Between the upright of the back seat and the forward wall of the trunk
you mean the manual one on the throttle linkage?
Yes, a Mercedes 300D has a fuel tank.
In the 1984 300 TD, the capacity is 18.5 gallons of which 2.5 gallons are reserve. It should be the same for the '85.
Approx 18 gallons
You don't need to convert it. You only need to add the conversion kit if you choose to run veggie oil. Remember though that BioDiesel cleans out sludge from regular diesel fuel and if you try to run just pure Bio you may experience plugged fuel filters. Just buy a couple spare filters and carry them with you. I have a 82 300D Turbo I run on Bio. For a conversion kit look to LoveCraft in Hollywood California. One of the leaders in the industry. Keep on keeping on, billy 1982 300D Turbo "Joon" 260,000 miles!
Mercedes goes by body styles, yet classifies them by engine type. For instance, the 300D and the 280E are both W123 body styles. D is obviously for diesel and E is for gasoline fuel injected. The only differences between the 300D and 280E are the engines, electrical wiring, and fuel systems.
There are rubber hoses that will eventually leak when using biodiesel which, I assume, is why you ask. The injectors and injection pumps don't seem to have any such difficulties.
need to replace them with new ones
The fuel pump on a 1982 Mercedes 240D is located inside the fuel tank. A replacement fuel pump can be purchased at any auto parts store.
The glow plugs are screwed into the side of the engine, directly underneath the fuel injectors.
I had a comparable issue with my 1983 300 SD. It went to the point where the fuel gauge read full all the time, the temperature read far over 120
yes, if the fuel filter is clogged causing high back pressure on the fuel return it can pop off the injector fuel return lines [bypass hoses]