I have installed a couple of clear inline filters to help. One after the fuel pump in rear and another in the engine bay. I think the tank is the culprit because the rear one was pretty dirty looking after only a few hundred miles. I was thinking what your thoughts are on just flushing and cleaning the tank without putting in any after market sealing products? I am worried they could cause trounle down the road. I have read various reviews and im thinking I may just clean and reinstall.
From my understanding they are like that in stock form.
I have lots of thoughts on this.
1. Clear in-line fuel filters - great for cars with regular fuel pumps and carbs. I'm not so sure about for cars with mechanical fuel injection. All of the fuel filters that I've seen for mechanical injection cars are BIG and have lots of filter capacity for lots of gas to flow through with fairly little restriction. So even though it is useful from a diagnostics standpoint, it might cause performance problems.
2. You are still dealing with a recirculating fuel system that has presumably ancient fuel lines between front and back of car that have never been cleaned. If the return line is dumping stuff back into the tank it isn't the tank's fault and cleaning the tank won't really solve the problem. But more likely, there is crap in the tank that needs to be cleaned out.
If it were me, and I just went through this on my 1963 220 S fuel tank and fuel lines, I would drop the tank (which is supper easy on a W111 and I think it's the same tank on a W108) and take it out of the car. There should be a forward and return fuel line on the left side of the tank, disconnect those and run those lines with more new hose into empty 2 liter clear plastic bottles. On the under-the-hood side of things try to fill up the fuel lines with carb cleaner and then abruptly try to blow the line clean and have stuff shoot into the plastic bottle on the other end. BE CAREFUL DOING THIS as you don't want to end up with carb cleaner in your face. Do this over and over until you are no longer getting particles coming out of the lines.
I found it useful to use a bass guitar string connected to an electric drill (or dremel) and try to feed it into the fuel line as far as I could and have it spin around with carb cleaner in it... this seemed to loosen stuff. Because a guitar string isn't that long, you might want to try this from front and from back on both forward and return lines. I would probably follow this by flushing the lines with gasoline instead of carb cleaner to get that stuff out of the lines. On a W111 there are rubber fittings that connect pieces of rigid line - on my car I couldn't easily replace them. In a perfect world I would have replaced them and more thoroughly cleaned the metal lines.
As a separate issue you have your tank, now out of the car. You can see a LOT about the inside of the tank by taking out the screen drain, the fuel sender, and the gas cap. I have a tiny-size Mag lite that is LED that is designed such that you can take off the head of the flashlight and just have a bulb for general illumination (camping, etc). You can suspend that from a string into the (empty) tank and see pretty well from the fuel sender opening. Also you might be able to get some pretty amazing pictures using a cell phone. My Droid has the lens and flash close enough together that I am able to take very good pictures of limited portions of the inside of a tank (very limited portions but very high resolution) and you may get some answers there.
My tank cleaning involved taking it to the self-service spray car wash and spending a few dollars in quarters sticking the spray wand inside each opening and having water flow out the other openings. The wand at my car wash was rubber hose with a high pressure tip so I literally could get the wand all the way into the fuel filler and spray around, then stick it in the fuel sender and spray around, etc.
Then I plugged the tank back up with the old screen, capped hoses on the forward/return lines, a gas cap, and a butchered fuel sender that I used to seal the top. I used acetone (but you could use lacquer thinner) and put about 10 feet of lightweight chain (not the really heavy stuff) into the tank and sloshed it around and left it for hours, sloshed it around and left it, etc and then drained it again and rinsed it out again.
For drying the water out of the tank I found it worked quite well to put a shop vac in suction mode on the fuel filler and leave the other openings all open and let it set there sucking for 20 minutes.
You'll need a new screen and screen rubber ring (might be able to re-use old screen but they are cheap, a new fuel sender gasket, and new hoses to connect the forward/return lines back to the tank. On a W111 there is a short hose that goes from one size to another (8 mm to 6 mm or something) - if your old one was in good condition use it again, if not then buy it, it's cheap.