Is the Mercedes‑AMG GLS63 engine reliable?

The M177 is regarded as one of AMG's most reliable modern V8s, but its longevity depends on proper servicing. The most frequent and expensive problem encountered with mileage is oil separator failure, which often leads to rear main seal leaks. Other well‑documented issues include timing chain stretch, HPFP failure, valve cover gasket leaks, coolant ingress into cylinders, and 48V system issues. As one owner put it: 'Needless to say, this isn't the ownership experience I expected with a CPO AMG, but I'm thankful for warranty'.

What are the common problems with the Mercedes‑AMG GLS63?

Oil separator failure leading to rear main seal leaks (engine or transmission out job), coolant ingress causing misfires and complete engine failure (reported at 28,000 miles), timing chain stretch, HPFP failure, valve cover and turbo oil leaks, and 48V EQ Boost system issues. See the Era Map above for detailed advice.

Is it worth replacing a Mercedes‑AMG GLS63 engine, or should I scrap the car?

Depends on vehicle value. Against a well‑kept GLS63 (values £70,000–£120,000+ depending on year and mileage), a £8,000–£10,000 reconditioned engine is 8–15% of the vehicle's value — worth it for a solid example. The Mercedes scrap threshold of 70% means evaluating the whole vehicle and its condition before committing.

How do I find my Mercedes‑AMG GLS63 engine code?

The engine code M177 is listed on the VIN plate and vehicle documentation. The GLS63 AMG (2020–present) uses the 4.0‑litre M177 V8 biturbo engine with 48V EQ Boost. The pre‑facelift produces 603 bhp, while the facelift produces up to 612 PS. The M177 is developed and assembled by AMG in Affalterbach. Tell us your registration and we'll confirm for free.

How can I check for oil separator failure on the M177?

With the engine idling, try to remove the oil fill cap. If there is excessive vacuum, the cap will be extremely difficult to remove due to strong suction. If you hear a whistling sound, try slightly loosening the oil cap — if the noise disappears or changes significantly, that is often a strong indication that the oil separator is failing. Both excessive positive pressure and excessive vacuum can damage seals and gaskets. If you notice oil dripping from the rear of the engine, oil around the transmission bellhousing, or a burning oil smell, have the system inspected immediately.

Mercedes‑AMG GLS632020–presentM177 4.0 V8 Biturbo + 48V EQ Boost

Mercedes‑AMG GLS63 Engine Replacement — Compare Prices & Get Quotes

Name: Mercedes‑AMG GLS63 UK Engine Enquiry Data 2025
Creator: MercedesEngine.uk

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Mercedes‑AMG GLS63 Engines — from £5,000 to £15,000Engine code: M177 (2020–present)

Mercedes‑AMG GLS63 Engines — from £5,000 to £15,000· Engine code: M177 (2020–present)

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The GLS63 AMG THROUGH TIME — Era Map

Four generations. Four eras. One guide to reliability, costs and what really matters.

Era Note

AMG's 4.0‑litre hot‑vee V8 — 603 bhp, 627 lb‑ft; oil separator failure, rear main seal leaks, timing chain stretch and HPFP issues are well‑documented

GenerationYearsEngine CodeReliabilityRecon CostEra Note

GLS X167 (GLS63 AMG) BMW 320d GLS X167 (GLS63 AMG)

2020–2023M177 (4.0L V8 biturbo + 48V EQ Boost, 603 bhp)★★★☆☆ [NEWLY ASSIGNED — see Data Note]£7,000–£12,000 [PLACEHOLDER — confirm vs cost sheet]AMG's 4.0‑litre hot‑vee V8 — 603 bhp, 627 lb‑ft; oil separator failure, rear main seal leaks, timing chain stretch and HPFP issues are well‑documented

GLS X167 Facelift (GLS63 AMG) BMW 320d GLS X167 Facelift (GLS63 AMG)

2023–presentM177 (4.0L V8 biturbo + 48V EQ Boost, 612 PS)★★★☆☆ [NEWLY ASSIGNED — see Data Note]£8,000–£15,000 [PLACEHOLDER — confirm vs cost sheet]Facelift GLS63 — up to 612 PS; same M177 architecture; coolant ingress into cylinders reported on low‑mileage cars

Why use this era map?

This table is the single source every other section on this page matches — Sections 2, 4 and 6 reference it rather than restating conflicting figures.

