Some engine noises can confuse the driver and lead to misdiagnosis. With so many engine noises that can differ in loudness, frequency, location and severity, the layman is hard-pressed to tell the difference between them and locate the accessory or component responsible for the noise. Normally a smooth running engine will produce a moderately quiet "rushing" sound when heard from the engine compartment. With the windows up and cruising down the highway, a normal running engine will make almost no perceptible noise. If a definite knocking, clicking or "plapping" noise is heard from the engine compartment, or with the window down while driving, it indicates that a part is failing or has already failed. Fine-tuning your ear can help you find the damaged or failing component.
- Skill level:
Things you need
- Long-shaft screwdriver
- Automotive stethoscope (if applicable)
- Large sheet of cardboard
Park the vehicle inside a garage, with the garage door open. (Sound amplifies better in enclosed areas, particularly over solid floors.) Place the transmission in neutral or park, with the emergency brake on. Make sure the tailpipe exhaust flows out the door. Lift the bonnet of the vehicle.
Position yourself on the side of the vehicle that holds the exhaust manifold. Small four-cylinder engines are located on one side---dual exhaust systems occupy both sides. Listen carefully for a sound that resembles a "slapping" or "plapping" noise that has a puff of escaping air quality to it. Leaking exhaust noises come in a variety of frequencies, but mostly commonly sound like a "plap-plap-plap" or a muffled ticking. Have an assistant raise the engine RPMs and hold it above idle. The sound repetitions will increase.
Have an assistant hold a large piece of cardboard over the exhaust manifold, or one exhaust side on a dual exhaust vehicle. With the engine idling, have the assistant momentarily pull the cardboard away from the manifold. Any indication in volume change would point to a leaking exhaust gasket.
Note: Do not position the cardboard over the intake manifold (unless four-cylinder) or the valve covers (older model).
Have the assistant momentarily restrict the exhaust pipe opening by using a rag to cut off the airflow. Apply the rag for a few seconds, then take it away. Repeat several times. While you are at the front of the vehicle, listen for a volume change--the exhaust leak will increase in volume with the tailpipe restricted. The noise will lessen in volume when the rag is removed.
Lean over the fender and try to detect the smell of raw gas. A leaking exhaust will expel traces of unburned fuel that has not cycled through the catalytic converter. The smell is sometimes accompanied by the sight of dirty, brown exhaust gases. Sometimes the gases will appear greyish or even black, with a very strong odour of fuel.
Position yourself over the fender and lean over the engine compartment. Listen for a "metallic" tapping sound, sometimes a light knocking sound. Move around the vehicle to determine which side of the engine the noise emanates from. On older model vehicles with a V-6 or V-8, equipped with valve covers, the noise will come from the top of the valve cover. On overhead cam engines, the noise will originate from the top of the intake manifold.
Place the handle of a long-shaft screwdriver to your ear, with the tip placed over the suspected location of the noise. Change the position of the screwdriver, moving it from the back of the engine to the front. An interior lifter noise will produce a metal-to-metal clacking or knocking sound when the screwdriver tip is directly over it. Use an automotive stethoscope to hear a clearer indication of the noise. A metallic vibration often accompanies lifter noise that you can feel through the listening tool.
Remove the air cleaner or air cleaner box intake hose. Momentarily cover the air intake (without choking off the engine) with a rag. Remove and replace and listen carefully. With the rag removed, a lifter noise will increase in volume.
Remove the oil filler cap from the filler tube pipe or valve cover (depending upon age and model). A lifter noise increases dramatically when the cap is off.
Note: A collapsed or severely worn lifter can cause a detectable engine miss.
Tips and warnings
- Remember that a lifter noise will produce a metallic clack, clicking or knocking--not to be mistaken for the "plapping" or hissing type noise associated with an exhaust leak.
- Do not touch the exhaust manifolds with bare hands.
- Perform the inspection in a well-ventilated area.
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