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.
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.
