Introduction: How to Do a Tune-Up

About: The official instructable for Popular Mechanics magazine, reporting on the DIY world since 1902.

There’s the thruway exit, but you’re stuck in the left lane. Hey—a break in the traffic. You flip up the turn signal, floor the gas pedal and try to edge into the right lane. Whoops! Your V8 is embarrassed by a 4-cylinder econobox with the same objective. It was behind you, but it apparently has better acceleration. You hit the brakes, crank the wheel to get back into the left lane, cruise to the next exit miles and miles ahead, and make the time-wasting U-turn. The Wife suggests a tuneup. But we don’t tune up today’s engines, right? They’re computerized and there’s nothing to adjust.

Wrong. It’s true we stopped replacing ignition points almost 20 years ago. If it’s really a late-model vehicle, it may not even have a replaceable fuel filter under the hood. The battery is a fill-free design. And there’s no ignition timing to adjust. However, while it has changed significantly—even in the last seven or eight years—the concept of a tuneup itself is anything but gone. Emissions regulations may have tightened, but ambient air is still dirty. So the air filter still plugs up, and the engine gasps for breath until a clean new filter is installed. Gasoline is unleaded, so spark plugs don’t lead-foul anymore, and the new precious-metal designs last much longer. But they aren’t guaranteed to last a lifetime, and engine performance often can improve when plugs are replaced well before the owner’s manual dictates.

This project was originally published in the September 2002 issue of Popular Mechanics. You can find more great projects at Popular Mechanics DIY Central.

Step 1: Underhood Checks

A good way to start today’s tuneup is to look for trouble codes, using a scan tool. The only codes that turn on a Check Engine light are those that directly impact emissions, provided the computer can detect them. To keep people from getting nervous, most codes show up only with a scan tool. On the other hand, “no code” driveability problems are extremely common. If you find a trouble code, you should trace the circuit to pinpoint the problem. Sure, it could be just a bad sensor, but it may be a bad wiring connection, chafed wire or damaged hose. You’ll need the factory service information to be able to do this.

No trouble code? Inspect under the hood. Look for damaged vacuum hoses that should be replaced, loose connections at any of the sensors and solenoids, tears in the air cleaner ductwork, a disconnected duct or poor spark plug wiring connections. Test the behavior of the engine controls and their sensors. There still are timing marks on many late models and if your vehicle is one of them, you can check basic timing (at idle) with a timing light. No marks? You still can check ignition timing on the scan tool by reading the ignition advance data item. Look for a steady increase in ignition timing as the throttle is gradually opened from idle to a midthrottle position.

Even if the timing is all right, check the throttle position sensor and intake airflow (mass airflow) sensor readings, which also should show gradual increases as the accelerator pedal is depressed. At the same time, tap on the mass airflow sensor with a screwdriver handle and if the engine hiccups or the scan tool reading spikes, the sensor is defective. A coolant temperature sensor should show a continuous increase in the reading until the engine is fully warm (195˚F to 230˚F). A MAP (manifold absolute pressure) engine vacuum sensor should show changes when a pinched-closed hose supplying vacuum to it is released.

The engine should read about 750 to 850 rpm at idle with the engine warmed, and increase gradually as the gas pedal is depressed. Still no reason for a driveability glitch? You need to access the powertrain computer. It may be located under the hood or under the dash. The most common under-dash location is behind the passenger-side kickpad. Remove the kickpad and, while a helper is slowly accelerating the engine, flex the computer wiring connectors and tap on the computer housing. If the engine hiccups, there’s a bad connection or possibly cracked solder joints on the circuit board. You may need a replacement.

Step 2: Replacing Spark Plugs

Time was, spark plugs were always visible, even if it took a struggle to reach them. Now they’re often recessed into the head, and in many cases there are no plug wires. A mini ignition module, perhaps with the coil built into it (or into the plug boot), is used instead. You may have to remove a cover that holds the ignition modules and plug boots for access. Whatever the design, very carefully make any necessary electrical disconnection at the plug prior to plug removal. If there are plug wires, grasp and lift by the plug boots. Never pull on a plug wire or it may separate internally.

Look inside the boot and if it appears soaked in oil or cracked by heat, replace it. If the ignition coil is built in, expect it to be pricey. Inspect the plugs. If they all have a coating of black carbon, that indicates a rich fuel mixture and likely a bad oxygen sensor. A scan tool should be able to pinpoint a malfunctioning oxygen sensor even if it didn’t log a trouble code. If there’s just a single carbon-blackened plug, you prob-ably have a leaking fuel injector. When you’re ready to install the new plug, apply a thin film of antiseize compound so the new plug doesn’t heat-seal in place. Use a torque wrench to tighten the plug.

Step 3: Battery Terminals

Clean the battery terminals, then reinstall and tighten. Today’s batteries—top-post and side-terminal—are subject to continuous drain with the engine off, to keep alive the memories of many computers (from powertrain to car radio). Just a slight coat of corrosion—perhaps too subtle to be visible—can reduce battery charge. Disconnect the cable terminals.

On a top-post battery, brush around the post and inside the cable terminal. If the post or cable terminal is badly corroded, replace it with a premium terminal, the kind that often includes a cable section. Forget the cheap screw-together terminals—they’ll become severely corroded and cause more problems than they solve. With a side-terminal battery, brush both contact faces even if they look clean. Make sure the bolt threads in without a feeling of looseness, and if there is any, don’t try to muscle the bolt supertight with a wrench. You’re more likely to make things worse.

Step 4: Where to Get Service Information

Diagnosing failures that produce trouble codes, and finding out normal readings for engine sensors, is not subject to rule of thumb pro-cedures. You need the latest factory diagnostic sequences and specifications. Vehicle makers have Web sites with this service information, and you’ll be able to access the data for a day at a time. Many sites charge for this information—you can pay using a credit card. At present, General Motors information is available for $15 per day at (click on ACDelco TechConnect). Or you can get a low-cost subscription from (a leading source for professionals, with information on all makes). In addition, AutoZone offers free scan tool connections and readouts at its stores.

Step 5: Exhaust Gas Recirculation

Exhaust gas recirculation (EGR) valves may stick open, fail to close completely or just not operate smoothly. If so, the engine will hesitate and may stall, fuel economy may drop, and emissions may be affected—even if you pass the state inspection test. The EGR valve meters some exhaust gas back into the cylinders to lower peak temperatures of the air-fuel charge during combustion. This not only reduces a key pollutant (oxides of nitrogen) but often improves fuel economy.

The EGR valve, typically a diaphragm-actuated device with a pin-type valve, is pulled open by engine vacuum, but that vacuum must be precisely regulated. Too much vacuum results in too much exhaust gas flow, which can cause the engine to lose power, even stall. Late models with OBD II computers have sophisticated strategies to detect severely malfunctioning EGR. However, this basic check will work on all vacuum-controlled EGR systems.

Locate the EGR valve and if it has a vacuum hose connection, unplug it. Run the engine at idle, and connect a spare hose from a source of engine vacuum or use a manual vacuum pump and apply vacuum to the hose neck. The engine should slow significantly, probably even stall, if you apply full engine vacuum (17 to 21 in.). If there’s no significant change in engine idle—in fact, if the engine has been idling rough—the valve may be sticking open. Remove it and if you can see a heavy accumulation of deposits in the port, and if it’s an exposed pin-type valve, clean it with a wire brush. If the end of the pin is recessed in the port, it can’t be cleaned. Replace it.