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Engine Tune-Up

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. tuneup, spark plugs

The word Tune-up is an oputdated and obsolete term. The need for tune-ups went out back in the mid 1980s when fuel injection replaced carburetors and conventional spark plugs were replaced with long life platinum spark plugs. Yet many people think their engine still needs a tune-up.

What is needed is preventive maintenance. Or, if your Check Engine Light is on, what you need is a diagnostic scan to determine what is causing the fault.

If your engine is hard to start, stalls, runs rough, gets poor fuel mileage, doesn't run right, or is experiencing any other kind of driveability or emissions problem, you don't need a tune-up. You need a diagnostic check to find out what's causing the problem.

The only engines that still need a tune-up today are old ones from the early 1980s and back that have carburetors and distributors. Yet the tune-up myth persists, and may people still think it is some kind of "cure-all" for what ails their engine. To make matters worse, many new car dealers tell their customers they need a 60,000 mile "major" tune-up (whatever that is).

Tune-Up Definition is Undefined

There's no common definition of what exactly a tune-up should include, but most would agree that it involves replacing the spark plugs and performing other adjustments to the idle speed, fuel mixture and spark timing that are necessary to maintain or restore like-new engine performance. The problem is there is almost nothing that can adjusted or "tuned" under the hood on late model engines with computerized engine controls. Ignition timing is fixed and controlled by the engine computer, as is idle speed and the fuel mixture. Base timing can be checked with a scan tool, but cannot be adjusted on most engines. The same goes for idle speed and various emission functions. A scan tool can reveal if the systems are functioning normally, but in most cases no adjustments are possible because the adjustments are programmed into the computer.

A simple maintenance type tune-up (a new set of plugs) may make an engine easier to start, improve fuel economy, lower emissions, restore lost pep and power if the old spark plugs are worn or fouled. But if the problem is due to something else, a new set of plugs alone won't help. A tune-up under these circumstances is a waste of time and money. The engine needs to be diagnosed to find out what is wrong.


An engine check-up should start with a scan for any current, pending or past fault codes. This requires plugging a scan tool or code reader into the vehicle diagnostic connector so the tool can communicate with the powertrain control module (PCM). The onboard diagnostic system does an excellent job of monitoring all the key systems, and on most 1996 and newer vehicles it can even detect engine misfires.

If no faults are found, and the engine is running normally, the check-up is not over because there are additional things that should also be checked (especially if the engine is NOT running normally or any fault codes were found with a scan tool):


In addition to these performance checks, hoses and belts should be visually inspected.

All fluids (oil, coolant, automatic transmission fluid, power steering fluid and brake fluid) should also be inspected to make sure all are at the proper level, and that the appearance and condition of each is acceptable. There should be no sludge in the oil, the ATF should not smell like burnt toast, the coolant should have the proper concentration of antifreeze and not be full of rust or sediment, the brake fluid should be clear and not full of muck, etc.


If the tune-up checks find no major faults, the following items can be replaced for preventive maintenance:


Oxygen sensors on late model vehicles should last 100,000 to 150,000 miles under normal driving and operating conditions (which does NOT include an engine that burns oil, or vehicles that have been under water!). The oxygen sensor is a key sensor that can hurt fuel economy if it is getting old or has failed. One EPA study found that up to 70% of high mileage vehicles that fail an emissions test need a new O2 sensor.

So does that mean the oxygen sensors should be replaced as part of a tune-up? Not unless they are defective or are acting very sluggish. Oxygen sensor performance can be verified with a scan tool, and a bad oxygen sensor will usually set a fault code and turn on the Check Engine Light, but not always. If an oxygen sensor fails or is getting sluggish, it will usually cause the engine to run rich. This causes an increase in fuel consumption and emissions. It usually does not hurt performance or cause other driveability issues.

Oxygen sensors are expensive to replace. They typically cost $35 to $70 each, and some may cost upwards of $200 or more depending on the application. In addition, V6 and V8 engines have one oxygen sensor for each cylinder bank, and some have two. There are also one or more oxygen sensors in the exhaust system to monitor the catalytic converter(s). So oxygen sensors are not something you want to replace unless it is absolutely necessary.

