Removed Throttle Spring - wow, big difference!
 

I have really enjoyed working on my "new" 1983 300D non turbo BUT I have not enjoyed driving it because it was so hard to push in the accelerator. I mean, I drive big diesel school buses from the 90s that are easy, and this thing was wearing my leg out everytime I drove it.
And, it always felt like the car was towing a weight behind it. You had to keep pushing on the accelerator or it would decelerate (is that a word?) much faster than you would expect .
Anyway, I tried everything I could think of. There was nothing wrong with the accelerator itself. I lubed the throttle linkage and adjusted the bolt on it. Finally I read that some have taken off a spring and saw wonders, so I looked for it and finally found it against the firewall under the linkage. it was hard to find and I needed a light to see it. I used a screwdriver to pop it free.
I was AMAZED at the difference. It now drives like any other car. It even seems to go faster, and when I take my foot off it doesn't quickly decelerate like before.
I think that the spring must have been pulling on the throttle so that when I took my foot off it began pulling it back down.
If you are going to try to do this make sure you have the right spring. I saw another spring further away from the firewall, near the throttle linkage - that's not it.
Anyway, hope that helps someone else.

None of that sounds right. I think you should get the diagram or some pictures of the linkage setup and recheck everything.

I agree with skippy. If you don't have the throttle spring in place the throttle could potentially stick wide open even though your foot is not on the pedal which would cause a very dangerous situation when you least expect it.

This sounds extremely dangerous. I suggest that you find a spring with less tension and install that in place of the old one.

It may not be the correct spring. The return force on my W123 accelerator pedal never struck me as high.

This is the discussion I read.
Gas pedal hard as a rock | Mercedes-Benz Club of America
Note response #11 especially.
Just to show I didn't come up with it myself.
Although you have me concerned now. :(

I've seen videos of how some car commenters believe the gas pedal is too 'disconnected' from the engine, IE it is too hard to push. I personally believe mine is harder than I would Like, but I doubt I will go that far.

on my chassis, there are two different holes on a little bracket below that throttle spring. when the spring is in one position the throttle is too loose and won't return to idle, in the other it works right. perhaps your spring was merely stretched too tight to the farther hole, and you can try the nearer one.

Well, I pulled the spring on my 77 300D. First thing is that the throttle still returns to idle and the accelerator pedal returns to it's "non-pressed" position. The accelerator is not very soft and easy to press. The accelerator pedal is now much more like every other car I've ever owned. The biggest difference is that the engine response now feels actually connected to the pedal input. There is no longer what seems to be lag from the pressing of the pedal to the response of the engine. Everything I see so far says that this in not "the" throttle spring (based on the fact that the throttle still returns promptly and easily to idle) but is rather a spring put in place to make the accelerator pedal more firm and less sensitive to minor inputs.

I went for about a half hour drive that included highway, country roads and town. I have detected nothing negative about the whole experience so far. I am, however, carrying the springs and tools to reinstall it if things "go pear shaped".

I checked my wagon and it has three different holes at the bottom where you can attach the spring. I kind of wonder if this was a "personal preference" item on these, where they could be adjusted (or removed?), depending on what type of pedal feel you like. I happen to like a firm accelerator pedal and a soft brake pedal, for instance. I can't stand driving a car where simply touching your foot to the accelerator causes it to lurch forward. But I can see a wicked foot cramp on a long trip if it was constantly fighting back.

-Rog

There's two springs, one on the pedal and one on the injection pump.

My pedal has always been stiff since the mechanic swapped in nice heavy duty handbrake cable after I snapped the original (you LHD folks won't have that?).

Also in my country it's actually a legal requirement to have two means of returning the accelerator to 'idle'. Failsafe sorta thing...

Well, this is the one thing that I do have concerns about. I may try the local junk yards for a less strong spring to put on the car. I do like the idea of having two springs, but I don't think the stock spring is ever going back on my car. The difference I've experienced in "drive-ability" in the last 100 miles or so is nothing short of extraordinary (in my humble opinion).

The one behind the injection pump is actually two springs, one inside the other.

Totally agree though, those springs are very small and prone to rust. I would not make them the sole throttle return mechanism, especially with how much action they now have to move alone

I'm in the camp with those who think this spring is required. Inspect / lubricate all the linkage points, starting from the pedal and working to the injection pump. Replace any worn or missing components (that rubber pivot point on the firewall is notorious). Read up on the throttle linkage adjustment in the factory service manual, I've found that ignorant "mechanics" will sometimes adjust the linkage to adjust the idle speed. Check for collapsed motor mounts, which may also affect how the throttle linkage performs. If the spring in question has several different mount points, try them all. Finally, try a different (weaker) spring.

It has been said several times that a throttle stuck forward would be a major safety hazard, I'd hate to be that gal/guy who removed the spring and then crashed his/her baby.

this thread has some pics that may be of interest-

http://www.peachparts.com/shopforum/diesel-discussion/337755-question-throttle-assembly-return-springs.html