Mercedes-Benz S350 Problems

RepairPal has identified the most common problems with the Mercedes-Benz S350 as reported by actual vehicle owners. We'll tell you what the problem is and what it'll take to fix it.

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Known Problems

The Mercedes Benz S350 may have engine oil seeping or leaking from the back of the cylinder heads. This is commonly misdiagnosed as leaking valve covers.

On the back of the cylinder heads there are three, in total, plastic expansion plugs that plug access ports to the camshafts. These plugs are well known for seeping oil, and leaking if left unattended. The oil will run down the back of the engine and eventually make it to the ground. 

Replacement of these three plastic plugs is extremely simple, and should be done as regular maintenance. The recommended interval for replacement is every 60,000 miles.

The engine in the 2005-2008 Mercedes S350 is known by Mercedes Benz to have issues due to a poorly manufactured gear on the balance shaft (also known as the counter-rotating assembly). Since the balance shaft gear is driven by the timing chain, which keeps the engine rotating synchronously, when it fails, the timing of the engine will change causing:

-Illumination of the Check Engine Light

-Rough Running Conditions

-Stalling

-Loud Rattle or Slapping Noise

-OBD Trouble Code P1200 and P1208

Correction for this issue is replacement of the balance shaft, or balance shaft gear. Replacement of the timing chain and tensioner should be done as well. The Mercedes Benz dealer may cover some or all of the repair cost on select vehicles as a result of a class action settlement.

When starting the engine after sitting for several hours, a knocking sound may be heard for several seconds. This is a common issue with the 2005-2010 Mercedes Benz S350. There are three common problems that can cause this sound to occur:

-Oil pressure building too slowly, allowing for movement between the crankshaft and crankshaft bearings

- Stretch or wear of timing chain and components

- Balance shaft (counter rotating shaft) gear wear

The remediation for these problems can be crankshaft bearing replacement with correct size, replacement of timing components, or balance shaft replacement.

The camshaft adjuster solenoid (which is related to the variable valve timing system) may fail or timing chain/balance shaft components may wear, resulting in illumination of the Check Engine Light and various drivability issues for the 2005-2010 S350. Mercedes-Benz has released a service bulletin outlining specific repair instructions depending on fault codes stored.

The engine may develop a stalling and/or no start condition due to a failed crankshaft position sensor.

Other than a defective motor, a bad switch or blower motor resistor assembly can cause an inoperative heater blower motor.

The evaporator temperature sensor can fail, causing the AC compressor not to cycle. If the AC compressor does not turn on, the AC system will not blow cold air. If the compressor is stuck on, the air will be very cold at first, before warming up.

A refrigerant leak may develop from one or more of the following AC components: AC compressor, hose manifold for the compressor, or the evaporator core. The loss of refrigerant can cause poor AC performance. Our technicians tell us that proper leak diagnoses should be performed in order to determine the cause of any refrigerant loss.

Front thrust arm and control arm bushings can crack and sometimes tear completely. If this is not repaired quickly, damage to the front subframe unit will occur, which is very expensive to repair. Updated control arms (that prevent damage to the subframe when the bushings wear out) are available.

An engine Oil leak may develop from the PCV vent housing on the valve cover and/or the inspection plate on the front of the engine.

Some models have tandem hydraulic pumps—one section for power steering and the other for suspension adjustment. These pumps are prone to leakage from either section.

A cracked or broken heater core outlet housing can cause a coolant leak near the bulkhead (firewall). Our technicians remind us that coolant leaks should be repaired in order to avoid overheating the engine due to coolant loss.

One or more door windows may stop working due to a failed window regulator. Replacement of the failed regulator will be necessary to correct this concern.

The lower control arm bushings can wear and crack, resulting excessive movement at the control arm pivot point, leading to irregular and premature tire wear. Worn Ball joints can also cause similar tire wear symptoms. I some cases it may be necessary to replace the lower control arm, including bushings & ball joint to correct this concern.

Due to wear, high usage vehicles can experience ignition lock and tumbler failures. The ignition lock cylinder will generally require replacement to correct this concern.