Oil pressure light flickering? Engine knock? Both? Major repair problem or minor annoyance? Either way, you should always quickly investigate the source before it becomes an even bigger problem.
Oil pressure - or more precisely the lack of it - in certain parts of your Mercedes' engine can become a major repair nightmare. All engines lose a certain amount of oil pressure over time as normal wear increases bearing clearances. But unusually low oil pressure in an engine, regardless of mileage, is often an indication that something is seriously wrong and requires immediate repairs.
That "tappet" noise may be only one sticking lifter but it may also indicate an oil flow problem that will eventually cause damage to at least one valve.
A flickering oil light is more difficult to troubleshoot if your engine is not obviously in need of major repair work.
Modern engines with hydraulic lifters, tight tolerance bearings and miniature oil filters require conscientious monitoring of oil pressure.
The following diagnostic tips (excerpted from "Troubleshooting Low Oil Pressure", Underhood Service, 10/97) will help you determine whether you have a major repair problem or just a minor annoyance.
Mercedes Oil Pressure Diagnostic Tips
A good place to start your diagnosis of a low pressure condition is at the dipstick. Check the oil to see that it's at the proper level (not low nor overfilled). If low, the engine may be burning or leaking oil. Adding oil may temporarily remedy the low pressure condition, but unless the level is properly maintained, the problem may recur.
If the engine is leaking oil, try new Mercedes head gasket or seals to fix the leak. If the engine is burning oil, the Mercedes valve guides and seals are most likely worn, but the rings and cylinders might be bad, too. A wet compression test and/or leakdown test will tell you if it's the Mercedes valve guides or rings and cylinders that are worn.
The least expensive fix in the case of worn guides is to install new Mercedes valve guide seals (if possible) without pulling the head.
The best fix is to pull the heads and have the guides lined, knurled, replaced or reamed for oversized Mercedes valve stems. Worn rings and cylinders would call for a complete overhaul.
Also note the condition of the oil and make sure it's the correct viscosity for your Mercedes and climate.
Heavier viscosities, such as 20W-50, straight 30W or 40W, may help maintain good pressure in hot weather but are too thick for cold weather driving and may cause start-up lubrication problems - especially in overhead cam engines.
Light viscosities, on the other hand, such as straight 10W or 5W-20, may improve cold weather starting and lubrication but may be too thin in hot weather driving to maintain good pressure.
That's why most car and OE parts manufacturers recommend 5W-30 in modern engines for year-round driving.
If the level is okay, the next thing to check would probably be the pressure sending unit. Disconnect the unit and check the warning lamp or gauge reading.
If the warning light remains on with the sending unit disconnected, there's probably a short to ground in the warning lamp circuit. Likewise, if there's no change in a gauge reading, the problem is in the instrumentation, not the engine.
Bad sending units are quite common, so many mechanics replace the unit without checking anything else to see if that cures the problem. This approach might save time, but it's risky because unless you measure pressure directly with a gauge attached to the engine, you have no way of knowing if pressure is within specifications or not.
Most warning lamps won't come on until pressure is dangerously low (less than four or five pounds). So don't assume the absence of a warning lamp means pressure is okay, especially if the engine is making any valve or bearing noise.
One Final Warning
If you insist on driving blind and deaf to the obvious warnings coming from under your Mercedes' hood, the next sound you hear may be a rapping or knock noise from the rod bearings - which will eventually be followed by dead silence as your engine seizes and your Mercedes coasts to a dead stop.