Engine block too corroded?
 

I'm in the process of trying to replace my leaky old Oberdorfer water pump on my late model, raw water cooled, A4. The aft portion of the block has been shedding LOTS of rust onto the bottom of the engine compartment for years. I was using a screwdriver to scrape away piles of rust surrounding the bottom bolt of the pump, and larger pieces of metal started coming off. I was able to pull off several clumps of metal under the pump by hand. It is soft, wet, and spongy. It seems to be a mounting flange, but the engine mounts are several inches below. I pulled out a piece 2.5 inches long, and I can crumble it in my hand.

I'm attaching photos. Is this engine block fatally rusted out? Assuming I can get the last pump bolt out and replace the pump with my newly-purchased MMI pump, can I trust this engine block? By the way, except for the leaky pump, it runs GREAT!

Phil
A Worried man. :(

Oh my!
You have some serious active corrosion going on! You didn't say, but I'll guess salt water cooled, no heat exchanger. Worry is good, but don't despair - yet. A little iron will make a LOT of rust.
Replacing the water pump is not a bad move - the old pumps do wear out and many folks spend way too much time and effort trying to bring a dead pump back to life (I'm one of them).
First thing I suggest you do is see how much good metal is left in the block. Pull that cover plate behind the starter/alternator and see what's left of the block - 3/16" or more is OK. Take some pictures while things are open and post them so we can see how things look inside the water passages. Don't reinstall the cover plate yet - if the block looks like a keeper, you'll want to install studs there to eliminate leaks in that area.
I also recommend you remove the manifold. Then pull the manifold studs and use a hooked piece of wire to check block thickness there. See if you're getting down towards 3/16".
Final check is the head studs, but let's see what you get in these two locations before you remove the head.
If you don't have the Moyer Overhaul and Repair manual, I recommend you buy a copy from our host. It's good reading, and folks responding to posts like yours will tend to refer to it.

The side plate rusted through a couple of years ago. Photo attached. I had to drill out almost all of the studs so I could get the new MMI plate and new studs in. I manually cleaned out the block and took the attached photo before I put it back together and flushed the block. I am very hesitant to take it apart again.

I had the manifold out about three years ago because it was blocked. I had it ultrasonically cleaned and it looked very good afterwards. It still looks good on the outside.

I also have the MMI overhaul manual. I have been using it for several years.

Your photos show a lot of corrosion at the water pump. A new pump will stop leakage in that area.
You also have a lot of corrosion at the water jacket cover plate, usually caused by leaking bolts and corrected by the installation of studs to secure the cover plate. You say you had to drill out studs - is this correct? That area should be dry now if you installed studs.
The heavy corrosion under the water pump is likely in the flange of the oil pan, not the block.

Thanks, Al. You're right, it must be an oil pan flange and not the block. I feel better about that. If I can find a way to remove the bottom bolt of the old pump, I'll install the replacement and the whole thing should dry up. Then I'll treat the rust.

Here's a photo of my replacement side plate. No leaks there since I installed it about two years ago.

Yep, nice dry joint. That's the way they should be.
I replaced my water pump with the Moyer ball bearing job when I went with glycol cooling/heat exchanger. With raw water, a bit of leakage was no biggie - plenty where that came from. With glycol, it's like leaking blood - you've got a limited amount!
If you're tempted to chip at the rust with a small chisel and hammer (very effective), think eye protection. Full face shield or safety glasses with side shields. You get a chip of that stuff in your eye, and you're going to become acquainted with your local ophthalmic surgeon.
Once you get it cleaned up, you're going to need a good primer. I've been using the Interlux barrier coat stuff - a 2 part epoxy. Rather a pain to mix in small quantities, as the packaging is designed to mix the whole lot at one time. I mix by weight - the MSDS data sheet will give densities so you can get from volume ratio to weight ratio. Once you get it primed, you can use whatever you want for a top coat - even latex.