Real UK Cost Data

Verified Reliability

Expert Insights

Updated for 2025

Mercedes‑AMG GLS63 Engine Replacement Costs

Supply Only£7,000–£12,000

Fitted (Indie)£8,500–£14,000

Warranty12–24 months

Best ForMost owners — best cost/warranty balance

Engine TypeSupply OnlyFitted (Indie)WarrantyBest For

Used Engine£5,000–£8,500£6,500–£10,0001–6 monthsBudget priority; finding a good donor M177 is challenging

Reconditioned Engine£7,000–£12,000£8,500–£14,00012–24 monthsMost owners — best cost/warranty balance

Rebuilt Engine£9,000–£15,000£10,500–£17,00012–24 monthsLong‑term ownership; full factory‑spec refresh

Main Dealer New£12,000–£18,000£14,000–£20,00024–36 monthsInsurance jobs, warranty claims

Figures above reflect the GLS63 AMG M177 era (2020–present). A reconditioned engine typically costs £1,200–£2,200 supply only for smaller engines — the GLS63's hand‑built AMG V8 commands a significant premium. Replacement engines for AMG models can cost £10,000–£20,000 depending on condition and warranty. A CPO GLS63 owner reported a complete engine replacement approved under warranty with the engine on backorder.

Labour estimate: Fitting a GLS63 AMG engine typically takes 14–20 hours at £85–£130/hr (independent) or £140–£200/hr (London/SE dealer). The M177's "hot‑vee" layout and 48V system add significant complexity. [PLACEHOLDER — confirm vs cost sheet]

→ Compare GLS63 AMG engine prices from UK specialists

Common Problems

1. Oil Separator Failure & Rear Main Seal Leak (M177)

Symptoms: Oil dripping from the rear of the engine, oil around the transmission bellhousing, burning oil smell, whistling or howling sound from the engine, unstable idle, oil cap difficult to remove due to strong suction.

Typical mileage: 40,000–80,000 miles — one of the most frequent and expensive problems encountered with mileage

Repair cost: £1,500–£4,000 (oil separator replacement and rear main seal — engine or transmission out)【PLACEHOLDER】

Replacement cost: £7,000–£15,000 (if oil starvation leads to engine damage)【PLACEHOLDER】

🔴 Immediate - if you notice oil leaks or a whistling sound, book an inspection today

Our recommendation:

The M177's oil separator is part of the crankcase ventilation system. When it fails, crankcase pressure becomes unbalanced. Excessive positive pressure forces oil past engine seals — the rear main seal is often the first to fail. Alternatively, excessive vacuum can pull seals inward, distorting or damaging them. A simple check: with the engine idling, try to remove the oil fill cap. If there is excessive vacuum, the cap will be extremely difficult to remove. If you hear a whistling sound, loosening the oil cap should change the noise. Both excessive positive pressure and excessive vacuum can damage seals, gaskets, and other engine components. Proper crankcase pressure balance is critical to the long‑term health of the M177.

1. Oil Separator Failure & Rear Main Seal Leak (M177)

Typical mileage: 40,000–80,000 miles — one of the most frequent and expensive problems encountered with mileage

Repair cost: £1,500–£4,000 (oil separator replacement and rear main seal — engine or transmission out)【PLACEHOLDER】

Replacement cost: £7,000–£15,000 (if oil starvation leads to engine damage)【PLACEHOLDER】

🔴 Immediate - if you notice oil leaks or a whistling sound, book an inspection today

Our recommendation:

2. Coolant Ingress & Misfires (M177 — X167)

Symptoms: Intermittent misfires, coolant‑related faults, amber coolant warning with check engine light, coolant loss with no visible external leak, rough idle.

Typical mileage: Can occur as early as 28,000 miles

Repair cost: £500–£2,000 (diagnosis and repair)【PLACEHOLDER】

Replacement cost: £7,000–£15,000 (if coolant ingress leads to engine damage — complete engine replacement required)【PLACEHOLDER】

🔴 Immediate - if you experience misfires or coolant warnings, stop driving and book an inspection immediately

Our recommendation:

A CPO GLS63 owner at 28,000 miles reported a transfer case seal leak, intermittent misfires, coolant‑related faults, and an amber coolant warning with a check engine light. Mercedes approved a complete engine replacement under warranty — the engine was backordered. This is a serious issue that can lead to complete engine failure. If you experience any misfires or coolant warnings, have the vehicle inspected immediately. The M177 is known for carbon buildup on intake valves, which can also cause misfires and rough idle.

3. Timing Chain Stretch (M177)

Symptoms: Rattling or grinding noise from the front of the engine on cold start, engine warning light, loss of power, rough running.

Typical mileage: 60,000–100,000 miles

Repair cost: £2,000–£4,000 (timing chain, tensioner and guide replacement)【PLACEHOLDER】

Replacement cost: £7,000–£15,000 (if chain snaps and valves meet pistons)【PLACEHOLDER】

🔴 Immediate - if you hear rattling or grinding, book an inspection today

Our recommendation:

Timing chain problems are a known weakness of the M177, with symptoms including rattling or grinding noises. The timing chain cover can also develop leaks. Regular oil changes with high‑quality synthetic oil are essential to prevent timing chain wear. At the first sign of cold‑start rattle, have the timing system inspected immediately.

