How to Troubleshoot a Popping Noise in the Engine

by Chris Stevenson
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Auto Engine image by Andrew Breeden from

Engine noises come in a variety of sounds and locations. Most engine noises have specific tones and rhythms associated with individual parts. Some noises will be heard during start-up in cold engines. Other heavier knocks will announce themselves under acceleration and heavy load. Sometimes clicking and clacking sounds will be constant and rise with engine rpm. Intermittent noises can be the most difficult of all to solve since they come and go with no regularity. Knowing how the internal reciprocating engine works helps the vehicle owner know what parts produce which sounds. The vehicle owner should use a process of elimination to pinpoint the exact source of the noise.

Step 1

Place the vehicle in park or neutral with the emergency brake set. Start the engine and open the hood. Try and determine where the general location of the noise comes from. Check the belts on all the pulleys. Make sure the belts rotate on each of their pulleys, with no slack. Check inside the cooling fan shroud to make sure the fan does not make contact with the shroud while rotating. Examine the radiator cap for a tight seal.

Step 2

Don a stethoscope and move it slowly over the top of the engine, listening for any internal knocking or clacking sound. Place the stethoscope probe over the valve covers, if so equipped, or the top of the intake manifold. Listen for any clacking or clicking. Such noises will indicate problems with the hydraulic lifters or rocker arm tappets.

Step 3

Use a floor jack to lift the vehicle, and place two jack stands under the front frame and two at the rear. Slide under the vehicle and place the stethoscope up against the oil pan or underside of the engine block. Listen for dull metallic clunking sounds that have a regular rhythm. Such a noise will point to a bad connecting rod bearing, and possibly a worn piston pin.

Step 4

Lower the vehicle from the stands. If you have found a clunking sound at the bottom of the engine, you will need to confirm it and locate the cylinder. With the engine running at idle, use the insulated plug wire pliers to remove one plug wire at a time at the spark plug location. Listen for any change in the clunking noise. If the noise disappears or lessens substantially when a wire has been removed, that cylinder will be the likely suspect for a bad rod bearing.

Step 5

Connect a vacuum gauge to an intake manifold hose. The best source will be a hose coming from the base of the throttle body or sensor on top of the intake manifold. Refer to your owner's manual for the correct vacuum reading of your vehicle at idle. Any deviation in the vacuum reading could indicated a blown head gasket. A blown head gasket at its outer perimeter on the edge of the cylinder will make a "plapping" or popping noise. Any evidence of oil in the radiator or coolant in the oil will confirm a blown head gasket. Blue-white smoke exiting the exhaust pipe means oil has crossed into the cylinder because of a bad head gasket seal.

Step 6

Listen very carefully to the exhaust manifold. A popping noise coming from it will indicate a blown or cracked exhaust manifold gasket. A continuous popping or plapping sound will be evident every time the engine cylinders fire.

Step 7

Connect a timing light up to the vehicle. Remove the air cleaner housing or the cold air intake hose to the throttle body. Refer to the owner's manual for the correct timing number in degrees. Any deviation from the manufacturer's specification will indicate a too far advanced or retarded condition. Incorrect timing will produce a popping backfire noise originating from the intake manifold, or from the throat of a carburetor. A popping backfire will increase when "reving" the engine.

Step 8

Let the engine warm up to normal operating temperature. Shut the engine off and examine the coolant overflow reservoir. Remove the reservoir cap and listen for popping or bubbling noises. Such sounds indicate a release of cylinder pressure into a coolant jacket in the head. This points to a bad head gasket.

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