Rattling Belt Tensioner - Again

2000 Olds Silhouette; 129K

About a year ago my wife reported hearing an intermittent rattle, especially at idle. I never heard it but had my belt tensioner pulley seize & blow one night. (Dead batteries in the flashlight too - I hate that). The tensioner itself was in pretty bad shape so I just replaced the whole tensioner along with a new belt. (And then no more reports of a rattle from my wife).

The replacements were both Dayco.

It is now about a year and 18K miles later, and the tensioner is rattling again. It is intermittent and only at idle and only when warm. Even the slightest change from idle (about 750rpm) silences it). It is more likely with a load (e.g. with the AC on). Sounds terrible.

If I apply pressure to the tensioner it stops the rattle.

I have pulled the belt - all of the pulleys spin quietly and without resistance or binding. (I checked all of that last year too). Nothing is out of alignment and everything seems to go perfectly smoothly if you just watch the belt spin. The only thing I really wonder about is steering pump whine which has been there the whole 18 mos we’ve owned the van. But that pulley, like all the rest, doesn’t show any obvious sign of a problem.

I did try a new belt - no change. (I measured them both and the old was still the same size as the new).

So I need some voices of experience. Is it likely that this tensioner is already worn out? Or is it sometimes the case that one of my components has a problem that can’t be found by sight, feel, sound? If so, any tricks for figuring that out? I just don’t want to have to keep wearing out a new tensioner every year if that is the story.


(FYI: the components here are AC compressor, alternator, water pump, and ps pump. There are 2 idler pulleys).

Well, the ideas didn’t come flying out of the woodwork or anything, but I’m just coming back to report that I figured this out. My wife had to call in sick to work so I had the van this morning and went back over everything.

Basically, the rattle was the stops on the tensioner hitting each other. The specs here are such that even with a new belt the tensioner is pretty much all the way at its stop - as in really applying just enough tension to make the belt work. I probably could have gotten the belt on without operating it. As soon as it heated up a little and then had to do any load work it any movement would have it rattling against the stop.

I actually got on the phone with a Dayco tech guy and explained the whole thing. He seemed surprised but didn’t give me any lines about it. As far as I know he is going to look into it.

But anyway, I went and exchanged the belt for one 1/2" shorter. This keeps the tensioner off the stops without getting in the way of anything else. (I tried 1" shorter but it put the tensioner pulley up on my power steering line). The GM belt length spec is 87.25" - Dayco’s spec for this car is 87.5" - I’m now running 87" - with no rattle.