Mercedes‑Benz GLS X166 Engines — The Complete UK Guide
Every engine fitted to the X166 SUV — from the 3.0‑litre OM642 diesel to the 5.5‑litre M157 V8 in the AMG GLS 63. Real UK replacement costs, honest reliability data, and the full‑size SUV ownership realities that define this generation. Backed by real UK X166 enquiries in 2025 [MB‑VERIFIED].
Explore X166 Engines
Mercedes‑Benz GLS X166 Engine Codes — The Complete Database
Every engine fitted to the X166 SUV specifically — spanning the GL‑Class (2012–2015) and GLS (2015–2019) eras.
Overview
Mercedes‑Benz GLS X166 — Overview
The X166 is the second‑generation full‑size luxury SUV from Mercedes‑Benz, launched at the 2012 New York International Auto Show as the GL‑Class and renamed GLS at the 2015 facelift.
It replaced the X164 GL‑Class and positioned itself as the "S‑Class among SUVs" — a seven‑seat luxury flagship with air suspension, 4MATIC all‑wheel drive, and a range of V6 and V8 engines. A significant 2015 facelift brought the GLS name, 9G‑Tronic transmission (replacing the earlier 7G‑Tronic), and revised engine line‑up.
Key Facts
Platform
Mercedes‑Benz W164/W166
Predecessor
X164
Successor
X167
Sibling body styles in this generation
None (standalone full‑size SUV)
Market Intelligence
The GLS/GL‑Class range (X166 combined) generated real UK engine enquiries in 2025 [MB‑VERIFIED], with the OM642 diesel and AMG M157 V8 commanding the strongest specialist interest.
Best & Worst Engines
Best Diesel Overall
OM642 3.0‑litre V6 diesel (GLS 350 d, 258hp)
“The proven 3.0‑litre V6 turbo diesel that powered the X166 across its entire production run — 258hp, 620Nm of torque, and a documented reputation for longevity. Oil cooler seals are the main watchpoint, but the engine itself is robust and well‑supported.”
Who it's for: buyers who want effortless V6 diesel torque without the M278's petrol‑engine risks.
Best Petrol Overall
M276 3.0‑litre V6 bi‑turbo (GLS 400, 367hp)
“The facelift‑era V6 bi‑turbo delivers 367hp and 500Nm from a 3.0‑litre V6 — smooth, responsive, and without the cylinder scoring risks that plague the M278 V8. It's the sensible petrol choice in the X166 range.”
Who it's for: buyers who want V6 petrol refinement without the M278 V8's reliability concerns.
Highest Risk
M278 4.7‑litre V8 twin‑turbo (GLS 500, 435–455hp)
“The 4.7‑litre V8 is the most contentious engine in the X166 range — cylinder scoring is a well‑documented pattern, particularly in the right cylinder head, caused by heat concentration in a thermally overloaded design. A full engine replacement is a five‑figure bill that can appear at almost any mileage.”
Who it's for: buyers who understand the risk and are prepared for a potential £6,000–£10,000 rebuild.
Most Expensive Failure
M157 5.5‑litre V8 twin‑turbo (GLS 63 AMG, 557–577hp)
“The hand‑built AMG V8 delivers 577hp in facelift guise — and with it comes AMG‑level running costs. Carbon buildup, oil consumption, valve cover gasket leaks, and cooling system failures are all documented on higher‑mileage examples. A full recon is a five‑figure bill, and parts availability can be a headache.”
Who it's for: AMG enthusiasts with deep pockets and a trusted specialist.
Best Value
M276 3.0‑litre V6 bi‑turbo (GL 400 / GLS 400)
“The V6 bi‑turbo offers genuine performance without the M278's risks or the M157's running costs — 333–367hp, 480–500Nm, and a proven track record across the Mercedes range. It's the sweet spot in the X166 petrol line‑up.”
Who it's for: buyers who want V6 performance without the V8's reliability concerns.
Most Overlooked
OM642 3.0‑litre V6 diesel (GL 350 CDI)
“The pre‑facelift OM642 diesel is often overlooked in favour of the GLS 350 d's 9G‑Tronic car — but the 7G‑Tronic GL 350 CDI offers the same 258hp and 620Nm, often at a lower purchase price, with the same robust diesel V6 underneath.”
Who it's for: buyers who want X166 capability on a tighter budget.
Mercedes‑Benz GLS X166 — Ownership Economics
The X166 rule
A £3,200–£5,500 OM642 recon against a £12,000–£25,000 X166 sits at 13–46% of the car's value — well inside the 70% Mercedes‑Benz threshold. Against a £10,000 high‑mileage example, the same recon reaches 32–55% — still viable, but a significant proportion of the car's residual value. However, a £6,000–£10,000 M278 recon against a £12,000 X166 is 50–83% — on a tired example, that's the borderline of the 70% threshold. Value the car and its engine history first.
Common Problems
X166 Engine Replacement Costs — By Engine and Condition
Core Variants
Core Variants
Diesel Variants
Scope note: All variants listed are X166 SUV only. The X166 was a standalone full‑size SUV with no direct sibling body styles in this generation (unlike the smaller GLE, which had a coupé variant).
Market Intelligence
Most Requested E90 Engines (2025)
Most Requested E90 Variants
Average E90 Replacement Cost
Most Common E90 Failures
FAQ
Why UK Mercedes‑Benz GLS X166 Owners Trust MercedesEngines.uk
Trusted X166 Engine Suppliers
every garage and engine rebuilder in our network is vetted, rated and reviewed by real X166 owners, as part of the Engine Finders network.
Compare X166 Engine Replacement Companies
see prices, warranty terms and supply‑and‑fit options side‑by‑side before you commit.
UK Engine Replacement Services for X166
covering every variant from GL 350 CDI to GLS 63 AMG, with supply‑only or full supply‑and‑fit available nationwide.
Find the best Mercedes‑Benz GLS X166 engine replacement UK drivers rely on. Compare real prices from trusted specialists on EnginesMarket today and get back on the road with confidence.
