Timing belt sliding forward

[KCHead]

Just replaced the timing belt (and 3 bearings) in my 94 Cutlass with 3.4. Within 500 miles the belt had moved forward off the gears enough to chew it up and slip. I'm back to replacing the belt again. The bearings look fine and the tensioner appears to be working. Any suggestions on why the belt (both old and new but I assummed the old one was really old) keeps moving away from the engine and getting the side chewed up? Thanks in advance.

[DEI37]

I know I have seen Audi and VW engines do that. Though not to the point that it would appear you are describing. I'm talking about less than 1/8th of an inch. Do you have an pictures? Maybe get it running with the timing belt exposed, and see if anything is amiss.

[OldSkoolGP]

More than likely the tensioner pulley is pushed into the plate all the way or you have a worn idfler pulley that's flexing and allowing slop. When you have the idler pullies and tensioner out, hold them by their bearings and sping them. If they freewheel, get new ones. If they feel pretty stiff and don't spin much, you can reuse them.

[KCHead]

Thanks, but I put all new pulleys on. They appeared to be lined up and no obvious problems. When I run the car with the timing covers off, the belt goes right to the outer edge of the cam gears and appears to rub on the raised part of the pulleys. The rubbing is probably causing the failure, but what is causing the belt to move? I did not pull the valve covers off to move the cam to flat and reseat the cam gears since I was able to use the old timing marks and it worked fine. Can leaving the cam gears in their old position cause this? My goal this weekend is to pull everything apart, including the valve covers and cam gears, get the cams to flat and reassemble.

[OldSkoolGP]

Something's not lined up or screwed in all the way. The idler pullies have inner and outer lips so they do the majority of keeping the belt aligned. Make absolutely sure that that are bolted in all the way.