M103 water pump still leaking from block
 

I pulled the water pump and replaced the hard o-ring against the block, filled up coolant through the thermostat opening and it's still dripping down the block.

It's not coming from the weep hole it's dry - definitely coming from the seal to the block.

Going to have to pull it off again. Can I try a dab of engine sealant on the o-ring?
Pump is only 6 years old so I don't see why I'd need to replace it (handles AZ 120F summers fine!)...will a dab of sealant maybe do the trick

When you pull the pump, check the pump face and the matching facing surface on the block for corrosion or pitting. With aluminum parts it is common to have some corrosion following a leak (especially if it leaked for a long period of time).

Your only solution may be a replacement part or getting liberal with some Permatex.

No corrosion at all as I've been really good with coolant.
I lightly sanded any old residue off and it was very clean.

I guess it's always a possibility the aluminum pump face has warped slightly over time and in this heat. I'll give the sealant a try and if no luck, I'll just have to replace it.

ps2cho, before pulling the water pump again, check the O-ring on the coolant pipe that attaches to the water pump and runs across the front of the engine to the auxiliary water pump. The last time I replaced the water pump on our 1995 E320 I inadvertently damaged that O-ring and coolant appeared to be leaking at the joint between the water pump and block. I really did not want to remove the water pump again, so I examined closer and noticed coolant leaking from the upper end of the coolant pipe. Simply replacing the O-ring on the end of the coolant pipe fixed the leak.

Pulled the pump today -- did it in record time! 45mins to have it in my hands.

I inspected everything, couldn't find much. Most likely is O-ring.
I did a dab of sealant on the block o-ring and will let it sit overnight to cure.

I'll fill up the pump tomorrow and then check the ground a few hours later to see if any signs that its coming out. I'm hoping this does the trick. I made sure all the bolts were really snug as well just in case one wasn't quite there before. Before it would just drip, drip, drip down the block onto the ground, so it should be obvious.