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Just did the motor mounts on my 2000 C280, 90k. I had the rough idle while stopped and it would almost go away when slipped into neutral. Car ran fine and the vibration was not noticeable at speed. The definitive test is to jack up the engine a little bit to see if the vibration goes away which it did in my case.
Here are a few things that might make it a little easier for you. All the usual caveats. Work safe!
You don't want to do this repair again anytime soon so buy good parts, OEM if possible, I used Corteco. Stay away from ebay parts!!!!!!
Put front of car up on jack stands (Important! not ramps, wheels need to be free to turn). Chock rear wheels.
Unlock steering wheel and THEN unhook battery.
Remove 13mm lower mount bolt both sides.
Passenger Mount:
Removal is to the front so alternator needs to come out, but check my later thoughts below.
Release fan belt at the idler pulley with E10 torx on the belt tensioner, CCW. Remove fan belt from around alternator pulley and rest on dip stick.
From below, remove splash guard to the right of alternator, 2x8mm washer bolts. (Alternator will not drop with the slash guard in place). Disconnect two wires to back of alternator. Remove lower E14 torx alternator bolt.
From above remove upper E14 torx alternator bolt. No chance of the alternator dropping as it is held tight by its bracket. From below, pry it loose, or you might get it started from above. If you have around 100k and haven't yet changed your voltage regulator, now is a good time. Three screws for the black cap and some clips, then gently pry off. Voltage regulator just 2 screws.
Top mount bolt is 16mm, kind of an odd size. SAE 5/8" works well. Lot of different tools that you can use here, stubby combo wrench, saltus wrench, low profile flex socket with short extension as a handle. Plenty of room to get both hands on the bolt. You may be strong enough to break the nut as is, if not, a good method is to pry against something solid and your wrench. All you need is one good pry to break the bolt free. I put the upper alternator bolt back in a few turns and that provided a nice slot for the pry bar. Remove the 16mm bolt and the heat shield. Then jack up the motor with a broad piece of wood under the oil pan. I do this in small increments, but this is probably not crucial as an old mount is about a 1/2" shorter than a new mount and you'll have to raise the motor the 1/2" and then some to get the new mount in. Remove old mount, sweep away grit in the mount seat. Note that the slot in the mount bracket (attached to the block) is facing away from the motor. Raise the motor until you can get the new mount in, locating pin of the new mount in the mount bracket, and the heat shield and put in the upper bolt finger tight and the lower bolt in a few turns.
Driver mount.
I read one version where the DIYer eased the AC compressor aside to remove this mount to the front like the passenger side mount and another DIYer who did it thru the rear by removing some hose brackets. I really didn't want to mess with the compressor, but it looked pretty tight from the rear. I had time so I decided to monkey with the rear removal. Release two metal hose brackets, one on the tranny pan and one on the oil pan. There is a heat shield to the left that you may be tempted to remove but the upper bolt is really tough so I didn't bother, and turns out it didn't need to be removed. Same as with the other side, easy access with both hands to the top mount bolt, use a short wrench and pry against something. Raise engine enough to get the mount out of its seat. Sweep out grit. To get the mount out, manipulate the left front wheel to move the idler rod and tie rod into the perfect position to let the old mount pop out. IIRC it came out tail end first. Work in the new mount the same way.
Button everything back up. Drivers side routine. The alternator is a real pill. Pay attention to the bushings in the brackets where the alternator is held. I started with the bottom bolt and with a stoke of luck the alternator slipped in for the lower bolt. Start with the side opposite the side with the bushing. There must be a reason those bushings are there. For a small pry bar? I then held the alternator aside and quickly put in the splash shield. This prevents the alternator from rolling down. I found it just about impossible to get the alternator into the top part of the bracket. I tried and tried but nothing worked. Finally in looking at the bracket I see the three bolts just to the left of the water pump pulley. I eased all three of them and the alternator slipped right in. I don't know if this is a good procedure as these bolts must hold some part of the water pump on, but I eased them as little as possible.
Additional thought: Book time is 2.5 hours so a tech could most certainly be able to do it in less time. I'm guessing that the correct procedure is to remove both mounts to the rear and the time consumed in removing and replacing the alternator is not necessary. On the passenger side there is just the heat shield for the o2 sensors with 2 easy bolts. I already had the alternator out, but I probably should have tried the same method of moving the right wheel around to allow the mount to slip thru as on the drivers side.
Either way, not too bad a repair and an easy way to save a little money.
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