I noticed a few drops of oil on my garage floor the other day, so I put my car onto jackstands and got under to look around. Both the front and middle underbody sections showed signs of oil, primarily on the passenger (right) side of the car. I removed both of them and found more oil on the top side of these panels. Then I focused my attention on the engine oil cooler and the source of the oil seems to be from the cooler drain plug. Oil all around it, and even a few drops of oil on the head of the drain plug itself. I said a little prayer then put a socket wrench on it to see if it was just a touch loose. AFAIK tell, it wasn't (sigh). I don't see or feel any cracks in the plastic housing around the drain hole. I suppose there's an outside chance that I used a bad crush washer (stranger things have happened, ask me how I know) however I feel like it's probably a tiny crack in the housing. Altogether we're talking about somewhere around 2 tablespoons of oil (for the leak and residue everywhere).

Has anyone had an OEM oil cooler crack on them? They are plastic, and my car is now over 10 years old. I idled the car for 15 minutes while lifted and no leaking was observed (I realize oil pressure isn't super high during idle). Oil change was last done 3-4 months ago. I plan to drive around a few hundred miles this weekend and will inspect the car again on Monday. If oil seepage/leaking is observed again I'll start by removing the drain plug, cleaning the plug area, then reinstall plug with a new crush washer. If it still leaks after that, then I'm guessing I won't have any options besides replacing the oil cooler (looks like a sub $450 part when purchased online).

Any advice, similar situations, etc would be appreciated. Thanks fellas.

 

Hey man, really sorry to hear this. I don’t have any experience with this, but I’d also check the banjo fittings on the inlet/outlet. Tape a towel to the tray and go for a drive, may be able to pinpoint the source. Hope it’s not bad but luckily the replacement cost isn’t prohibitive - you might look at replacing with the larger BS cooler (which is all metal).

I’m not saying this to be irritating, but this is exactly why you should extract oil from the top and never touch those plugs. I would weld mine shut if possible. There’s just too high of a chance of screwing up the thread, cracking the housing, messing up a crush washer, under/over torquing, etc. Going forward I would suggest using a MityVac for changes.

Good luck

 

Yeah...that thought definitely crossed my mind lol

 

I I think it is important to really clean it really well, I would inspect where the oil hoses are attached for dampness at the unions.

If after your drive dampness reappears then it will be easy to locate, it might be a case of vibration fatigue on the cooler and oil is collecting at the lowest point ie the drain plug.

 

Thanks for the tip. Will do.

 

Do what @NZ-Merc and @BLKROKT wrote... clean up the area really well so you can determine where the leak is coming from. Once you find it and fix it - whatever that may entail - in the future use a vacuum method to extract the oil through the dipstick tube.

 

So it just occurred to me, as I was taking a shower earlier (lol), that when I did my last oil change I didn't replace the crush washer on the oil cooler drain plug. I don't always go this route, but for the last oil change I used FCP Euro's oil change "kit" for the C63. Which came with 2 new drain plugs but only one washer.

Hopefully this is a much cheaper/easier fix than I thought.

 

Drove 400 miles over the weekend. Everything from putting around town, cruising at 80 on the interstate, and a few full throttle runs up to 125ish. Removed the underbody covers last night and found zero oil drops. One of the hoses going into the oil cooler had some oil residue on it (driver's side hose), but I cannot recall with 100% certainty if I wiped it clean last week. I wiped it clean this time, and will reexamine everything in a week or two when I perform the next oil change.

 

Hmm, to follow up with this. I'm hopeful that it was much ado about nothing. Over the weekend I changed the oil. Since the last time I checked under the car for signs of oil residue I've since driven around 500 additional miles. And nothing. All underbody panels were bone dry. Furthermore, the oil cooler drain plug and hoses associated with it were all dry and still clean as well. So we are good!

Now, as to how oil ended up there in the first place, I have a pretty strong theory. Back in December I accidentally drove a few (okay, was like 10) miles WITHOUT the engine oil cap installed. It was a mess. Oil everywhere under the hood. God it was a massacre. I thought I had cleaned up all the oil pretty well (topside and bottomside) but perhaps not. Additionally, I didn't mention this initially because all the oil that I found (which prompted this thread) was on the passenger side of the car, whereas the engine oil filler is on the driver's side (and most of the topside carnage was localized on left side). However, some oil was "transported" to the passenger side (and all over really) by the serpentine belt.

So it looks like that's the answer to this whole mystery.

 

The first time I added oil to a car was a 1981 300SD that was a quart low. My Dad told me where to add it. A few minutes later after running the car we popped the hood again. BLACK oil EVERYWHERE! He looked at me and said, everyone does this... ONCE.

 

LOL. Yup. And you nailed it. Was shortly after I had just topped off the oil.