Mercedes AMG M157 M278 Upgraded Rear Coolant Hose & Connector Set

Mercedes M157 & M278 Upgraded Rear Coolant Hose and Connector Set

We supply 2128302300 and 2782002152 together as a two-piece set for the rear coolant and heater connection on the Mercedes-Benz applications listed below. The set includes the engine-to-heater feed hose and the coolant feed connector fitted at the rear of the left cylinder head.

An upgraded replacement for the stock Mercedes rear coolant hose and connector, featuring PA66-GF30 glass-fibre-reinforced plastic fittings. Compared with standard unreinforced PA66, this reinforced material offers greater rigidity and improved resistance to deformation under load.

Suitable applications include selected E63 AMG and CLS63 AMG 5.5 V8 biturbo models, including applicable S and 4MATIC versions, alongside E500, E550, CLS500 and CLS550 4.7 V8 biturbo models.

What Is Included?

• 1 × heater feed hose: replacement reference A2128302300 / 2128302300.
• 1 × rear cylinder-head coolant feed connector: replacement reference A2782002152 / 2782002152.

Both components are supplied together as one set. The smaller component is a rigid coolant feed fitting rather than a second flexible hose. Mercedes-Benz part numbers are provided for identification and compatibility.

PA66-GF30 Reinforced Plastic Upgrade

The plastic fittings in this set are manufactured from PA66-GF30. PA66 is polyamide 66, commonly known as nylon 66, while GF30 identifies reinforcement with approximately 30% glass fibre by weight.

The glass fibres reinforce the plastic structure. Compared with an equivalent unreinforced PA66 grade, this provides several useful benefits:

• Greater stiffness: helps the fitting resist flexing under mechanical loads.
• Improved dimensional stability: helps maintain the connector's shape as operating conditions change.
• Better resistance to sustained-load deformation: particularly useful around hose connections and mounting points.
• Improved resistance to heat-induced deflection: reinforced grades generally retain their shape better under load at elevated temperatures.

This makes PA66-GF30 a reinforced-material upgrade over standard unfilled PA66. The reinforcement applies to the plastic fittings, rather than the flexible hose itself.

Why Owners Replace These Rear Coolant Connections

Owners of affected Mercedes V8 models have reported leaks from the rear heater-hose coupling and cylinder-head feed fitting. Reported symptoms range from gradual coolant loss to a sudden leak, sometimes accompanied by a coolant warning, steam around the back of the engine or coolant running onto the underbody.

The flexible hose is not always the source of the problem. A deteriorated plastic fitting, damaged coupling or leaking connection seal can also allow coolant to escape. Because the connection is tucked away behind the engine, identifying and reaching the source can be more difficult than the size of the component suggests.

Inspect both mating components when repairing a leak. Replacing the hose alone may leave an ageing connector in place, while replacing only the connector may leave a damaged hose coupling unresolved. Supplying both components together allows this section of the cooling system to be renewed as a set.

Recommended During an Engine Rebuild

We recommend replacing this set while the engine is out for rebuilding or major repairs, especially if the existing components are original or their replacement history is unknown.

The coolant feed connector is positioned at the rear of the left cylinder head, close to the bulkhead. Once the engine is installed, working space is restricted. Depending on the vehicle and repair method, access may involve removing surrounding components such as the vacuum pump, bulkhead panels or intake components.

Renewing these parts during an engine rebuild takes advantage of the improved access and can avoid repeating substantial dismantling work if an old connection starts leaking after the engine has been refitted.

Engine removal is not necessarily required solely to replace these parts. The recommendation is to renew them while access is already available during rebuilding or other major work.

Compatible Engine Codes

The M278 engine may be described as either 4.6 or 4.7 litres in parts catalogues. An M157 or M278 engine designation alone does not confirm compatibility with this complete set: the vehicle must use both component references, or their applicable Mercedes supersessions.

European and UK Model Applications

The following table provides application-year guidance for the relevant V8 models. Earlier vehicles may use the superseded part numbers listed below. Confirm both parts against the VIN before ordering.

North American Model Names and Model Years

• E550 4MATIC saloon: W212, 2012–2014 — M278.922.
• E550 coupé and cabriolet: C207/A207, 2012–2017 — M278.922.
• CLS550, rear-wheel drive and 4MATIC: C218, 2012–2018 — M278.922.
• E63 AMG, rear-wheel-drive saloon and estate: 2012–2013 — M157.981.
• E63 AMG 4MATIC saloon: 2014–2015 — M157.981.
• E63 AMG S 4MATIC saloon and estate: 2014–2016 — M157.981.
• CLS63 AMG, rear-wheel drive: 2012–2013 — M157.981.
• CLS63 AMG 4MATIC: 2014 — M157.981.
• CLS63 AMG S 4MATIC: 2014–2018 — M157.981.

Fitment notes: auxiliary-heater equipment, including option code 228, can change the required hose arrangement and requires an individual catalogue check. Production dates, model years and registration years may differ.

This set is listed for the relevant biturbo V8 applications only. It is not listed for the earlier naturally aspirated M273 5.5-litre or M156 6.2-litre engines, or later 4.0-litre AMG engines.

OE Part Numbers and Cross-References

Earlier valve references A2782000531 and A2782000631 also appear in the M157 catalogue supersession chain leading to A2782000831. These identify an earlier valve design; conversion to the later feed connector must follow the applicable VIN-specific parts instructions.

Installation Advice

• Check the earlier valve arrangement: A2782002152 is the later feed fitting and differs from the earlier electrically connected valve. When converting, check whether electrical connector dust cap A0025457383 is required. This is an associated service item rather than one of the two main components listed in this set.
• Inspect the surrounding connections: check hose couplings, retaining clips, mounting points and sealing surfaces before installation.
• Renew disturbed seals as required: confirm the correct seals and any additional service parts for the vehicle before starting.
• Pressure-test before completing reassembly: test to the vehicle manufacturer's specification and inspect the rear connection before refitting components that restrict access.
• Use the correct coolant: refill and bleed the cooling system according to the vehicle manufacturer's procedure.