Some manufacturers do recommend replacing oxygen sensors for preventive maintenance, however. The recommended replacement interval for unheated 1 or 2 wire wire O2 sensors on 1976 through early 1990s applications is 30,000 to 50,000 miles. Heated 3 and 4-wire O2 sensors on mid-1980s through mid-1990s applications should be changed every 60,000 miles. And on OBD II equipped vehicles (all 1996 and newer), some recommended replacing the oxygen sensors at 100,000 mile intervals.


Dirty fuel injectors are a common problem that can hurt engine performance, fuel economy and emissions. Many experts recommend cleaning the fuel injectors and intake system as part of a tune-up. The need for injector cleaning isn't as great as it once was thanks to improved fuel additives and redesigned injectors. But in areas that have gone to reformulated gasoline, injector clogging is more of an issue.

Fuel varnish deposits that form in injectors restrict fuel delivery, which has a leaning effect on the air/fuel mixture. The result can be lean misfire and a general deterioration in engine performance and responsiveness. Deposits can also build up on the backs of intake valves, causing cold hesitation problems in many engines.

The cure is to clean the injectors and valves. Cleaning is recommended for any engine that is suffering a performance complaint or has more than 50,000 miles on the odometer. Cleaning the throttle body can also help eliminate idle and stalling problems that plague many of today's engines.


The spark plug replacedment interval on most late model engines with platinum or iridium spark plugs is 100,000 miles. But that does not mean the engine requires no maintenance whatsoever for 100,000 miles.

Regular oil and filter changes are still necessary to maintain proper engine lubrication. Most experts still recommend changing the oil and filter 3,000 miles or three to six months. The oil change interval can be stretched out to reduce maintenance costs if a vehicle is driven under ideal conditions (no extremely hot or cold weather, no short trip, stop-and-go driving, no excessive idling, no extremely dusty road conditions, no trailer towing, no turbocharging). But the average driver is more often than not a "severe service" driver so should follow the 3,000 mile change interval.

Today's 100,000 mile tune-up interval also skirts around the issue of fuel filter and air filter replacement. A number of new cars and trucks now have "lifetime" fuel filters, most of which are located inside the fuel tank with the electric fuel pump. Such a filter might go 100,000 miles. Then again, it might not. A couple of tanks of bad gas or some corrosion caused by accumulated moisture can cut short the life of any filter, even a so-called lifetime filter. Sooner or later even a lifetime fuel filter will have to be replaced.

As for air filters, the service life depends more on environmental factors rather than time or mileage. If a vehicle is driven on gravel roads, filter life may only be a few months or few thousand miles.

Many vehicles also have a cabin air filter for the passenger compartment. This also needs to be replaced at specific intervals (see your owners manual for the location and recommended service interval).

Repairs are also inevitable regardless of what the tune-up interval is supposed to be. It's pretty unlikely that a set of front disc brake pads will go 100,000 miles in city driving -- 20,000 to 30,000 miles is a more realistic figure. The same goes for belts, hoses, the battery, water pump, exhaust system and many other parts. No vehicle that's yet been built can even come close to going 100,000 miles without needing some type of maintenance or repair.


Some car dealers continue to promote a 60,000 mile major tune-up that includes a long list of items they supposedly check, and may also include normal tune-up replacement parts such as spark plugs and filters. They may also do a coolant flush, transmission flush and/or brake fluid flush. The service is fairly expensive ($250 to $1000 or more!), and is probably unnecessary provided you have changed your oil regularly, you have kept your fluid levels full, and the engine is not experiencing any problems. I think people who spend this kind of money to have their engine examined should also have their head examined. But if you want to pay for a "piece-of-mind" check-up, your new car dealer will be more than happy to accommodate you.

Related Articles:

5 Things Your Car May Need Now!

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Recommended Service Intervals from the Car Care Council

Don't Neglect the Spark Plugs

Why Spark Plugs Still Need To Be Replaced

Car Maintenance: Changing Fluids, Filters, Belts & Hoses

Common Car Problems

Analyzing Ignition Misfires

Spark Plugs & Ignition Performance

Chevy Firing Orders

Chrysler Firing Orders

Ford Firing Orders

Distributor Ignition Systems

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Coil-Over-Plug Ignition Systems

Multi-Coil Ignition Systems

Ignition Coil Diagnosis & Testing

Diagnosing An Engine that Won't Crank or Start

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Fuel Saving Tips from the Car Care Council

Warning Lights (TEMP, OIL, ALT/GEN, BRAKES, ABS, AIR BAGS, etc.)

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