How to Troubleshoot a Rough Idle

An engine can develop rough idle for several different reasons. Rough idle is often derived from wrong fuel and air delivery or poor spark delivery. Generally, rough idle can be diagnosed by visual checks of certain systems.

Instructions

Air Filter

1

Remove the air filter cover with a screwdriver. Pull the air filter from the air filter housing.

2

Hold the air filter up to the sky. Look through the filter in the direction air would travel. If you see little to no light, your air filter is plugged and your engine is starved for air. Replace the air filter if necessary.

3

Place the air filter back into the housing as it was removed, and tighten the screws until tight. If the filter was replaced, start the vehicle and check the idle.

Plugs and Wires

4

Put on safety glasses. Remove one spark plug wire from the coil and the spark plug. Inspect the plug wire visibly for any damage to the boots or the wire itself. Look inside the boot with the flashlight. Inspect the connectors inside both boots for corrosion or damage. Repeat for all remaining wires and replace damaged wires as necessary.

5

Remove a single plug wire and set it aside. Remove the spark plug from the engine head using the spark plug socket-and-ratchet set.

6

Inspect the tip of the spark plug starting with the terminal on top. It should be clean and have a silver color with no buildup. Ensure that there are not cracks to the porcelain housing. Check the center of the electrode and the ground strap. They should be clean with very little buildup. The center electrode should come to a point. Place the spark plug back into the spark plug hole and tighten lightly with the spark plug socket-and-ratchet set. Connect the plug wire back onto the spark plug. If there seems to be excessive wear or damage to any spark plug, replace the worn plugs with new ones.

7

Repeat the inspection of each individual spark plug. If there seems to be excessive wear or damage to any spark plug, replace the worn plugs with new ones. Once the process is complete, start the engine and check to see if the rough idle problem has gone away. If the engine still suffers from a rough idle, the fuel system may be at fault.

Fuel Filter

8

Back the vehicle up onto car ramps and set the parking brake. Put on the safety glasses.

9

Check the fuel lines from the top of the fuel tank to the fuel filter, and from the filter to the engine. Look for any leaks around screw fittings or in areas of excessive corrosion. Fuel leaks create a loss of pressure in the fuel system and can create a rough idle. Replace any fuel lines that are leaky or damaged.

10

Remove the gas line fittings on both sides of the fuel filter with the line wrench set. Use the line wrench set for both the fitting on the fuel filter and on the fuel line to prevent twisting or breaking the fuel line.

11

Pull the filter away from the fuel filter clamp and remove it from underneath the car. Wipe down the fuel filter and remove and gas or road film that may be on the filter or filter fittings. Blow through the filter in the direction gas travels through the filter. If air flows freely, the filter is in good condition; if air does not pass through the filter easily, the filter is clogged and is most likely the cause of the rough idle. The filter needs to be replaced.

12

Place the fuel filter back into its clamp and install the fuel lines with the line wrench set. Start the engine and check for any leaks at the fuel filter. If the vehicle still has a rough idle, either the fuel injectors are clogged or there is a faulty sensor on the engine and needs to be checked with a code scanner for trouble codes.