Just wanted to share my experience with the Benz on my latest trip. I drove from Phoenix to Palm Springs and back this past weekend. I traveled the whole time at 65mph with the cruise set. On the way there, the cruise kept slightly surging which was really annoying. Not sure if it was the buffeting wind or just that the linkages were stiff, but it didn't do it on the way back.
It only used a little bit of oil there and back, like maybe from half way on the stick to a quarter of the way down from the low mark, each way. Some things I want to upgrade or fix for next time; I need to fix the ACC vacuum pods and the A/C system, as it was getting hot, even in February. Also, I need to beef up the seat springs a bit, maybe with the M.S. kit. They aren't dead flat, but they do need some attention. Also, I'd like to get the 2.88 diff at some point to lower the RPMs. I made it to Palm Springs on half a tank, which is awesome. I ran into a guy at a truck stop off of the I-10 checking the fluids on his '85 300D, which was pretty neat. Oh, weirdly, I thought I was going to have a really big issue when I went to re-fuel, because the fuel door was locked! It has never done that and it didn't matter if the door locks were up or down. I found the release in the trunk though and got it open.

I love driving the 84 & 85SDs now that the front suspension has been renewed. The next comfort item on the list is the seats. Broken springs make long trips .......long.

I made another list & it is still full. Hopefully by the time summer is over.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

I fixed the broken seat spring in my 240d with a pool noodle.. worked excellent.. it would bottom out before on the way down the hill i live on.. every bump.. but after the noodle fix, the seat is nice and firm and comfy.. might work for you?

I fixed mine with pool noodles too. Lasted about a year and now I am back in the same situation as before. Better to actually fix the seat springs; assume they can be spot welded but not sure. There are a lot of threads on this.

Easiest way to fix broken seat springs is to slide each end into a section of metal tubing, filled w/ epoxy. I recall using copper tube, but steel tubing (ex. brake) would be better. Spring steel is very high tensile strength, so may not be reliable to weld, but just guessing.

I recall posting my lower seat repairs, but if not I give a brief description. I removed the torn cushion material from the springs. I recall it was horse-hair in my 1985 & cotton or foam in my 1984. Both had been cut into by the springs, but still fairly intact. In the firrst car, I beefed up the springs by winding a few coils from fairly thick spring wire (ebay) and adding in. That was because the factory springs seemed too soft. I skipped that the 2nd time because seems I made it almost too stiff. I strung ~1/4" nylon cord across the top for more cushion support, then a layer of carpet protector (Home Depot). The spikes keep it from sliding off the top. In at least one car, I covered the cushion w/ thick canvas that I hog-crimped to the metal top. I glued the felt strips back to the plastic seat cover.

The seats are nice and firm now and in the 1984 one seems to ride almost too high, even w/ the height adjustment all the way down. I am 6'1" and my hair would rub the headliner if I didn't usually have a crew-cut. I also removed the seat tracks and worked hard to free them up after years of spilt sodas and such. They slide easily now. I used much silicone lube.

One quick fix is to fill the seat from underneath w/ Dow Great Stuff foam. Some gomer did that "fix'ed er" in my 1964 Valiant. Actually seems pretty firm, but I am changing to modern seats w/ shoulder belt anyway. I don't know if they added any tensile support across the springs.

The one missing link is new seat covers. I have seen links to some, at exorbitant price (~$1000 leather). It would be nice to find something better than the cheap tie-on covers. Someday someone will find one for a common car that fit close enough. In classic car world, they aren't cheap being ~$500 for vinyl front covers for even common Mustang and Camaro.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

The Cruise on that car was mediocre when it was new, I took a number of road trips in one of those in the 80's. Awesome car, nothing better than getting in the fast lane going 60mph for a while waiting for somebody in a poorly ventilated car to get behind you then stepping in the turbo. i have very fond memories of the 1981 anthracite gray 300SD.

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1981 240D Four on the floor, Orient Red over Parchment, bought with 154,000 but it's a daily driver and up to 180,000 miles, mostly original paint and all original interior.

LOL! I did exactly that to a treehugger in a Volt (doing 15mph below the speed limit) just this afternoon. Oh to be a fly on the window inside that car...

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Makes the day worth while doesn't it? LOL

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1981 240D Four on the floor, Orient Red over Parchment, bought with 154,000 but it's a daily driver and up to 180,000 miles, mostly original paint and all original interior.

I'm planning a trip in my 83 SD from Wichita to Seattle and back this summer. Already pulled it out of rotation to change to 2.88 diff, reboot axles, rebuild trans, etc. Hoping to have 290,000 miles on it when I get back.

Regarding your cruise control "surging" issue, it's possible that it could be the speedometer cable. If your speedo waggles around a lot, it may very well be the culprit. After I had my cruise control box redone by JamesDean (here on the forums), above 65, the cruise is rock stable. That said, my speedo cable is rock stable at that speed as well. Between 55-60, it will bounce all over the place. As JamesDean told me, the cruise does play off the speedo cable indicator, which can cause the surging issue. This spring I plan on replacing the speedo cable with new...from Pelican!

I was under the impression that all W126s had an electronic speedometer. There is no cable. - Mike