'00 Mustang 3.8L Belt Tensioner Problem

I've got some issue with my belt tensioner (it's spring-loaded) on my 2000 3.8L mustang...
The old fan belt started squeaking incredibly and it had been
stretched I think... I bought a new one, it doesn't just have the pure longitudinal ribs but has additional cross-hatching... anyway compared the two, the new one seemed about an inch shorter in circumference, put it on and it's quite a bit tighter. The range indicator for the tensioner seems like it's ok; it's not pulled somewhat not too little and not to the max.
My system looks like Figure 11 in http://www.autozone.com/az/cds/en_us/0900823d/80/1d/a4/de/0900823d801da4de/repairInfoPages.htm (or google ford mustang belt tensioner and pick the autozone link)
when the car first turns on, it goes through a series of RPM changes, it revs up and down as it idles. in the lower RPM state things are fine, but in the higher RPM idling state, the tensioner arm becomes unstable, and it goes back and forth and the whole system makes a "tic tic tic" sound, about 3 tic's per second. The instability in the belt runs all the way around the alternator (?? upper-left, with the triangle--it kicks on and off), and the two pulleys on the upper right. The smaller pulley on the upper right, which has no grooves (touches the back of the belt) and may have no real function, has the belt sort of move forward (toward the front of the car) and back. if the belt moved forward too much, it would slip off.
the pulley on the tensioner arm, i found out, had a hard ball-like mass inside one of the grooves that was large enough to possibly cause a problem. i picked it out and the system seemed to be a little bit smoother, but still did the tic-tic-tic.
I revved the engine and things seemed OK. the system doesn't squeak BTW, apart from the tic.
in the lower RPM state, things are very smooth. there is no tic sound or visible instability.
any idea what causes the instability? I assume a back-and-forth on the tensioner arm can stretch out whatever serpentine belt is placed there and could have been the cause of the previous belt's deterioration?
i am worried about difficulty in replacing the belt tensioner, the T50 (?) nut keyhole wasn't budging, I tried hard to turn it and it started stripping.
probably I will trust it enough to drive it in town to the shop but maybe there is some insight.
greg
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 13 Apr 2008 22:43:21 -0700 (PDT), Greg of KURST

Your car should have both a belt tensioner which controls belt tension and an idler which looks a lot like the tensioner but does not move in or out. They both have bearing in them which tend to wear about as long as a belt. If you belt is running back and forth over the idler or tensioner, there is undesireable play in a pully or sheave somewhere. It is most likely the idler or tensioner. It has been my experience that they both should be replaced along with the belt. They will cause belt damage or accelerated wear if bad. They are not terrible expensive and are in stock at most decent parts stores. If you still have noise or funky belt tracking, you may have problems in the driven components that need attention. If you have run the new belt with a bad tensioner and/or idler for more than a very short period, you may need another new belt. A good experienced tecnician should be able to diagnose any drive or belt problems up front. Belt changing is not quite as simple as it may look. If your car has many miles or has the original water pump, now is a good time to just replace it. It is also good to check the alternator and other driven components carefully.
Lugnut
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
"Greg of KURST" wrote:

Belt manufacturers have different theories on which style works best.

As long as it's between the marks, the length is okay.

Not belt related. Probably a dirty IAC (Idle Air Control) valve.

That's the air conditioning compressor.

The idler and the alternator.

Reduces the overall "span" of the top belt run, to prevent whipping.

The belt shouldn't move much, but the pulley is wider than the belt to allow some misalignment without it coming off. With the belt off, the pulley should spin smoothly (without any grinding or "dry bearing" noises), and should not wobble or slide forward/back on its bolt.

Could have over-stressed one side of the belt a bit, and now it's not tracking quite straight, or maybe there's a tiny bit of it in one of the belt's grooves. That pulley (like the top idler) should also turn smoothly; no grinding, wobble or sliding. The arm should swing smoothly (with some force), with no excessive forward/back movement.

The arm's normal spring-loaded swinging movement won't cause the deterioration (apart from the natural stretching any tensioner will cause), but it can be an indicator that something else isn't quite right. No belt is perfect, and putting tension on it will cause some uneven stretching as it moves around the also-imperfect pulleys. When the A/C compressor kicks on, that also puts more stress on the belt, and can increase the amount of belt bouncing and arm movement.

Torx bolts are great, as long as the hole is new, and the bit is a perfect fit, but once things get worn or loose...
If the arm isn't wobbling forward/back, I wouldn't worry about it. Usually it's the pulley bearings that go bad, and they can be replaced without removing the tensioner arm from the car.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.