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Tune-up Results In Engine Running Worse? Here's Why

Spark plug

Those of us who maintain our own vehicles or who want them to run top-notch at all times will do regular "tune ups" to keep things going strong. The term "tune up" is a generalized one used to describe replacing small parts associated with the fuel-air-spark (ignition) system of the vehicle and cleaning out some items which could affect those three things. The actual process of a tune-up will vary depending on the vehicle at hand.

For example, an older vehicle will have a distributor cap and rotor replacement whereas a newer vehicle which does not have a mechanical distributor will not require this. Spark plug wires are another common tune-up change, as are the spark plugs themselves. Some mechanics also replace spark plug solenoids. Other items such as engine air filter changes, radiator cleaning (cleaning the outside of the radiator to clear the fins of debris), and so forth are also common tune up items. Remember to replace the fuel filter at recommended intervals - this is one of the most commonly overlooked maintenance items on a vehicle.

Sometimes, though, a tune-up goes wrong and the car actually runs worse after it's been done. This usually means one of two things:

  1. One of the replacement parts is not working properly.
  2. Something was installed incorrectly.

If a distributor cap and rotor were replaced, make sure they're seated properly and that there are no cracks or breaks in the cap. Be sure the cap is not loose and the rotor is correct for the vehicle.

Check the spark plug wires, one by one, to be sure they're seated and plugged into the distributor cap and/or ECU correctly, then trace each of them to check the same where they connect to the spark plug and/or its solenoid. If all is well, proceed to the next step.

Pull each spark plug individually and make sure it's not damaged and was seated correctly. It may be a good idea to carefully re-thread the spark plug seat with a proper die in order to clean the threads to ensure a proper fit.

Test resistance on all electrical items. Begin with the plug wires, using a multimeter to make sure resistance is to spec (usually 10,000/15,000 ohms per foot).

In most cases, one of the above will remedy the problem. If the vehicle is high mileage, you may want to remove and check the exhaust gas recirculation valve (EGR) and/or the mass airflow filter (MAF).