Here are some potential culprits:
* Significant internal engine wear: High levels of metal particles from worn piston rings, bearings, or camshaft lobes are the most likely cause. These particles will quickly darken the new oil.
* Fuel dilution: Diesel fuel leaking into the crankcase (a common problem in older diesels) will drastically darken the oil. This can be caused by worn injector seals, cracked injector tips, or a faulty fuel pump.
* Contamination from previous oil: If the oil change wasn't performed perfectly, residue from the old, dirty oil could remain in the engine, quickly darkening the new oil. Though unlikely to turn it completely black in a day, some residual discoloration is possible.
* Cooling system leak: If there's a leak between the cooling system and the engine, coolant (often containing antifreeze) could mix with the oil, causing a darkening and potentially a milky appearance. This is a serious issue.
What you should do:
1. Check the oil again: Make absolutely sure the oil is indeed black. A slightly darker than expected shade is normal in the first few hours. True jet black within a day is alarming.
2. Inspect the oil for other signs: Look for unusual smells (fuel, antifreeze), milky consistency, or metallic grit.
3. Check the dipstick again and also the oil filter for unusual contents or characteristics: Carefully remove the filter and inspect the oil coming from the filter, paying attention to its texture and color.
4. Get the oil analyzed: A professional oil analysis can reveal the presence of contaminants such as metal particles, fuel, or coolant. This is the best way to pinpoint the cause.
5. Consider a compression test: This will help assess the condition of your engine's cylinders and piston rings.
6. Have a mechanic inspect the car: Given the age of the vehicle and the severity of the problem, professional diagnosis is crucial. Ignoring this could lead to catastrophic engine failure.
The rapid darkening of your oil is a serious warning sign that requires immediate attention to prevent further damage to your engine. Don't drive the car extensively until you've had it checked by a mechanic experienced with older Mercedes diesel engines.
