Hello,

Following the repair of engine oil drain plug (see this post) , I checked other parts underneath the car, which I have never paid attention on before. I was able to find that the real differential box was covered with thick oil. Given the thick oil covered around the fill plug, I believe that the fill plug was leaking the oil. The workshop that made the mistake on engine oil drain plug gasket also checked differential oil, I believe that they didn't apply enough torque when they closing the fill plug.

Anyway, to prove that my hypothesis that the fill plug was leaking oil was correct, I cleaned around the fill plug with WD-40 to dissolve the thick oil and scrubbed with blue shop towel... oil was dissolved by WD-40 then it showed yellowish coat. I thought that it was also residue of the oil so scrubbed it as well, but it was not easy to remove all of them. At this point, I started feeling that it might not be oil, but a coat and stopped cleaning it up. See the picture; the fill plug area was where I completely removed the yellowish coat, and you can see that some coats are still remain as I stopped removing it.

Do you know if that is coat or residue of oil? I hope that I didn't make a bad mistake which is not reversible... if that is the coat, what was the purpose and what would be the consequence not having it on the differential box?

Thank you in advance!

 

You mean the differential was covered in leaking oil, so you don’t know if it has any oil, but you don’t check if it has oil , instead you’re worried about something yellow. Wow .

 

Looks like a wax coat, similar to what they put on the steering box bolt after the recall. If it feels tacky to the touch, it's definitely wax and NOT oil.

The wax coat is likely an additional layer of protection against corrosion. I'd leave it alone.

 

Cosmoline

 

You didn’t hurt anything. But remove the differential fill plug to double check the oil level.

 

If it’s empty ? So don’t say he didn’t hurt anything.

 

Thank you all for the advice. Yes indeed it felt like wax. I may need to spray wax after checking the oil level. Regarding the worry about oil level, I didn’t check it yet as I didn’t have 14mm hex (going to get one today) but not worrying too much as the side where drain plug located was clean.

 

I don’t know what made you become a social misfit today, but this sarcastic response does not help anyone and it is not best use of your and others’ time. Basically what you are pointing out is the same as what Odd Piggy's response (which was engaging and providing actionable advice), but nobody will pay attention to your sarcastic response. You might think that you would look cool though.

 

Any excuse for not checking oil fill is moronic

 

the yellow stuff does look like cosmoline, but I'm surprised that rear diff would be covered in it that much.

How many miles? Anything above 80k and you should replace the fluid. It's so easy, you 50% there...drain and fill until it leaks out of fill plug.

 

tiny leak (if that's even happened) from fill plug is not going to empty 1 quart from the diff. You are overreacting a bit.
Your front diff has a vent and is probably covered in diff oil and only has like half a quart to begin with.

 

Yes, I was surprised too. So the fill plug side was completely covered by dried oil and even formed a lump of dried oil in dimples. The drain plug side was relatively clean. So I could see that the oil was leaked by fill plug. I also checked for other possibilities, like lubricant from main drive shaft, but they were clean. Mine just passed 86,000 miles. I didn't consider to change the oil as the dealership said that differential oil for GLK was filled for life, but now I am thinking to change it. Thank you so much for encouraging.

 

This is to an extent social media, so it gets anti social occasionally. I’ve been accused of mansplaining when trying to provide details. What is a site like this worth without details? I’ve even been cussed out when expressing an opinion. How does that add value? I’ve had people complain to the mods that I am over posting. That’s what we do here. No big deal. MMR1 gets a bit sarcastic at times, but many of his posts are helpful and right on target. Maybe he thought I was being critical of his reply. IDK. He’s not one of the trolls. Stick around, there’s good stuff inside.

 

Another hypothesis is that the oil that covered differential assembly was engine oil, not differential oil.
My car leaked a huge amount of engine oil recently due to the wrong drain plug gasket used (see this post).
The oil has contaminated entire splash cover (the inside of the splash cover was like Lake Superior), so it could cover the differential too. Weird thing is that other parts around differential were clean.

Is there a way to identify if a certain oil residue is differential or engine oil?

 

Maybe with gas chromatography. Engine oil and differential oil have different profiles. But I doubt it’s worth that.
Anyone else?

 

Another clue was that, although I forgot to take a photo before cleaning the differential assembly, the oil that covered it was completely black and dried up like lump of particles. My car is a Bluetec diesel, so the engine oil is also black unlike gas cars. I haven't seen differential oil residue or stain, but I assume that it should not be completely black...

 

Only a personal opinion ... overthinking.

Clean up, check levels, correct if necessary, done 👍

 

Yeah, it really does sound like you're making excuses not to check the level. It's easy enough to do and, worst case, you'll find it's OK. On the other hand, you might find it's nearly empty and save yourself a couple of thousand dollars.

 

Waiting for the delivery of a 3/8 drive 14mm hexbit. :-)

 

A bit? Buy a SET and be done with it. There are so many inexpensive options these days. You don't need big name, expensive tools for occasional use.

 

and, if you already have a SET, but without 14mm, then buy more complete sets 😀 13mm fasteners are extremely common in the world but, having said that, 14mm is far from rare.

 

Kids these days...

Any parts store should have them. Or, buy a coupling nut from the hardware store with 9/16 flats (3/8" nut, IIRC), stick it in the 14mm hole and use a 9/16 or 14mm socket on it.

 

Now there is a person who has done field work!

 

Reporting the outcome here. TL;DR - the differential oil was full and clean.

Received the hex bit set today and opened the fill plug. I prepared a screw driver to "dipstick" into the differential, but it was not needed. Some amount of oil (< 1ml) immediately spilled out as I opened the fill plug. I assume that it was because the car was on the Rhinoramp slope backward, or the manufacturer filled the oil more than needed. At least the oil is full tank. The spilled oil was also very clean with some good odor (smelled like cooking oil mmm), but anyway I will change the oil soon. For now, I just fastened the plug with a torque wrench (50Nm). Now I think that it was dried engine oil that was covering differential assembly.

Thank you all for the advices!

Right. I didn't have any hex bit, so got a SET. Finding a 3/8 driver hex bit set with 14mm was very difficult. A hex bit set with 14mm was rare, and for 3/8 driver was even rarer. Some auto parts shops had only SAE ones. I tried 9/16 hex bit on the drain plug, but it didn't fit... Using coupling nut is a great idea. If I would need another size of hex bit that I don't have in the set, I will try that!

By the way, I read from many articles that said GLK front differential would not be changeable. Is that correct?

 

To check the oil level properly, the vehicle must be level. And that's for any type of oil check: engine, tranny, differential, and so on.

Admittedly, the angle might be slight ... but I have no experience with how much of a rise the "rhino" ramps provide. Be that as it may, I'd not check oil levels if the vehicle is on a slope. 👍