Black Oil...a Diesel thing? - PeachParts Mercedes-Benz Forum

acastell · #1 12-14-2003, 02:02 PM

OK, I haved been a searching machine these past 24 hours. I learned about Temp guages, Tach fixes, Poil change intervsals/T/C change intervals (or stretch measurement), 116vs123vs126vs...ect...

But, I was unable to find the answer to this question.

As you may know, I am the new owner of the Maple Yellow 79300sd. Now over 500 miles on it and not a single problem (go I hope not).

Anyway, when I purchase the car, the seller and I changed the Oil (as well as every other fluid and filter). The we also switched to Mobil 1 15-50 Syn. First Syn oil added to the car, ever!

So, in my Gasser experience, a quick lift of the dip stick, after 100 miles, would show fairly clean oil (still amber clear).

Well, when I checked the oil after ~100 miles, it was BLACK. Not dark, buy black! Questions:

1) Is this a Diesel thing? Oil gets dirty fast..
2) Was the engine so dirty that the oil is already black?
3) Is the Syn detergent so effective that it cleaned the engine already?

If this is normal, then no need to perform the next steps. If not normal, then:
1) Would it be helpful to do another oil change. Forget cost (50$-oil and filter) is nothing in the name of preventative maintenance.

2) If you recommend another oil change, do you also recommend I change the filter. Forget cost again...no concerned with a few extra buck to gain piece of mind.

Thank you all....

By the way, if this has been answered, I will not take offense if you send me the link to the post or simply profide a query string. I tried "Blakd Oil"...too vague!

Anthony

Kyle Blackmore · #2 12-14-2003, 02:11 PM

All is well with BLACK diesel motor oil. I believe it takes a nano-second for it to change colour after it's been started. Change your oil every 5000 miles or 3 months and you'll be fine.

#3 12-14-2003, 02:59 PM

Quote:

Originally posted by acastell
I tried "Blakd Oil"...too vague!

That is your problem right there

#4 12-14-2003, 03:05 PM

There are several reasons why your Mercedes diesel oil became black ALMOST instantly. None of them are "bad" and you do NOT have any problem!

Diesel engines produce LOTS of soot - 10-100X as much as a gasoline engine does, and while MUCH of that soot goes out the exhaust, some remains in the oil and SOME may be recirculated back into the intake (if you car has and EGR system that is functioning). So your car makes more soot than a gasoline car!

Then there is the fact that Mercedes uses an oil cooler in it's engines. That cooler holds about a pint of oil and that oil can't be easily extracted without a chance of damaging the cooler fittings. Then there are the other passages of the engine that hold anothor pint or so of oil. So after you drain ALL of the oil you can (without turning the car upside down and shaking it) there is about a qt of dirty oil in the engine.

It takes about 0.1-0.2% soot (give or take a few tenths of a percent ;-) to make the oil appear black (not just dark, but BLACK), and so if you run the oil until soot reaches maybe 2%, and then drain out 7 qt (leaving one in the engine and then add 7 qts of new to the one qt of old oil left, the concentration of soot in the newly changed oil (once it's all mixed together) will be about 0.25% or high enough to make the oil appear BLACK.

But there is no problem with the oil appearing black - Mercedes counts on that! That's NOT a flaw - it's a feature! That's how they remove the soot from the engine. The oil holds the soot in suspension until it can be removed when the oil is changed. It's just important to remove the soot before it exceeds 2%. Newer API rated CG-4, CH-4 & CI-4 oils are required to hold even higher concentation of soot in suspension (Mercedes re4qires CF rated oils that will usually suspend 2% or a bit more soot), but Mercedes requires that the oil be changed when the soot reaches 2% no matter which oil you use. There are other oil characteritics that shpuld be monitored to establish oil change interval, but soot is the one that will usually limit oil longevity in a Mercedes diesel.

Some manufacturers use filters to extract the soot, and when such filters ARE used, the oil will remain "honey colored" sometimes for 10s or even 100 thousand miles. Problem with that (providing the additive package hold up that long) is that with the kind of oil system and system size that Mercedes uses, the soot filter (the Mercedes filter does NOT filter out soot) would require changing about every 2000-2500 miles and about a qt of oil would need to be added in addition to the filter with each filter change (to replace the oil trapped in the discarded filter).

