How to Tell the Difference Between an Exhaust Leak and a Valve Lifterby Jack Hathcoat
Engine noise can be hard to identify and even harder to isolate. Sometimes the noise can be frighteningly loud and sound ominous--only to be engine belt noise. Other times the noise can be a tiny ticking that leads to a costly, major repair. Distinguishing between the assorted rattles, squeals, knocks and ticks is somewhat of an art form. Exhaust leaks sputter and are very different than lifter noise or any abnormal engine sounds at all. To accurately diagnose under-hood noise, here are a few guidelines for getting started.
Locate the noise. At times it is difficult to zero-in on the noise, especially since sound will carry through metal. It helps to twist an old newspaper into a cone and listen. The general area of the sound can then be more easily located.
Isolate the sound. A mechanic's stethoscope will help pin-point the noise; however, it's not foolproof. It's great for distinguishing between a bearing hum or squeal in an alternator or an air conditioning compressor. It will also localize an engine knock to the point that there is no question that the noise is coming from the engine. It cannot, however, be stated with absolute certainty precisely what inside the engine is knocking or where in the engine the knock is located.
Test for under-hood exhaust leaks. Exhaust leaks are distinct and easily checked. As the vehicle warms up, these leaks get louder. As the metal expands, exhaust manifold cracks and flanges expand, allowing more exhaust gases to escape. They make a sputtering sound as opposed to a lifter with a ticking sound. Exhaust leaks also leave a trail of black soot. It is also easy to smell exhaust fumes under the hood, but they do present the danger of carbon monoxide poisoning.
- Lifter noise is caused by the valve train components "lashing" back and forth. Older engines require periodically setting the lash to specifications. New cars use hydraulic components to cushion the lash. When these fail, there is a ticking at idle which will often quiet down as the engine revs up. This is caused by the accompanying increase in oil pressure, allowing a slight "float" in the hydraulic system. The ticking immediately reappears when the engine is returned to idle.
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