Engine rough idle can be caused by a series of problems, or just one faulty component or adjustment. Finding the problem can be daunting, especially with so many sensors and switching valves involved. However, the most likely causes come from the basic operating components that control fuel and ignition. Compression does not usually show up as a problem until later in the vehicle's lifespan. Fixing a rough idle requires a number of sequential check procedures, eliminating them one by one until the problem has been located.
Drive the vehicle to a reputable service centre that has a code scanner. If your vehicle has a universal scanner jack installed under the dashboard, the technician will hook a code scanner up to it and retrieve data from any failing component or system.
Place the transmission in Neutral or Park. Apply the emergency brake and raise the hood. Look for any disconnected or cracked vacuum lines attached to the carburettor or intake manifold. Refer to your owner's manual for the location of the EGR vacuum hose, PVC vacuum hose and the hose leading to the brake booster diaphragm. Check all rubber vacuum lines that lead to engine compartment switching valves. Check for tight connections on any sensors, such as the electronic choke and throttle positioner.
Start the engine and let it warm up. Make sure the choke linkage deactivates completely upon engine warming. Use a pair of insulated plug wire pliers to pull each plug wire from the plug location. Note any drop in engine speed -- this will be normal. No drop indicates a problem with a plug wire or spark plug. Shut the engine off. Use a plug socket to remove each plug from the head.
Check the condition of each plug. They should be light brown or tan in colour. Any plug that shows heavy encrustation (white or brown), black colouring or appears wet must be replaced. Check the gaps of the plugs using a feeler gauge. Refer to your owner's service manual for the correct plug gap. Depress the spark plug electrode down to the proper thickness of the appropriate gauge width.
Reinstall the plugs and start the engine. Recheck your plug wires again with the insulated pliers. Hold the spark plug end connector against the engine block. No spark indicates a bad wire. Replace the wire by disconnecting it from the spark plug tip and the coil or coil pack. Check your service manual for the proper procedure. Some plug wires connect underneath a plenum cover, or down within the valve cover.
Locate the idle speed and idle mixture screws on the carburettor, if so equipped. The speed screw will attach to the carburettor linkage and activate a cam. The mixture screws will sit below it and be attached to the base of the carburettor. To increase the idle speed, turn the speed screw inward (clockwise) to raise the idle. Use a screwdriver to turn the mixture screws in or out (one at a time) to achieve the highest rpm speed. Adjusting the idle speed or mixture will sometimes straighten out a rough idle.
Shut the engine off. Hook up the positive and negative leads of a timing light to the positive and negative leads of the vehicle battery. Attach the plug clip to the number one cylinder. See your service manual for the number one plug position on your vehicle, as well as the proper timing setting, measured in degrees.
Start the engine and aim the timing light at the steering damper (crankcase pulley), and read the degrees on the damper and tab marks. If the timing marks do not align, loosen the distributor base bolt with a socket and turn the distributor to achieve the correct degrees, according to your service manual specifications. If you have a vacuum advance hose leading to the distributor, disconnect it before checking the timing. Reconnect it after the timing check. Tighten the distributor base bolt with a socket.
Turn off the engine. Look for an in-line fuel filter on your vehicle. Refer to your service manual for its location. It could be in the engine compartment or routed under or alongside the chassis frame leading to the gas tank. Use a screwdriver to remove the hose clamps on the fuel filter. Plug both hose ends with spare bolts. Blow through the fuel filter at both ends. If obstructed, replace it with a new one. Reconnect the hoses to the new fuel filter and tighten the clamps with a screwdriver.
Remove the lid to the cold air intake box or air cleaner housing using a socket or unclasping the snaps. Examine the air cleaner element by holding it up against the sunlight. You should be able to see daylight through the element fibres. Tap it firmly on the pavement to remove excess dust. If the filter has oil on it, or appears clogged, replace it.
Hold a stethoscope over each fuel injector if you have a fuel injected system. Fuel injectors normally buzz when the engine runs. No buzzing indicates a failed injector. Hook the negative lead of a voltmeter up to an engine ground source and the positive lead up to the electrical connector on the fuel injector. Refer to your owner's service manual for the correct injector voltage. No voltage at the injector indicates a problem with the wiring or vehicle computer.
Locate the EGR valve on your vehicle. Consult your owner's service manual for its location. Most EGR valves connect to the intake manifold directly by use of a hose and flange attachment. Look for the plunger mechanism on the EGR valve. Have an assistant rev the engine. The plunger should activate, moving in and out. No activations means a defective or clogged EGR valve.
Disconnect the hose at its farthest point from the EGR valve and feel the end for exhaust gas pressure with the engine idling. No pressure indicates a problem with the valve itself or the exhaust system.
Remember that a number of other components can cause your vehicle to have a rough idle. Perform a compression check on your cylinders if your ignition and fuel systems appear in proper working order. Bad compression can be caused by burnt valves, a worn cam lobe or worn piston rings. Check the exhaust for excessive back pressure, usually caused by a clogged catalytic converter. Check the oxygen sensors for proper physical operation and voltage. Check for bad gas, or low fuel pump pressure.