Just changed the differential fluid in my '83 300D (110,xxx miles). Both the drain plug and the fill plug appear never to have been disturbed, and were VERY tight. The fluid that came out was a brownish red and had some bubbles/foam in it. It's possible that it never has been changed. I refilled it with the Mobil 1 synthetic gear oil. While I had the plugs out I soaked them in a bit of brake cleaner and then cleaned the threads with a wire brush. They were a little gummed up with black stuff.

My problem is that the differential drain plug won't go all the way back in. I can screw it in some by hand and then some more with my socket wrench, but once it's probably 2/3 of the way in it just won't turn anymore. I tried removing it completely and trying again, but never could get it to go in all the way. I'm fairly certain that it was screwed all the way in before, but I didn't make a point of noticing and it was so gummed up and dark that I might be wrong about that.

Has anyone else had this problem, and do you think it's anything to worry about? I took the car for a little test drive after I finished the change and then parked it in the driveway about an hour ago, and so far there are no leaks.

Is it possible that the inner threads on the diff got buggered up or deformed?
If its not leaking don't worry just monitor it

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1981 240D 143k 4 spd manual -SOLD

2004 VW Jetta TDI 5 speed 300k -still driven daily

I suppose it's possible, but I'm not sure how that would have happened. Also, it's possible that some gunk that had been floating around in the old diff fluid got stuck on the inner threads while it was draining and is now preventing the plug from threading properly, although that seems unlikely. Unfortunately I'd have to remove the drain plug to check that hypothesis, thereby losing all of the fluid that I just put in. Although I guess I could always put that fluid back in once I (hopefully) fix the problem...

I don't mind leaving it as is and monitoring, but with all of the vibration in these cars I'm concerned that it will come out while driving and then I'll be in a real pickle.

If you are concerned about the plug coming out, make sure the diff housing is clean and drain it into a clean container to reuse and try and chase the threads with a small wire brush or the like.

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1981 240D 143k 4 spd manual -SOLD

2004 VW Jetta TDI 5 speed 300k -still driven daily

debris from inside is most likely stuck in the threads... start cleaning.

...you didn't switch the plugs, right? It should be obvious.

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If it was that bad... You should pull the cover and flush the entire diff. with brake clean. Scoop and blow out all the crud you can, wait for it to dry completely and re-fill.

More than likely that trash got on the threads.

Or, is it possible to get the plugs mixed up since you had both out? I always pull one at a time and then immediately temp. re-install. I never noticed if they were a different length or thread?

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AJ

1985 300D (SOLD)

Your drain plug is fine, its NPT so the threads are tapered and the plug will not go in all the way like the fill plug. Clean the threads with some brake clean, apply a little ultra gray or black silicone and tighten. It should be tight but not super tight, you do want to get it back out again in 30-50k miles.

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1998 Mercedes E300TD

1983 Mercedes 240D Turbo, 131bar injectors, Cosworth intercooler and 63' Ford Falcon radiator, Ardic Parking heater, Headlight wipers, Best 38.6mpg.

1973 Saab 96 Rally Car, 1.8l V4 with all the race bits

I found some photos of differential housings and the drain plugs all appear to be fairly flush with the housing, i.e. screwed in all or nearly all the way, or at least farther in than mine is currently. See here for example.

I'll remove the plug tomorrow and ensure that there is no gunk stuck to the inner threads. Hopefully that will take care of it.

I havn't done mine but that looks like an off plug.

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1983 300 SD. 157K.
1964 Studebaker GT Hawk
1972 Corvette, Bought new
2007 Toyota Tundra
2007 Toyota 4 Runner
1993 Toyota Sequoia
Press 1 to continue in English, Press 2 to disconnect and call back when you learn to speak English.

ROFL!! Thats a good one...

I have since replaced my diff. and have done everything short of replacing the gears, bearings and the pinion seal in the other. I'm sure there is a torque value on both plugs. I don't know what they are. But, I have opened and closed enough drain plugs in my lifetime to know that it is not that critical. As long as it is snug. Most people tend to "over torque" drain plugs (Jiffy Lube is a good example). It is usually measured in inch pounds instead of foot pounds. I don't do the metric thing (NM) and I always convert it because all of my wrenches read differently.

Anti seize goes a long way to preventing future plug removal problems...

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AJ

1985 300D (SOLD)

What criterion qualifies a plug as "off?"

Go click on "here" in post #8 and he is showing the arrows to the Diff. mounting bolts.

The pic shows the whole diff... BUT the pic has red arrows. It took me a second or two to catch on too.....

The mounting bolt would be an "OFF PLUG"....


Edit: Geez

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AJ

1985 300D (SOLD)