First of all, I am very worried about your using starting fluid. Some diesels use glow plugs as a starting aid and others don't. On some glow plug diesels (Ford Pickups as an example) there are decals in the engine compartment warning against the use of starting fluid. It sounds like your glow plugs are not working and that's why; a) you have to use the starting fluid, and b) you're getting by with it. If the glow plugs are working this may be a dangerous thing to do.
There is a simple way to check the glow plugs. It would best be done with a volt meter, but a test light would do. Follow the glow plug connection at the rear of the engine. There should be a wire from there up to a small box on the firewall behind the engine. Open this little box and first check to see if the fuse bar is intact. If not replace it and recheck your glow plug connections to see that the insulators are on correctly, if one is grounding, then you do not have all of them in series causing more current draw and a burned out fuse. With your volt meter or test light, test for voltage at the fuse block. If no voltage, find and check the glow plug relay, it may be inside behind the instrument cluster. If there is voltage, keep going from glow plug to glow plug testing voltage. This is a series circuit, so the voltage should progressively decrease by 1/4th as you move toward the front of the engine.
You also need to determine the first three digits of your vehicles serial number. It will be 123 or 115. I expect that it will be 123. The 123 series was sold in Europe for about a year before it was sold in the US. By finding out which series car you have, it will help everyone on this forum, to help you better.
If your 240D doesn't use oil and runs well when everything is right, I would encourage you to get to the bottom of your problems, and stay with it. The first 240D I bought many years ago was an old, tired car, but it was so great to drive and I loved it so much, I bought another that had 17,000 miles on it, and I now have over 500,000 miles on it. Their great.
Keep posting your problems and questions on here, and you'll find that you can get through whatever ails the car, and enjoy it for a long time. You will also probably make some cyber friends.
Best of luck,
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in