Spark Plugs: Spark plugs are good indicators of engine condition. Inspecting spark plugs
reveal a great deal about the condition of the engine. All spark plugs should be removed and checked
every 30,000 miles - even if your Mercedes is "low maintenance." Be sure to apply anti-seize to the spark
plug threads to prevent the plugs from seizing in the block, causing expensive repairs later on. Always
note which plug came from which cylinder. This tells you if a particular cylinder has a problem. The spark
plug insulator should be white/off white in color. A dark, sooty insulator indicates an overly rich fuel
mixture (caused by a problem in the ignition system, fuel injection system, or faulty sensors). BEWARE:
New spark plugs can make a difference in your Mercedes's performance but only if replaced with the
appropriate OE replacement plug.
Distributor Cap & Rotor: Both the distributor cap and the rotor are usually plastic and, thus,
deteriorate over time. Cracks may develop, allowing moisture in. The metal contacts on both components can
then corrode, leading to misfiring. The parts should be replaced at recommended intervals or if they show any
excessive wear.
Spark Plug Wire Sets: Spark plug wires have become more and more reliable. Hi tech materials and
better sizing have increased their useful life. Electronic ignitions have increased operating ranges. Spark
plug wires should be tested for resistance before replacement. They are no longer replaced routinely with
the cap, rotor and plugs.
Filters: Changing filters on a regular basis has more to do with your
Mercedes's longevity than any other single factor.
Oil Filter: Oil filters are easy to replace and help prevent unnecessary engine wear. The job of the
oil filter is to remove sooty particles (or "coke"), and other solid contaminants from the oil. Oil
filters should be replaced with engine oil every 3,000 miles. It is best to use an oil viscosity that is recommended
by the manufacturer.
Air Filter: Air filters remove dirt by trapping particles as air passes through the filter media.
The air filter protects the fuel injectors by preventing dirt from clogging the injectors. Air filters should be replaced
every 20,000 miles, but more often if you live or drive in dusty areas.
PCV Breather Filter: A clogged breather filter prevents the PCV from siphoning away the blow-by gases and
moisture created by engine combustion, resulting in oil breakdown and sludge buildup. The PCV breather
filter should be replaced every 30,000 miles, but yearly replacements are a good preventive measure.
Fuel Filter: Contaminants (particles) can get into your fuel system when refueling, and if not
trapped by the fuel filter, they can clog the injector inlet screens. If dirt reaches the injector itself, it can
clog or damage the pintle valve and seat. If the fuel filter is not replaced regularly, fuel flow to the engine will
become restricted, resulting in stalling, loss of high speed power, and hard starting. The fuel filter should be
replaced every 30,000 miles.
Automatic Transmission Filter: Properly filtered transmission fluid transmits energy, plus it cools
and lubricates the moving parts of the transmission. A clogged transmission filter can produce transmission
slippage, engagement problems and hesitation. It is recommended that the filter be changed every 12,000-15,000
miles to extend the life of your automatic transmission.
Oxygen Sensor (O2 Sensor): Your oxygen sensor(s) should be replaced at the
recommended intervals. A worn oxygen sensor drastically changes engine settings due false signals transmitted to the
ECU from the O2 sensor.
Vacuum Hoses: Many major systems depend on manifold vacuum for signals and function. All vacuum hoses
should be checked and replaced as needed. Signs to look for are: loose fitting hoses, brittle and cracked hoses, kinked
hoses, and stretched hoses (too short). Even a slight leak can cause problems with performance; in
certain cases, the car won't run if there's a vacuum leak.
Temperature Sensors: Temperature sensors for various engine functions ia also something to keep an eye on.
Sensors control the fuel injection system, cooling system and even the exhaust system. And they can
definitely be a cause of poor performance if they are not operating properly.
Here are a few tips for extending the life of your Mercedes and improving its overall performance (especially with gas
mileage and emissions):
Keep the engine of your Mercedes clean. A clean engine runs cooler and is much less likely to cause premature
failure of other parts. If an oil leak develops, it is easier to trace the source. It's also easier to work on.