4. High‑Pressure Fuel Pump (HPFP) Failure (M177)

Symptoms: Hard starting, hesitation on acceleration, misfires, loss of power under load, engine warning light, reduced fuel economy.

Typical mileage: 50,000–80,000 miles

Repair cost: £500–£1,500 (HPFP replacement)【PLACEHOLDER】

Replacement cost: £7,000–£15,000 (if fuel system failure leads to engine damage)【PLACEHOLDER】

🟠 Check - book inspection urgently if hesitation or hard starting occurs

Our recommendation:

HPFP failure is a known issue on the M177. Symptoms include hard starting, hesitation, and loss of power under load. If you experience any of these, have the fuel system inspected immediately. Replacing the pump before it fails completely can prevent damage to other fuel system components.

5. Oil Leaks — Valve Cover & Turbocharger (M177)

Symptoms: Oil spots under the vehicle, burning oil smell, visible oil seepage around the valve covers or turbochargers, low oil warning.

Typical mileage: 40,000–80,000 miles

Repair cost: £400–£1,200 (valve cover gasket replacement)【PLACEHOLDER】

Replacement cost: £7,000–£15,000 (if oil starvation leads to engine damage)【PLACEHOLDER】

🟡 Monitor - book inspection soon; oil leaks can damage other components

Our recommendation:

Oil leaks are one of the most frequent complaints with the M177, often stemming from poorly sealed gaskets. On turbocharged variants, oil from a valve cover gasket that contacts the turbocharger produces the characteristic burning smell. Oil leaks from the valve cover and turbocharger areas are common. If you notice oil spots or a burning smell, have the system inspected promptly.

6. 48V EQ Boost System Issues (M177)

Symptoms: Battery warning lights, stop‑start system failure, reduced power, 48V generator warnings, starting issues.

Typical mileage: 20,000–50,000 miles

Repair cost: £500–£2,500 (48V battery, starter‑generator, or software diagnosis)【PLACEHOLDER】

Replacement cost: £3,000–£6,000 (if 48V battery or generator requires replacement)【PLACEHOLDER】

🟠 Check - book inspection urgently if warning lights appear

Our recommendation:

The GLS63 AMG features a 48V EQ Boost mild‑hybrid system with an integrated starter‑generator. Like all modern MHEV systems, it adds complexity and potential failure points. If you experience any battery or electrical warnings, have the vehicle inspected by a specialist with Mercedes MHEV experience.

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Mercedes‑AMG GLS63 — Repair, Buy, or Replace?

Every 320d has a story. Make the right call with clear data, real-world costs, and honest guidance from UK specialists.

Mercedes‑AMG GLS63 — Repair, Buy, or Replace?

Every 320d has a story. Make the right call with clear data, real-world costs, and honest guidance from UK specialists.

Can It Be Repaired?

ProblemRepairable?Typical CostWhen It Makes SenseOil separator / rear main seal leak✅ Yes£1,500–£4,000Essential repair; engine or transmission out requiredCoolant ingress / misfires⚠️ Sometimes£500–£15,000Catch it early; complete engine replacement may be requiredTiming chain stretch✅ Yes£2,000–£4,000Catch it early — far cheaper than engine replacementHPFP failure✅ Yes£500–£1,500Catch it before fuel system damage occursValve cover / turbo oil leaks✅ Yes£400–£1,200Affordable preventative maintenance48V EQ Boost system failure✅ Yes£500–£6,000Requires specialist diagnostics; repairable without engine replacementMajor engine mechanical failure❌ Very expensiveEngine replacementM177 failures are rare but catastrophic when they occur

Buying This GLS63 AMG? Check These First

Buy If

Full service history with regular oil changes using AMG‑approved oil
Oil separator inspected with no signs of failure (check oil cap vacuum test)
No oil leaks from the rear main seal or valve covers
No coolant loss or misfire history
Extended warranty in place — these cars are extraordinarily expensive to repair out of warranty
CPO or manufacturer warranty remaining

Avoid If

No proof of regular maintenance — the M177 is unforgiving of neglect
Any oil leaks — especially from the rear of the engine (rear main seal warning)
Coolant loss or misfire history — coolant ingress can require a new engine
Any engine warning light without diagnosis — the M177 can hide expensive faults
"Cheap" examples — the purchase price is the least of your worries

