Tom:

The sending unit/fuel outlet on the W123 and W126 is accessable by removing the first aid kit floor in the rear package shelf. Take out the first aid kit and there are some screws that hold the trim panel down. Underneath is the top of the tank.

Needless to say, one should be somewhat careful not to liberally decorate the rear seat with diesel fuel!

I would be very cautious about adding "water absorbing" fuel treatments to diesel fuel -- if you add enough of something polar to the fuel to get it to absorb water, you've added enough to impair the lubricating qualities. I've never seen one or heard from a reliable source that it happened, but the thought of wrecking the IP by running fuel with insufficient lubricating power through it is frightening!

If the drying agent doesn't actually dissolve the water in the fuel, it will absorb into the water, increasing the volume of the water portion, and if THAT gets through the filters and into the IP, it is replacement time.

Ditto for adding gas treatment to a tank with sugar in it -- the only two times I've seen actual damage from sugar in the gas were a small engine carb completely plugged with syrup and the roasted crap from the combustion chambers of a Taurus that had sugar in the tank -- when the car ran like crap from fuel starvation, Dad put in de-icer (it was winter) thinking it was ice in the fuel line.
Dissolved the sugar in the methanol, ran through the filters and injection system, and turned into caramel in the combustion chambers. Ugly, had to get a new engine! Bent rods, bent valves, scratched cylinder walls.

Normally, if there is no water in the tank, sugar won't dissolve in gasoline -- just sits there and plugs up the fuel inlet. Sand will do the same thing!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!