Thermostat M278 for Mercedes-Benz

OE spec engine thermostat assembly, engineered for Mercedes Benz vehicles with the M278 4.7L V8 biturbo engine. Restores correct warm up time and stable operating temperature, protects against overheating and weak heater performance. Sold per unit, variant specific as supplied with housing and sensor where applicable.

What this is / isn’t:
Included: 1× thermostat assembly

Important: Match by VIN and engine code M278. Multiple sub variants and model years use housings with different outlet angles and electrical connectors. AMG and emissions packages can change the part. Always confirm by OE part number.

Key Features:

• Precise temperature control: OE matched opening temp and flow for stable engine and cabin heat.

• Integrated housing and seal: Direct bolt on replacement to minimise leaks and comeback repairs.

• Correct sensor or actuator logic: Works with the ECU for accurate warm up and efficiency.

• Durable materials: High temp plastics and elastomers resist glycol and thermal cycling.

• Exact fitment: Factory mounting points for a no mod install.

Compatibility:
Common applications with the M278 V8 biturbo, confirm by VIN or OE:

• S Class W221 late, W222 early S500, S550

• E Class W212 E500, E550

• CLS C218, X218 CLS500, CLS550

• GL W166, GLS X166 GL500, GLS500, GLS550

• ML W166, GLE W166 ML500, ML550, GLE500 where fitted

• G Class W463 select trims G500, G550 in some markets
  Drivetrain: 2WD and 4MATIC where applicable, market years vary, always match by VIN.

Options, engine or sensor variations:

• Standard M278 thermostat with integrated housing and sensor.

• Model year revisions may change connector style, outlet angle or hose size, verify supersessions.

• AMG related packages can alter part numbers, confirm before ordering.

Typical OE reference families, guide only:
A278-200-… , A278-203-… with supersessions by year and market.
Always order by the exact OE part number on your vehicle or confirm via VIN.

Symptoms of a faulty thermostat:

• Slow warm up, temperature gauge low or fluctuating.

• Overheating in traffic or under load.

• Weak cabin heat, fault code P0128 or related temperature errors.

• Coolant seepage around the housing, dried crust at joints.

Installation notes, pro tips:

• Work on a cold engine. Drain coolant to below the housing level.

• Replace the seal or O ring, clean mating faces, torque bolts evenly to spec.

• Refill with Mercedes approved coolant, MB 325.x spec, bleed the system thoroughly.

• Verify fan operation and hose temperatures on first heat cycle, clear stored codes after a road test.

• Inspect adjacent hoses, clamps and plastic tees for age related brittleness.

What’s included / not included:
✅ 1× Thermostat assembly with housing and seal, sensor or actuator if supplied
❌ Coolant, hoses, clamps, water pump, radiator, extra sensors, hardware

Q&A
Q1: How do I confirm I have the M278 engine?
Check your VIN data card or engine cover. Your VIN will map to the exact thermostat part number.
Q2: Do I need coding after replacement?
Coding is usually not required. Clear any fault codes and verify operating temperature with a scan tool.
Q3: Which coolant should I use?
Use Mercedes coolant that meets MB 325.x specification, mixed to the correct ratio.
Q4: What else should I replace?
Consider a coolant service, inspect hoses and clamps, replace any brittle plastic fittings.

Why choose TOParts:
OE grade engine components for Mercedes Benz with precise fitment, correct specs and long term reliability, trusted by panel shops and independent Mercedes specialists across South Africa.