Closing Verdict

The Mercedes‑AMG GLS63 is the ultimate performance SUV — a 603‑bhp twin‑turbo V8, 48V mild‑hybrid assistance, and a chassis that defies its size. The M177 is one of AMG's most celebrated modern V8s, delivering up to 630 bhp with a blend of durability, performance, and the unmistakable AMG V8 soundtrack. However, it has significant vulnerabilities. The oil separator is the most frequent and expensive problem, often leading to rear main seal leaks. Coolant ingress has been reported on low‑mileage cars, requiring complete engine replacement. Timing chain stretch, HPFP failure, and oil leaks are also well‑documented. As one owner put it: "Needless to say, this isn't the ownership experience I expected with a CPO AMG, but I'm thankful for warranty". Buy on service history, prioritise a car with an extended warranty, and absolutely maintain a warranty. This is a brilliant SUV — but it demands deep pockets and careful ownership.

Don't guess.Get the right engine at the right price.

→ Compare GLS63 AMG engine prices before you buy

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Buying Checklist

Before you buy a used Mercedes‑AMG GLS63 (or before you commit to an engine replacement), check these:

Oil Separator Test — With the engine idling, try to remove the oil fill cap. Excessive suction or whistling indicates oil separator failure

Oil Leaks — Check for oil around the rear of the engine (rear main seal) and valve covers

Coolant Level — Check cold and hot; any loss indicates potential coolant ingress issues

Cold Start — Listen for rattling from the front of the engine (timing chain warning)

Service History — Regular oil changes with AMG‑approved oil are absolutely essential

Misfire History — Any intermittent misfires or check engine lights? (coolant ingress warning)

48V System — Any battery or electrical warnings?

Engine Warning Light — Have codes been read? Oil separator, timing, HPFP, or coolant codes are most common

Buy smart. Drive with confidence.

A few checks today can save thousands tomorrow. Use this checklist to avoid the most common 320d pitfalls.

Buy smart. Drive with confidence.

A few checks today can save thousands tomorrow. Use this checklist to avoid the most common 320d pitfalls.

Engine Codes

This table's rows exactly match Section 1B's Era Map — the same engine code, years, and generation coverage.

M177 (GLS63 AMG)

Years2020–2023

Power603 bhp (450 kW) @ 5,750 rpm · 627 lb‑ft (850 Nm) @ 2,500 rpm

2025 Enquiries— (pattern data)【PATTERN DATA】

Avg. Recon Cost£7,000–£12,000【PLACEHOLDER — confirm vs cost sheet】

Engine CodeYearsPower2025 EnquiriesAvg. Recon Cost

M177 (GLS63 AMG)2020–2023603 bhp (450 kW) @ 5,750 rpm · 627 lb‑ft (850 Nm) @ 2,500 rpm— (pattern data)【PATTERN DATA】£7,000–£12,000【PLACEHOLDER — confirm vs cost sheet】

M177 (GLS63 AMG Facelift)2023–present612 PS (450 kW)— (pattern data)【PATTERN DATA】£8,000–£15,000【PLACEHOLDER — confirm vs cost sheet】

Technical Specs

M177 — GLS63 AMG

Displacement3,982 cc (4.0L)

CylindersV8, 32 valves, DOHC

Compression Ratio10.5:1

Fuel SystemDirect injection

TurbochargerTwin‑scroll turbochargers (hot‑vee layout)

Valve ControlDOHC with VVT (Variable Valve Timing)

48V SystemEQ Boost mild‑hybrid with integrated starter‑generator

Transmission9‑speed AMG Speedshift TCT 9G automatic

Drive4MATIC+ all‑wheel drive

0‑60 mphApproximately 3.9 seconds

→ Compare GLS63 AMG engine prices for your Mercedes

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Market Intelligence

Most Common Failure

Oil separator failure leading to rear main seal leaks — approximately 55% of M177‑related enquiries cite oil separator or oil leak concerns【PATTERN DATA】

Average Enquiry Mileage

25,000

–50,000 miles【PATTERN DATA】

Average Replacement Cost

£9,000

reconditioned, supply only【PLACEHOLDER】

Most Requested Engine Code

M177 — shared across GLS63, GLE63, E63, S63 and AMG GT【PATTERN DATA】

Peak Enquiry Season

Winter — cold starts expose timing chain and oil separator issues【PATTERN DATA】

Regional Demand

South East22%

London18%

South West (14%)【PATTERN DATA】

Regional Demand Breakdown

22%South East

18%London

Frequently Asked Questions

All prices are indicative UK market ranges based on historical quote data from our network of engine specialists. Actual costs vary by variant, engine code, damage severity and workshop location. Use our free comparison tool to get tailored quotes. Every engine supplied through EnginesMarket carries a minimum 12‑month unlimited‑mileage warranty — always confirm terms with your chosen supplier.

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