Shake, Rattle & Roll: DIAGNOSING NVH COMPLAINTS

Motor,  Jun 2006  by Warren, Mark

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Every good diagnostician skips procedural steps once in a while. Why? Skipping ahead in a diagnosis often pays off. But when success eludes yon, you have to know when to circle back and do some homework. Sure, gathering information from technical service bulletins (TSBs), the International Automotive Technicians Network (www.iATN.net), tech hotlines, etc., is time-consuming. But knowing your options and flexibility are hallmarks of a top tech.

A while back a 2000 Mustang with a 3.8L engine came into the shop with a noise complaint. The customer described the noise as a loud chirp under the hood-intermittent, but sure to occur. The problem, we were told, occurred when the engine was hot or cold and at all engine speeds, and stopped in Neutral. Sure enough, we got the chirp to happen and started tracing it with a stethoscope. This is a slam dunk, we thought. The chirp was loudest at the water pump housing. So, was it the water pump or a belt slipping and making the noise? I could have easily removed the belt, but chose to spray water on it to see if the noise changed. With the belt soaking wet the noise staved the same. Turns out the noise was coming from the water pump bearing.

I've never heard a water pump bearing make a loud chirp like this one, but the noise was indisputably loudest there. We replaced the water pump, started the Mustang, and, sure enough, same noise. Further inspection with the stethoscope revealed the noise coming from the timing cover. One of my techs concluded it must be a bad front camshaft bearing. Ouch!

Time to do some research. A quick search of iATN yielded a wealth of posts on that exact problem. The camshaft synchronizer assembly (cam sensor housing) does not lubricate well and the shaft seizes, creating the chirp. Many customers ignore the chirp until the entire assembly is wasted. Heading the posts, I was not alone in replacing a perfectly good water pump. I wish I had removed the belt. The chirp would have still been there and I would have known it could not be the water pump. Alas, the path not taken.

The Right Equipment

The final part of any successful diagnostic equation is equipment. In diagnosing NVH issues, a standard automotive stethoscope (photo 1 on page 46) should he a part of every toolbox. It can he used as a listening tube, with the bell on or off, and with the vibration diaphragm. The next step up is an electronic stethoscope. This tool will expand your ability to pick out the vibration. The sensitivity and volume control take it a step above the standard stethoscope. I use the The Detective's Ear (photo 2) from Thompson Automotive Labs (www.accminftrc.com).

The OmiSonic from Omitec, Inc. (www.omitec.com), is an electronic stethoscope and ultrasound detector. You can see in the OmiSonic kit (photo 3 on page 48) the aluminum vibration probe and the black sound tube. Also, note the ultrasound generator for detecting air leaks. Ultrasound tools I've used in the past (which had an LED indicator only) were disappointing at best. But the OmiSonic has changed my mind about the effectiveness of ultrasound diagnostic tools. The main unit receives ultrasound, filters it, then slows the frequency down to an audible level heard through the headphones. With the volume, gain and bandwidth controls, you can effectively hear the ultrasound. Using the ultrasound generator, I easily located wind noises and whistle leaks that were impossible to find using other methods.