I think this is what happens when you have a bad diode (or diodes) in the alternator. Only way I know of to find out is with a scope. It'll charge and indicate something over 12 volts, but make a mess like you describe. Let a good shop look at it for you.

 

Still have the stock batteries in my '03.

 

Ditto.

Mine are going on 8 years, still strong and no acid leaks. I also expect them to die any time.

 

I'm still running original batteries in my 03 as-well (knock on wood). When you install new batteries clean every cable clamp, wire tap, nut and bolt that connects to the post with a wire brush and emery cloth any rinse with contact cleaner. Then use a contact grease when putting together to keep moisture out to stop oxidizing. Boiling could be bad batteries but over charging as-well. Good connections help power in to battery and out to starter/ECM for better starts and running truck

 

WoW, 4 in a row, mine are still stock and going strong...

 

Mine were OE at seven years and as a pre-emptory move, I replaced them with Interstates...I noticed an immediate improvement in starting and for some reason I'm getting better radio reception...They gave me no problems before the change, but were obviously getting weak......Ben

 

Still running the originals on my '03. Cranks a little slower in the cold now but seems to be fine since it's been warming up.

MikeyB

 

My 03 still on originals also!!

 

Originals on my '03 too and cranks well. Zero corrosion either.

 

Does anyone know if the brand of Costco batteries are US Batteries?

 

One more 03 on originals, 100K miles and 12.6VDC at rest. I change out at 12.2.

 

And squeaky-clean terminations are especially important on a dual battery set-up.

Wire brush/emery cloth. soak in baking soda/water, dielectric grease, firm terminations (2 wrenches!), battery paint if you wish. Clean the posts on the new battery too.