Replaced top, port idler pulley for serpentine belt last year due to a click sound - new one clicks too. Only for first 15 minutes, once warm - it stops.
Drop the belt off and turn the idler by hand, see if you can feel anything. Also check it there might be something touching it. Another place to look would be the belt itself. Check for any irregularities. Belts are cheap enough, and it's good to have a spare, just buy a new one (but not from Merc!) and see if it does the same thing.
I just rebuilt water pump and checked the spin of the idler pulley - all free and clear. The belt is not old either. I have a spare ( an older belt ) but would like to buy a new one. What is the non Mercury spec ? I've bought parts for my engine using a 1998 Chevy as a baseline - but the serpentine belt would we unique to a boat.
IF you have a high mount alternator, water pump and power steering pump as accessories, then the Gates number is 6PK1930. Should look something like this...
In future, posting the engine serial number takes a lot of the guess work out of questions like this.
The pix above is a raw water cooled - I have a closed system with a raw water pump and a heat exchanger -
I'll have to get in there to check. A quick glance shows the mercruiser and Sierra are about $95. On eBay, there's a guy selling based on the mercruiser model # for my year; closed cooling, power steering engine for around $35.
I'm an OEM guy unless I find a source that is equal to or better than OEM...
A Gates belt IS OEM.... Merc don't make a lot of things. Plenty of places to get 'original' equipment that hasn't been through the Merc markup system... Spark plugs are from AC-Delco or NGK, the trim pump is just an Oildyne 108D, engine parts (oil pan, timing cover, pistons, rods, bearings, etc) are GM, the alternator is from Mando or Motorola, the electrical system connectors are Delphi, the relays are OMRON, the pressure sensors are from MotoTron, even that tensioner you originally replaced is from Dayco.. The ignition coil and coil driver are GM. The fuel injectors are Bosch. In the drive, and the bearings are Timken... And there's plenty more. Buy OEM parts from the same place Merc buy them, and save yourself a TON of money!
AMEN ! Chris. I agree. I bought all ignition coil parts from the manufacturers and saved soooo much $$$. That list you provided is priceless - thank you.
