA, an application part number, and a manufacturing date.

What causes spark knock and how do you get rid of it

Drawing on left shows completion of normal combustion. Cutaway on right shows a detonating cylinder, where the last portion of the air/fuel mixture self-ignites and collides with the normal combustion front.

Spark knock (detonation) is an erratic form of combustion that occurs when multiple flame fronts occur simultaneously inside a combustion chamber. Detonation occurs because fuel is subjected to either too much pressure, too much heat or both. It usually happens during acceleration when the engine is heavily loaded and cylinder pressures are at their peak.

Instead of a single flame front growing outward smoothly like an expanding balloon from the point of ignition, multiple flame fronts are generated spontaneously throughout the combustion chamber as the fuel automatically ignites from heat and pressure. The multiple flame fronts collide, creating shock waves that produce a sharp metallic pinging or knocking noise.

Mild detonation can occur in almost any engine and will not cause damage. Prolonged heavy detonation can crack pistons and rings, blow out head gaskets, damage spark plugs and valves, and flatten rod bearings.

Any of the following can cause detonation:


  • Too Much Compression: An accumulation of carbon deposits in the combustion chambers, on piston tops and valves can increase compression to the point where it exceeds fuel octane rating. If a top cleaner fuel additive fails to remove deposits, a new alternative is to blast the deposits loose by blowing crushed walnut shells through the spark plug hole. Otherwise, the head will have to be removed so the deposits can be scraped off.

  • Overadvanced Ignition Timing: Too much spark advance causes cylinder pressure to rise too rapidly. If resetting the timing to stock specifications does not help, retarding timing a couple of degrees may be necessary to eliminate knock.

  • Engine Overheating: A hot engine is more likely to suffer spark knock than one which runs at normal temperature. Overheating can be caused by low coolant, a defective fan clutch, too hot a thermostat, a bad water pump, etc. A buildup of lime and rust deposits in the head and block can also reduce heat transfer

  • Overheated Air: The thermostatically controlled air cleaner provides the carburetor with hot air to aid fuel vaporization during engine warm-up. If the air control door sticks shut so that the carburetor continues to receive heated air after the engine is warm, detonation may occur, especially during hot weather. Check the operation of the air flow control door in the air cleaner to see that it opens as the engine warms up. No movement may mean a loose vacuum hose or a defective vacuum motor or thermostat.

  • Lean Fuel Mixture: Rich fuel mixtures resist detonation while lean ones do not. Air leaks in vacuum lines, intake manifold gaskets, carburetor gaskets or fuel injection intake plumbing downstream of the throttle can all admit extra air into the engine and lean out the fuel mixture. Lean mixtures can also be caused by dirty fuel injectors, carburetor jets clogged with fuel deposits or dirt, a restricted fuel filter, or a weak fuel pump.

The air/fuel ratio can also be affected by changes in altitude. A carburetor calibrated for high altitude driving will run too lean if driven at a lower elevation. Altitude changes are generally compensated for on computer cars by the barometric pressure sensor.

A lean fuel condition can be diagnosed by watching for lean misfire on an ignition scope, or by using a four-gas infrared analyzer and watching exhaust oxygen levels. A reading over about 3% to 4% oxygen would indicate a lean fuel condition.


  • Spark Plug Too Hot: The wrong heat range plug can cause detonation as well as pre-ignition. Copper core plugs are less likely to cause detonation than standard spark plugs.

  • Loss of Exhaust Gas Recirculation (EGR): EGR keeps combustion temperatures down, reducing the tendency to detonate. If the EGR valve is inoperative or someone has disconnected or plugged its vacuum hose, higher combustion temperatures can cause pinging.

  • Low Octane Fuel: Burning cheap gas may be one way to save pennies, but switching to a higher grade of fuel may be necessary to eliminate a persistent knock problem.

  • Defective Knock Sensor: The knock sensor responds to frequency vibrations produced by detonation (typically 6 - 8 kHz), and signals the computer to momentarily retard ignition timing until detonation stops. A knock sensor can usually be tested by rapping a wrench on the manifold near the sensor (never hit the sensor itself). If there is no timing retard, the sensor may be defective.

What kind of fuel hose is best

Fuel, emissions and vacuum hoses are made of special materials to handle the liquids and vapors they carry. Fuel hose, for example, is made to withstand gasoline and alcohol (up to a certain percentage concentration). It is also reinforced to withstand internal pressure.

It is extremely important not to use any other type of hose for a fuel line. Use only approved fuel hose that is rated for carrying gasoline.

It is also extremely important to use hose with the correct pressure rating for the application. Hose for electronic fuel injection (EFI) must have a higher pressure rating (35 to 70 psi or higher) than that approved for carbureted engines (7 to 10 psi).

You can always use higher rated EFI hose on a carbureted application, but never the reverse. If lower pressure carburetor fuel hose is used on a high pressure EFI engine, it may burst and cause a fire.

High pressure EFI hoses often require special high pressure rolled-edge clamps rather than standard clamps or spring clamps. Some EFI hose also uses special connectors. Ford and Mercury fuel injected engines, for example, have hard nylon fuel lines held in place by push connectors.

What should a complete tune-up include


  • Electronic ignition, computerized engine controls, and electronic fuel injection have eliminated many adjustments that were once part of a "traditional" tune-up. Most would agree that a tune-up today is a preventive maintenance service and engine performance check.

  • Call it what you will, a complete tune-up should combine elements of preventive maintenance, adjustment and performance analysis. One of the main reasons people bring a vehicle in for a tune-up is because they are experiencing some kind of driveability problem.