Regular routine replacement of all filters, lubricants, coolant and the other parts noted on this page are critical.
Use the mileage guidelines shown in your owner's manual as your benchmark.
Its good to know your Mercedes's systems and particular requirements before starting any project, but do not attempt to fix what
you don't understand - seek a Knowledgeable mechanic in such cases.
If your Mercedes's computer is designed to learn and adjust, then some improvements may not take effect right away.
The computer may need to see various parameters before making any permanent setting changes.
Oil Pressure
You should always quickly investigate the source of unusual noises before they become a bigger problem. Lack of oil
pressure in your Mercedes's engine will lead to major engine problems (seized engine or severely worn engine components).
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.
Oil Pressure Issues
Check the oil level to see that it's at the proper level (neither 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 gaskets or seals to fix
the leak. If the engine is burning oil, the valve guides and seals are most likely worn, but the rings and
cylinders might be bad, as well. A wet compression test and/or leakdown test will tell you if it's the valve
guides or rings and cylinders that are worn. Also note the condition of the oil and make sure it's the correct
viscosity for your Mercedes and climate (see the recommendations in your owner's manual). 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. Its common for
pressure sending units to go bad, so don't assume the absence of a warning lamp means the oil pressure
is okay, especially if the engine is making any valve or bearing noise.
Cooling System Maintenance
Keep your engine compartment clean. A clean engine runs much cooler - and it's much easier to work on.
Replace coolant at or before factory recommended intervals with the proper type, mixture and volume
of coolant. Always allow the coolant system to rid itself of air before installing the radiator cap.
Replace all cooling system hoses - upper and lower radiator hoses, bypass hoses, heater hoses,
manifold coolant hoses and any other hoses on your vehicle - whenever you even suspect there may be a problem.
All hoses should be replaced at least every two years.
Replace the thermostat with the original temperature setting equivalent. The electronics in your vehicle
may use that setting for other controls. Do not substitute under any circumstances.
Replace the radiator/expansion tank cap with the original pressure setting and OE-type equivalent. Some
aftermarket substitutions do not seal and hold pressure properly on foreign-manufactured cars. Again, don't substitute.
Adjust or replace the water pump drive belt (external) at recommended intervals or more frequently, if required.
Check belts whenever you're working on any coolant system components.
Replace your water pump with an OEM/OES pump at the first signs of trouble or when your timing belt and tensioner
are replaced. Watch for signs of overheating - you don't want to break down in the hot sun when your water pump fails.
Replace the fan clutch and/or fan blade as needed (if applicable). Your Mercedes's temperature gauge is often
your best guide as to when your fan clutch needs attention.
Replace temperature sensors as required by diagnosis. Leave troubleshooting of your sensors to Mercedes experts
who have the proper equipment and diagrams.
Keep your entire vehicle properly maintained because of the effect timing, idle speed, exhaust and other systems
have on your engine's temperature. Your Mercedes's cooling system is designed to function with all other systems operating
properly. It cannot make up for a poorly operating or overheating engine condition.
Ignition Key Problems
It is recommended that the ignition key be replaced on all Mercedes models at 60,000 mile intervals. As the ignition key wears,
it also wears the tumblers in the ignition lock. Eventually, the wear will progress enough so that the ignition lock will remain
locked. At this stage, the only way to repair the lock is to cut it off and replace it. Therefore, it is best (and cheaper) to
simply replace the key before the lock can freeze up altogether.
Transmission
A weak link in Mercedes electronic transmissions (722 series) is the conductor plate electrical coupler. We often find these couplers leaking transmission oil right into the electrical connector. The oil can cause havoc with the electrical signals to and from the transmission. We highly recommend replacing the coupler at every transmission sevice (every 50,000 miles). Also, there is special ATF for these transmissions. Other types of transmission fluids will cause harm to the transmission.
