1979 Porsche 928 with fuel issue
 

My mechanic says that sand is getting in and plugging up the fuel distributor. He says the gas tank needs to be replaced. That the interior has broken down and is sending sand to the fuel distributor. Has anyone ever heard of this problem?

There's a fuel filter (and 2 fuel pumps) between the tank and the fuel distributor. Not sure how sand is getting to the FD. And the tank is made of plastic, If it's not leaking fuel onto the ground it doesn't need to be replaced.

that sand could be broken bits of a crumbling fuel strainer or old fuel filter or corrosion from rusted internal fuel lines.
As Rob said the fuel tank is plastic
NOTE it wouldnt hurt to remove the fuel tank sender after the tank is close to empty to see whats going on in the bottom of your tank,
look inside the bottom of the sender tube,
it could be full of rust if so then it needs to be replaced

It's probably not sand, but rather crystallized fuel. I bought a used fuel tank for one of my 928s and it had old crystalized fuel in it I pressure washed the tank before installation, installed a new strainer, new fuel filter, new fuel accumulator, and new rubber hoses.

The fuel distributor has tiny little filters in each port that will clog up with the slightest debris. You're going to need to clean out the fuel system and replace some components, but you don't need a new tank, which is a good thing, because they're expensive.

It's also possible that on a 79 you have an in-tank pump and an external pump, and no strainer, but should still have a fuel filter next to the pump.

https://cimg4.ibsrv.net/gimg/rennlis...7bbb9d89f5.jpg
Crystallized fuel


https://cimg5.ibsrv.net/gimg/rennlis...dec707db42.jpg
If the threaded insert on the fuel tank comes out when you try to remove the strainer, Roger at 928sRus sells this assembly to salvage the tank and have it be leak free.

Pete in that picture I believe what your looking at is the rust from the fuel tank sender return line, and other metal parts of the fuel supply system.
you could confirm it with a magnet,
NOTE Dried out fuel usually turns to varnish sludge first then a hard mass like tar.

https://cimg5.ibsrv.net/gimg/rennlis...0e71cd03e2.jpg
removal of the pump or screen is made easier with an impact gun and adding a clamp around the collar to hold the threaded ring in place
https://cimg0.ibsrv.net/gimg/rennlis...461abae2b3.jpg
some of the nastiest sludge I have drained from a tank, once this stuff hit air and the remaining light ends bled off, it turned to hard mass

I don't think so. If you zoom in you can see the shape and the sheen of the material. It almost looked more like larger chunks of sandblasting media than like rust or like tiny pieces of glass. I didn't test it with a magnet, but don't believe it was rust.

Can the strainer be replaced without removing the tank?

To go deeper, the engine was removed and re-built. The fuel filter, fuel pump, and accumulator were replaced. The gas was replaced but the tank was never touched. It did sit for 2 years during this process. After install, it was running. Then it stopped running. He took apart the fuel distributor and saw it was plugged up with sand. He got it all out and the car would run again. Then it happened again. He said the car needs a new tank and that the plastic has broken down and is causing the sand.

Yes, and as MrMerlin mentioned the best technique for trying to get the strainer to unthread and still leave the threads in the tank is to put a hose clamp around the perimeter and use an impact gut vs trying to gently loosen it. The impact usually breaks the threads free and allows it to unthread rather than twisting the threaded insert that's bonded to the tank collar. But, if that happens, the assembly I pictured from 928sRus will allow you to salvage the tank. The other thing you should check and replace when the tank is empty is the hose that connects the upper and lower sections of the tank that actually goes through the frame rail. Now, maybe if that hose rotted, which I tends to do (then the tank leaks when above 3/4 full) dirt may have gotten in through that opening.

I've never seen the plastic on these tanks break down into a sand like substance. They crack and then leak, but I don't believe they do what he's suggesting.

How can "sand" make it from the fuel tank past the fuel filter to the fuel distributor?

My recollection is gas tank > in tank screen > in tank pump > (external pump/filter/accumulator, God knows in what order) > fuel distributor.

Drop the tank. Clean it and maintain everything else?

And replace all the soft lines.