  • Things like hard starting, stalling, hesitation, misfiring, poor fuel economy, or lack of power are seldom cured by a new set of spark plugs and a few turns of a screwdriver. Every tune-up should include a comprehensive performance check to verify that no driveability problems or trouble codes exist.

  • Another item that should be included is an emissions check. Thirty-five states now have some type of annual vehicle emissions inspection program, and all but two include a tailpipe emissions check. Most mechanics will check EGR valve operation, the PCV valve, and make a visual inspection of other emission control components and plumbing. But unless an actual emissions performance check is made at the tailpipe, there is no way to know whether or not the vehicle will meet applicable emission standards. An emissions check is a must.

  • Taking into account longer service intervals and reduced maintenance requirements of today's vehicles, a tune-up is probably only necessary every 30,000 miles, or once every two to three years. This is altered when a driveability or emissions problem arises that requires diagnosis and repair.

  • The best guide to tune-up frequency is probably the recommended spark plug replacement interval in a vehicle's owners manual.

  • Our list of items that should be included in a "complete" tune-up include:

  • Replace spark plugs

  • Replace rotor

  • Check distributor cap (replace if necessary)

  • Check timing (adjust if necessary)

  • Check ignition wires (replace if necessary)

  • Check ignition performance (firing voltage and ignition patterns)

  • Check idle speed (adjust if necessary)

  • Check choke (carbureted engines)

  • Clean fuel injectors

  • Check compression and/or power balance (identifies bad fuel injectors as well as compression problems)

  • Check manifold intake vacuum (reveals exhaust restrictions)

  • Check battery/charging voltage

  • Check exhaust emissions (verifies fuel mixture, ignition performance and emissions performance)

  • Check vehicle computer for trouble codes

  • Install new air filter

  • Replace fuel filter

  • Replace PCV valve

  • Check all emission controls (EGR valve, air pump, etc.)

  • Check all vital fluid levels (engine oil, transmission fluid, coolant, brakes, power steering)

  • Check belts and hoses

  • Check safety items such as lights, wipers, tires (including inflation pressure), horn, etc.

Why are there many different types of spark plugs

Spark plugs need 5,000 to 40,000 volts from the ignition coil before a spark will jump across its electrode gap. It takes a lot of volts to push the spark across the gap because air doesn't conduct electricity unless it is ionized first. The spark jumps from center electrode to side ground electrode.

The reason why a plug fires from center electrode to side ground electrode, instead of vice versa, is because it's easier for a spark to originate at a hot electrode than a cooler one.

The center electrode runs much hotter than the side electrode because the center electrode is encased in ceramic (a good insulator of heat as well as electricity). This slows down heat transfer from center electrode to cylinder head.

If ignition polarity is reversed, it can take up to 40% more firing voltage to send the spark from ground electrode to center electrode. The result can be misfiring under load and poor engine performance.

Keeping the center electrode hot also helps burn off fuel and oil deposits that form on the insulator tip. Deposits can conduct voltage away from the gap causing the plug to misfire, so keeping the center electrode hot helps prevent fouling.

If the plug is too hot for the application, it can become a source of pre-ignition. If the plug is too cold, it can experience fouling problems.

The operating temperature of a spark plug depends on a number of variables. The two most influential are cylinder head temperature and the relative richness or leanness of the fuel mixture. Given such variables, it is impossible to have a single spark plug that would work well in every application, even if thread sizes and reach were standardized.

Heat range is determined by several design features, one of which is the distance heat must travel from center electrode tip to the plug's shell. A plug with a short ceramic insulator between electrode tip and shell runs cooler than one with a long nose insulator.

A cold plug is good for high speed, high load operation because it sheds heat quickly and is less likely to overheat and cause pre-ignition. Colder heat ranges are used most often in high performance and turbocharged engines.

For short-trip, stop-and-go driving, a cold plug may not run hot enough to keep itself clean. A hotter heat range plug may be needed to resist fouling.

For sustained high speed or high load running, a hotter plug may become too hot and cause preignition. The trick is to use a plug hot enough to prevent fouling yet cold enough so there is no danger of pre-ignition.

One way to extend or broaden the heat range of a spark plug is to extend the tip of the plug further into the combustion chamber. The longer insulator makes the tip run hotter for better self-cleaning at low speeds and light loads. It also exposes the tip to more of the incoming air/fuel mixture, keeping it from overheating at high speeds and loads. An extended tip spark plug typically has a much broader heat range than a standard spark plug.

Another way to increase heat range is to use a center electrode with a copper core. Copper is an excellent conductor of both heat and electricity. With a copper core center electrode, heat is carried away from the plug tip through the electrode during high speed, high load operation. This allows the plug to dissipate heat more quickly like a colder plug, yet stay hot enough to burn off fouling deposits.

Because of the increased heat range copper core plugs offer, one plug can be used in applications formerly requiring several different plugs with narrower heat ranges.

The use of a platinum or gold palladium center electrode is another design innovation that improves fouling resistance while greatly extending plug life. The special alloy at the tip of the center electrode is more wear and corrosion resistant than standard electrode metal. It allows the use of a longer insulator, helping plugs reach a self-cleaning temperature of 750 degrees F in only a few seconds.

Spark plug manufacturers avoid making specific mileage claims for such premium plugs, but many experts say the plugs will often last up to 60,000 miles. Other benefits include better cold starting, less cold fouling, and improved operation during both stop-and-go and highway driving. These plugs are considerably more expensive than standard plugs.



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