9 Signs Your Car Needs a Tune-Up

A tune-up on a modern car is not what it was thirty years ago. There is no distributor to set and no carburetor to adjust, because the engine computer manages timing and fuel mixture continuously. What is left is spark plugs, ignition coils and filters, and those still wear out.

Here are the nine things we hear most from drivers whose vehicles turn out to be due. Any one of them is worth a scan before you spend money on parts, because several have causes that a tune-up will not fix.

A healthy engine at idle is steady. If you can feel it shaking through the steering wheel or the seat at a red light, one or more cylinders is not contributing properly. Worn plugs and a weak coil are the common causes, though a vacuum leak produces the same feeling.

You press the pedal and there is a pause, or a stumble, before the car responds. Under load the spark has to jump a larger gap, and a worn plug that works fine at idle can misfire exactly when you need the power. It is most noticeable pulling onto a road or climbing a ramp.

A misfiring cylinder wastes the fuel it was given. A clogged engine air filter makes the engine work harder to breathe. A lazy oxygen sensor keeps the mixture richer than it needs to be. All three show up as fewer miles per tank, and most people notice it before anything else.

Cold starts ask the most of the ignition system. If your car cranks longer than it used to on a January morning in Dearborn but starts fine once it is warm, worn plugs are a common cause. A weak battery produces a different symptom: slow cranking rather than long cranking.

Misfire codes point directly at the ignition system, and a code that names a specific cylinder is useful information. A flashing light is different and more urgent: it means raw fuel is reaching the catalytic converter and damaging it, so stop driving and call the shop.

A vibration that appears at a particular engine speed and then smooths out again often comes from a cylinder dropping out intermittently. It is worth describing exactly when it happens, because that detail narrows the diagnosis considerably.

Modern iridium and platinum plugs commonly last 60,000 to 100,000 miles. If you have passed that number, the plugs are due whether or not you have noticed a symptom yet. Replacing them on schedule is cheaper than replacing a coil that failed because it was working too hard against a worn plug.

That is the cabin air filter, and most vehicles have one that has never been changed. It does not affect how the engine runs, but it is part of the same service and it makes a real difference to how the car feels, particularly during pollen season.

This is the least scientific sign and one of the most reliable. People know their own cars. If it feels slower to respond and less willing than it did a year ago, and nothing else has changed, it is worth a scan and a look at the plugs.

If the scan shows the plugs are not the problem, we will tell you that rather than selling you the service. A rough idle can come from a vacuum leak, a dirty throttle body, low compression or a fuel delivery fault, and none of those is fixed by new plugs.