how can you test if motor mounts are shot?
 

does the engine actually sag?

i used to have a rough idle issue where the rpm would be bouncing around in D and sitting at traffic lights. but more recently, i noticed that when i put the car in P and let the car idle, i can feel the car jerking around which makes me suspect the motor mounts are shot. it's a 97 E420 with 109k miles.

is there a way to test the mounts before replacing them? btw, the motor mounts seem like something i gotta pay a shop to replace..

with engine running if you can feel it through the seat of your pants they are shot.i think that left side mount requires removal of the a/c compressor bolts.

I'm replacing mine this weekend.

Yes, it does. Place an old mount next to a new one and you'll see the difference. A good test is to check the idle quality in both D and R. If the mounts are collapsed, it should be noticably smoother in reverse. Engine mounts shouldn't cause the idle fluctuation you mention though.

Mounts
 

Another way to see it is to open the car hood where you can see the engine while on idle and shift the transmission to drive or reverse, the engine will leap towards the passenger side if the mounts are bad.

and if the car is ten years old, has over 100,000 miles and are original, then they need to be changed. :)

as do other rubber parts like the transmission mount.

Len
'99 E300TD 99,500 miles mounts just changed - big difference

haha, sounds good!

So what other parts do you recommend replacing at 100k miles, along with motor mounts & transmission mounts?

Replace both motor mounts. I had mine replaced about 2 weeks ago and it feels like a new car !!

After that and when waiting on a red light (idle in D), my 4 yr old son asked me: "Dad, why don't you start the engine?". I just feelt so good about those new mounts.:wacko:

Mine were so collapsed that the fan blades of the belt driven fan were digging into the shroud.

I'm also doing front and rear sway bar links and bushings. Some cars need other suspension parts replaced like ball joints, control arms, etc.

Len

mine knocked badly when i started the car and also when i made a right turn. the mount degradation was caused by oil leaking on to it and rotting it. cost me £10 to buy and an easy (bit dirty though) hour under the bonnet to replace

Timing chain, rails and guides, all hoses and belts.

I don't think there is any need to replace the timing chain that soon on these cars, especially if you're using the proper synthetic oil.

Len

Wow,engine mounts 101 here.You guys are great!I don't know alot about the newer MBZ's because mine are '89 and older.I haven't spent much time under the hood of my 88,or 89,My 79 and 81 are 300CD's.Are the mounts on the newer cars on some sort of shock obsorbers?I haven't seen them as I said.I know my two 300CD's have just a round,rubber hockey puck looking thing.My newer cars probably both need mounts by now though,if they're the style you guys have on yours..'89300,CE and '88 560SEC,AMG,Euro.(but only 4500km.on the 560.)Do you guys think I should throw a set of mounts,at each of them,and be done with it?

If you feel an unconfortable vibration inside the cabin, you probably should.