How to Tell If Your Car Needs a Tune Up


Tuneups on late-model vehicles are limited to ignition, intake plenum and cleaning of the fuel injectors. Sparkplug fouling is a thing of the past, which can only occur if there is a mechanical failure. Early-model vehicles used a carburetor, which needed to be adjusted during a tuneup. If the carburetor is dirty or maladjusted, the idle circuit and the mixture is affected, causing poor performance, rough idle, stalling and fouling of the plugs.

Things You'll Need

  • Sparkplug socket/ratchet
  • Spray bottle of water
  • Plug wire remover pliers
  • Common screwdriver
  • ¼-inch drive ratchet
  • Set of ¼-inch drive sockets
  • Sparkplug gap tool
  • Code scanner
  • Timing light
  • Check the mileage since the last tuneup. Check for sparkplug gap increase every 30,000 miles. As the plugs wear, the gap increases, which causes a weaker spark and creates more heat in the coil. It also contributes to poor combustion.

  • Take a sparkplug out and check the gap using a spark plug gap gauge. Check the plug for carbon fouling, burnt-on oil deposits or indications of a lean condition, which makes the plug white with blisters on the porcelain. Always look in the owner's manual or a service manual for the proper sparkplug gap for the vehicle being worked on. If the plug has black soot on the porcelain, check for a rich mixture. If the plug exhibits an oily, crusty look, check the compression with a compression gauge and check the valve oil seals. Both of these condition can cause oil burning.

  • Take notice of the instrument panel. If the check engine light is on, pull the codes with a code scanner. Most late-model vehicles have a misfire sensor. When the codes are pulled, there will be a code set which, when interpreted, will state which cylinder has experienced a misfire. For example, the code will state "cylinder number 4, misfire detected."

  • Listen to the exhaust for an uneven sound or irregular "putting" type noise indicating a misfire.

  • Attach a timing light by hooking the pickup to each plug wire and the negative to a good ground. Shine the light on a flat surface with the engine running. Watch for a flickering light, indicating a bad sparkplug. This procedure will only work on vehicles with accessible plug wires.

  • Spray the sparkplug wires with water while the engine is running. Look for arcing to the block when the wires are wet. Replace any wires that show arcing.

  • Drive the car in first gear and hold the gas pedal steady at 3,000 rpm. This only needs to be done for a few seconds to check for a misfire. The engine should be very steady with no glitches or hesitations. If any are found, a tuneup is necessary.

  • Pull each fuel injector electrical connection off one at a time and watch the rpm drop and the effect on the engine idle. Reinstall the connector and proceed to the next injector. A fuel injector could cause the same misfire as a sparkplug. In most cases, fuel injectors just get dirty and need to be cleaned.


  • Don Bowman, A.S.E. Certified Mechanic
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