Most MB diesels that are in good running condition will reach the 2% soot level somewhere between 5 and 20 kmi (depending on your driving style and whether its high speed highway driving or stop and go or short hop city driving and on climatic condition - bad weather USUALLY results in faster soot accumulation). Unless you bother to perform some oil analysis, it's best to stick with the change interval that the factory reommends, but if you DO perform oil ananlysis and don't do a lot of city driving, 10kmi or even longer oil change intervals are frequenty attainable. DO NOT simply assume that you can do that, however. If you want to extend the intervals, perform the analysis and use 2% soot levels to define the change interval. I usually run my mostly city driven cars between 5-7.5 kmi on an oil change and my highway cars to 10+kmi. The former (city cars) often approach or reach 2%, while the highway cars seldom exceed 1.25-1.5% soot.

Marshall

psfred · #5 12-14-2003, 03:31 PM

Black oil is normal. So are black fingernails from working on anything with diesel oil on them, the carbon won't wash off very well.

Change your filter again, no need to change the oil. All the crud washed loose in the engine will be sitting in the filter -- good place for it, but the filter will plug, if it isn't already, and you will be running unfiltered oil soon.

Happy driving!

Peter

stevelewis · #6 12-14-2003, 03:34 PM

My goodness Marshall, what a clear and consice[sp] explaination of the black oil question. Again, this forum blows my mind. thank you! steve 83sd

acastell · #7 12-14-2003, 04:09 PM

82-30TD...you got me! Actually, I stated this once before, but am amazed at the typos in my posts. I simply type as I am thinking...do not want to loose my train of thought. That haste, coupled with my pick and peck typing method leads to effors. Of course, I never re read my posts. Maybe I should start....

Marshall...I bow to you and your explaination! Top notch, thank you!!!!

Anthony

84300DT · #8 12-14-2003, 06:35 PM

black oil after a change is normal. i'm wondering why you would change a '79 car to synthetic?

#9 12-14-2003, 06:50 PM

Quote:

i'm wondering why you would change a '79 car to synthetic?

Well, synthetic oil can reduce the minimum cold start temperature by 10 degrees! Cold start friction is substantially reduced (my engines crank 40-50% faster using synthetic). Then on all of my cars, measured timing chain wear rate is reduced by more than 50% by changing to synthetic. I can use synthetic for more than 10kmi (before soot levels require changing the oil) in my highway driven cars.

No question, the oil is more expensive, but if we can start just one of my cars on just one cold day a year, when I couldn't before, I've saved much more than the added oil cost and the other benifits are all FREE!

Marshall

Gurkha · #10 12-14-2003, 08:58 PM

I wouldnt say it is a diesel thing but rather an engine thing as on my OM 616 engines, the freshly filled oil would turn black after 100Km wehreas the same oil would be almost as good as new on the Hino diesel engine even after 10,000km. I would definitely say it is an engine design issue.

psfred · #11 12-14-2003, 11:06 PM

It takes considerably longer for the oil to turn really black on the 300D. Better burn gives less soot, I suppose.

The Volvo TD gets black as fast as the old 220D -- and it's rather smoky, too.

Peter

84300DT · #12 12-15-2003, 11:52 AM

Quote:

Originally posted by Marshall Booth
Well, synthetic oil can reduce the minimum cold start temperature by 10 degrees! Cold start friction is substantially reduced (my engines crank 40-50% faster using synthetic). Then on all of my cars, measured timing chain wear rate is reduced by more than 50% by changing to synthetic. I can use synthetic for more than 10kmi (before soot levels require changing the oil) in my highway driven cars.

No question, the oil is more expensive, but if we can start just one of my cars on just one cold day a year, when I couldn't before, I've saved much more than the added oil cost and the other benifits are all FREE!

Marshall

thanks for the informative reply. to clarify, i was wondering about the leaks i have heard others experience after changing an older car that had run dino over to synthetic.

#13 12-15-2003, 05:27 PM

The full synthetics that I've used (as opposed to oil calling themselves synthetics, but actually containing largely hydrocracked oil) will quickly clean out sludge that oils using high quantities of viscosity improvers will often leave behind. If those deposits are plugging holes, then the engine COULD develop leaks. If the leaks are due to gasket or seals shirking however, Mobil synthetics will usually seal the leaks by causing the seal to swell slightly. Sometimes the leak will appear (as the sludge is cleaned out, but the seal doesn't swell quite as fast as the sludge disappears) but if you wait a few thousand miles, the seal WILL swell shut. I have NEVER had an engine leak only because I changed it to synthetic oil and I've change over diesel engines with more than 200kmi on them from conventional oil to synthetic oil. Such leaks WERE common with some early synthetics.

Marshall