Jim Grant's Tech Tips
'97 Mazda Millennia, Engine Knock
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.
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.