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Below is a collection of Jim Grant's Tech Tips sorted by Vehicle Make. These Tech Tips were answers to questions submitted to Jim by ALLDATAdiy.com users over the course of many years.
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  Jim Grant's Tech Tips

'97 Mazda Millennia, Engine Knock

Q: I have a great handling, fast little road car, an upscale '97 Mazda Millennia V6 with close to 80k miles on it. Here is my question: For the past year or more, after driving highway speeds for a time and I come in to gas up or let it idle, I get a LOUD repeated banging coming from the front of the engine. It almost sounds like a rod knocking (on my Dad's old '41 Chevy). But if I leave it idle for five minutes or so or if I turn it off and start it again after five or ten minutes, the sound has disappeared. I have had the fan belt replaced twice in the last year and a half, and re-tightened once. I don't want to mislead you, but it sounds to me like some kind of a belt may have stretched after extended use, but then shrink back to the right size again when cool. I can't isolate the noise by just listening by ear (I don't have a stethoscope), but are there timing belts or some belts just inside the front covers on my engine that it could be? I would hate to have one break on me or slip out of place or whatever and leave me stalled by the roadside.

- Mike

A: I think it would be a good idea to have the hydraulic belt tensioner for the timing belt inspected. Many, but not all engines that have timing belts, use an automatic tensioning device to assure that proper timing belt tension is maintained. This results in a quieter operating timing belt, allowing for stretching/growth of the timing belt with age and extends timing belt life. At high speeds the timing belt exerts an outward force as it travels its path. This outward force causes the tensioner to retract. If the tensioner is weak, after high speed operation, it will not correctly extend and remove the slack in the timing belt in a timely fashion. A loose timing belt can cause a slapping noise that sounds like an engine mechanical problem. Shutting the engine off allows the tensioner to extend and remove the slack in the timing belt. On start-up the noise is gone. With the mileage on your engine you're due for a timing belt replacement. When the timing belt is being replaced inspection of the tensioner should be performed. There should be no signs of oil leaks from the tensioner and there is a specified free length. If there are signs of oil leakage or the free length is too great the tensioner must be replaced! For folks that are going for the second timing belt replacement, I always replace the auto tensioner. There's too much at stake if the tensioner should fail. Once the timing covers are removed you'll likely find the cause for your noise.

 
     
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