I have an ML320 1998 that I purchased new in Jan. 1998. Today, Saturday, As I tried to load the SUV to leave, the cargo door would not stay up to load it. Since I was traveling alone, I loaded everything in the back seat. I just need to know do I have to replace these cylinders or can they be repaired or simply filled up with fluid
Those are called gas springs. And when they longer hold the hatch open, they require replacement.
They will need to be replaced for your liftgate to stay open on its own. New parts usually run $20-40 each and are easily replaced by all but the absolute least mechanically inclined. You will need a small flathead screwdriver and a helper to hold the liftgate open while you replace the gas springs. Use the screwdriver to pop the spring clip out and pull the cylinder out of its socket. Repeat for the other end of the cylinder. To install, just pop the new ones into the sockets on the vehicle. The clips do not need to be removed to install the new ones. A counter person at the auto parts store may be willing to assist you since replacing these is less of a job than replacing headlights on most cars. If in doubt, the new parts have instructions with complete with illustrations.
Alternatives to replacing these gas springs are cheap and ugly. I have seen people prop them open with a broomstick and, if a customer’s hood used these cylinders and it wouldn’t stay open, I would clamp a pair of Vise-Grips onto the strut rod to keep the hood open so I could work (it’s not like it will make it any worse, after all). I can’t condone doing this, though, because if you don’t clamp them on tight enough, they could slip and injure you. There is also a kit on the market that is supposed to be a sort of “prop rod” for the gas springs, but it’s made of cheap plastic and does not work well. I installed one for a customer for the hood on a Ford Explorer, and it was a disappointment. It couldn’t support the hood without buckling, and he had me uninstall it and discard it. I recommend replacing them for a couple reasons. One, the convenience is worth the relatively low cost of repair, and two, it’s a Mercedes-Benz, and having to find a way to hold the liftgate open sort of tarnishes the prestige of owning such a nice vehicle.
They can be pricey…A quick cure is to use a small (4") pair of ViseGrips to clamp on one of the struts to hold the hatch open…Another is to look at how those sliding catches work on screen door closers to hold the door open. Then fabricate one to fit on the strut rod…
I think Rock Auto has them for $16 bucks each http://www.rockauto.com/catalog/x,carcode,1316423,parttype,1404