300SE Oil Consumption
 

My parents have a 90 300SE that is using approximately 1 qt of oil every 400-500 miles. They just had new valve seals installed and are using 10W40 oil, but haven't seen a reduction in oil consumption. We have searched the archives for old posts and excessive oil consumption seems to be a recurring problem for these cars, but we aren't sure what to look for next or if anything else can be done. There do not seem to be any leaks...never any oil on the ground and the motor is dry and clean. There is no smoke on starting or during acceleration.

The mechanic did not do a complete valve job when replacing the seals, but indicated that the existing seals were not even in bad condition. This car has only 80K, runs great and has a great service history....what should we be looking at next? Any suggestions? Even my old diesel doesn't burn oil like this thing.....:p

You're right, the M103 motors are known to use a bit of oil.

Valve stem seals are a common problem; I'll assume the tech who did the work knows what's he's doing. There's a U-shaped gasket at the front of the motor that's known to leak, but the consumption you describe is high and if this item were the cause you would likely see an oily mess in that area. There should be a large plastic encapsulation panel underneath the motor. On your vehicle, there are 4 small sheet-metal type screws with a hex head holding the panel on. An 8mm socket gets them off. You can access this panel from underneath without lifting the front of the car. The back two bolts take a bit of a stretch to reach, but it's not difficult.

With this panel off, you might get a better idea about leakage. If it's wet, I'd clean the panel, reinstall and drive for a couple of hundred miles; then pull it again. This "might" give indication as to where leakage is coming from.

The rear corner of the cyl. head - passenger side is another area where leakage is common on M103 motors. If you see this going on, your head gasket needs changing. Normally you'll see oil on the garage floor in this situation, but it's worth looking at anyway.

MB upgraded the valve guides for the M103 motor in April of 1988, but you give hint as to what might be your problem - 80K miles. That's about 6,000 miles a yr. and I'd bet that a previous owner(s) had the car sit for who-knows-how long. Cyl. heads can dry out, better guides or not; this may be a big part of your problem.

There's always the valve cover gasket and the gasket on the oil filler cap - littles things add up.

Last but not least are the piston rings, but with that kind of consumption I'd expect to see some smoke.

In the end, I believe you'll need to find a qualified MB tech - not the "occasional" variety, but one who's spent many years working the motor you have in your 300.

Thanks for the tips...we'll check "under the belly" just to be sure we aren't missing a leak. Other than this, the car runs great, is in incredible condition and seems to be fine. With the number of posts on oil consumption in this car, I guess this is its weak spot. Have you had anything major with your SEL? My mother wants to sell this SE, but we're trying to convince her otherwise...;)

Sooner or later, you'll need a head gasket. When that time comes be very sure that a real MB tech does the work.

Other than that, the 88-91 300-SEL cars are very sturdy. Change fluids and filters on a regular basis and the car will last a long time.

I've had mine for 6 years and other than the head gasket, have had to do nothing more than routine maint.

Not bad for a 12 yr. old car.

My guess is that you are leaking it rather than burning it, unless it really burns clean!

Check the spark plugs for heavy ash deposits. If present, you are burning the oil.

Check the vent hose to the air cleaner, the large one on top, for oil. If present, you have a blowby problem, if not, it's either leaking it out or sucking it down the valve guides.

Vavle guide wear can get so bad the idle mixture goes out of control from air leaking down the guides, will drive you nuts, but this usually only happens at greater (150,000) milage.'

The oil really runs clean in this engine, so you may not notice leakage on a gravel drive, it won't be black at all.

Peter

Thanks, guys! I've been out of pocket for a couple of weeks and don't know if my dad has checked these replies. I'll pass this along. I'm sure he's checked the plugs (he's pretty meticulous), but you never know. We'll keep digging and I'll update this post with anything we find.

Thanks again....
Terry