Why does the engine speed increase after changing spark plugs?
3 Answers
Increased engine speed after changing spark plugs may be due to mismatched heat ratings. It is recommended to use the same model as the original. Spark plugs are crucial components in a gasoline engine's ignition system, which introduce high voltage electricity into the combustion chamber and generate sparks by jumping across the electrode gap, thereby igniting the air-fuel mixture in the cylinder. Main types of spark plugs include: Standard spark plugs, extended insulator spark plugs, electrode spark plugs, seat spark plugs, pole spark plugs, and surface gap spark plugs. Effects of faulty spark plugs: Damaged spark plugs can directly cause ignition system leakage, increased fuel consumption, reduced power output, and other issues. The most significant impact is difficulty in ignition, affecting the vehicle's normal operation. Severe spark plug erosion - including tip scarring, electrode damage/melting, or burn marks - indicates the spark plug is damaged and needs replacement.
I've been repairing cars for over 20 years and have seen many cases where replacing spark plugs caused RPM to rise. The most common reason is using incorrect spark plug models with mismatched heat ranges, which advances ignition timing and causes the engine to inject more fuel to increase RPM. Last time, an old Toyota had this issue because the owner bought cheap aftermarket parts, resulting in idle speed soaring to 1500 RPM. Installation problems could also be the cause, such as improperly seated ceramic bodies causing air leaks or excessively large gaps. If the RPM fluctuates, it's likely due to accidentally disturbing vacuum tubes during installation. I recommend immediately checking the spark plug model and torque, and reinstalling if necessary. Running the engine at high RPM is very damaging—don't wait until cylinder scoring occurs, as it'll be too late by then.
Be extra careful when changing spark plugs yourself! Once when I replaced the spark plugs with iridium ones on my Civic, the tachometer kept staying above 1200 RPM. After struggling for a while, I realized the heat range was too high, causing overly aggressive ignition which made the ECU misjudge and inject more fuel. Later, after checking the service manual, I learned that different engines have different requirements for spark plug gaps. Some cars need 0.8mm, and adjusting it to 1.1mm can affect the air-fuel ratio. After replacement, it's best to reset the ECU to let it learn the new parameters—just disconnect the negative battery terminal for five minutes. If the RPM is still high, check if the ignition coils are properly installed, unlike me who had to redo it twice.
