Van keeps throwing Serpentine belt - any ideas?

V8 5.7L engine. Mechanic friend thinks it may be the pulley attached to the water pump.

it makes a difference if it's got an AIR PUMP and if A/C or NOT. Dayco Serpentine/Belt Part Number: 5040940 Warranty: 3 YR Application: With air pump Without A/C Price: $20.99 Dayco Serpentine/Belt, Part Number: 5040880Vehicle Specific Dayco Serpentine/Belt Part Number: 5040880 Warranty: 3 YR Application: Without A/C Without air pump Price: $22.99 Dayco Serpentine/Belt, Part Number: 5061010Vehicle Specific Dayco Serpentine/Belt Part Number: 5061010 Warranty: 3 YR Application: With A/C With air pump Price: $33.99 Dayco Serpentine/Belt, Part Number: 5060945Vehicle Specific Dayco Serpentine/Belt Part Number: 5060945 Warranty: 3 YR Application: With A/C Without air pump

Wow, giving me a list of parts that I ALREADY HAVE was incredibly helpful. /sarcasm Now running under the assumption that I've already bought the serpentine belt and put it on, it's come off and shredded, and the new one won't stay on at all... any ideas on how to FIX THIS ISSUE?

didn't know if you were aware of SEVERAL different belts that would fit that engine.i'm sure it's routed correctly?don't know, unless someone has changed something different?may be a pulley getting in a bind?i assume you have had the belt off and turned the pulleys by hand to see if one was hard to turn?something is binding,got a burr on it or out of line if the belt is getting chewed up!........................................................................................................................................... How To Replace GM 5.7L V8 Truck Engine Serpentine Drive Belt greatautohelp.com/How-To/Replace-V8-Truck-Belt.html

Yes, I'm aware. Our original belt was a 945 - van with A/C, and put on as per the diagram in the manual. The first time that it came off, I hit standing water unexpectedly and the belt came loose. In researching, I found that that year of GMC products had an issue with the serpentine belt coming off if you hit water. So we put it back on - but each time it rained, even a small puddle would cause the belt to dislodge. So my father-in-law (very mechanically inclined but no longer available for assistance) checked the engine and said that there's an extra large alternator on this particular van due to the modifications done to the inside of the van (track lighting, satellite radio, all the bells & whistles) -- and that due to that extra-large alternator, we needed a longer belt. That's when we went to the 955 length belt that we have on now (or had on). It was running like a top until last Friday, when my sudden lack of power steering and sudden beginning of overheating told the tale - belt was off again. In retrieving that belt, I found that it had been shredded, and that there were "spaghetti" strings of the belt wrapped around the post leading to the exhaust fan. I pulled those out, and bought the replacement belt. When we put it on this morning, being 100% sure that it was properly seated in all grooved pulleys and across the pulley behind the exhaust fan, we released the tensioner pulley and started the van. There was an immediate "PING" of it hitting the fan blades and it came off. After talking to a mechanic friend, he's suggesting that the pulley behind the fan, at the end of the fan post, is the pulley that goes to the water pump, and that it's going to be a complete disassembly of the engine and replacement of the water pump to fix the issue (which is beyond my ability, means, and knowledge). I'm fairly sure that the pulley behind the fan is where the issue is, since all grooved pulleys are fine and the belt is sitting in them properly at the time of installation - but don't know if there's something I'm missing on why it may be coming off the pulley behind the fan now, if I need to change something in the installation process, or why this has gone from being a water-puddle related problem with a too-short belt to it coming loose almost instantly (it was on for about 30 minutes yesterday before it came loose, but was instant this morning). The auto parts stores here always triple-check to be sure we're getting the right part... and I had to fight to get them to sell me the 955 when all signs pointed to the 945 length when I put it on the first time, in Spring of this past year.

well you have a good parts store then,cause most don't check anything here,except NAPA.that's a one inch difference in the two belts.one is 94.5 inches and the other is a 95.5 inch belt.the tensioner should/may make up for that,but you know your van pretty good by now.stand by and maybe pushrod will see this.if anyone has seen or knows anything about this,it would be him.he has worked on about everything out there for a LONG time,and still does now.

Thanks - the only concern that we have is that in research, removing that radiator shroud means removing the radiator bolts and possibly the radiator. The tensioner pulley appears to be in the same condition it's been in since day one of us being in possession of the van. In the meantime, we'll double check the other pulleys. I'm getting advice from different sources - and nearly everyone is saying it sounds like the tensioner pulley is the issue, but I'm confused on how to the tensioner pulley is the only place it's NOT coming loose, if that's where the problem lies, but the belt is slipping forward rather than back and off (as it used to) so I'm a little puzzled. It's going to be Thursday before I can get in there to do any digging again, but will try the information that I've been given here and elsewhere at that time to see if we can fix it or what looks like the issue. Thanks again, all!

Tensioner *ASSEMBLY* not just the pulley is most likely the culprit... Radiator shroud should be a TWO piece unit and the TOP section will come right off.. after removing the bolts along both sides of it.