Used Mercedes-Benz GL/GLS X166 (2013-2019): Buyer's Guide

A 2014 GL550 owner on MBWorld posted a dealer estimate for $43,000 to replace the engine at 95,000 miles. The M278 V8 bore scoring failure is well-documented, affects early production vehicles, and carries no recall. The dealer considered it a normal wear item. The owner had no idea it was possible.

The GL/GLS X166 is one of the best three-row luxury SUVs ever made. Adult-sized third row, 7,700 lbs of towing capacity, air suspension that makes 6,000 lbs feel like a sedan. But powertrain selection and model year determine whether you get a reliable long-term driver or an expensive sequence of repair bills. This guide breaks it down by engine so you know exactly what you are buying.

The X166 Platform

The X166 platform ran from model year 2013 through 2019. Until 2016 it carried the GL-Class name. Mercedes renamed it the GLS for 2017 to mark its position as the brand's flagship SUV.

The 2017 model year was a genuine facelift, not just a name change. The old 7G-Tronic 7-speed automatic was replaced by a new 9G-Tronic 9-speed. Apple CarPlay and Android Auto arrived as standard. The suspension software was updated, the exterior received full LED lighting front and rear, and the infotainment moved to an 8-inch freestanding display. Mechanically, the engine family carried over.

Every X166 got 4MATIC all-wheel drive and AIRMATIC air suspension as standard equipment. There is no spring-suspension option in this generation. If you buy any GL or GLS from 2013-2019, you are buying a car with active air springs, an air compressor, and a valve block.

See current listings: GL450 (2013-2016) and GLS 450/550 (2017-2019).

Engines and Known Problems

3.0L M276 V6 Biturbo: 2015-2016 GL450 and 2017-2019 GLS 450

This is the engine to buy. The M276 puts out 362 hp and 369 lb-ft, returns 16 mpg combined, and has a far more manageable failure profile than the V8.

The M276 did not power the US GL450 until model year 2015. A 2013 or 2014 GL450 has the 4.7L M278 V8 despite the "450" badge. Mercedes published service information for startup rattle on some M276 applications involving oil check valves and chain tensioning. A brief startup noise needs diagnosis, but it does not justify claiming every engine requires a complete chain replacement at a fixed mileage.

A complete timing-chain repair can run $3,500-$5,000 when diagnosis confirms worn components. Ask a Mercedes specialist to evaluate a persistent cold-start rattle, check stored camshaft-correlation codes, and distinguish tensioner or check-valve behavior from actual chain elongation before pricing a repair.

The second documented issue is cam sensor oil leaks. The wiring harness connectors at the camshaft position sensors in both the M276 and M278 are known to allow oil intrusion over time. Symptoms include cam sensor fault codes and occasional rough running. Catching this early (before oil soaks deeper into the harness) keeps the repair to a couple hundred dollars.

Forum consensus on MBWorld.org is consistent: the GL450 with the M276 is the most reliable powertrain in the X166 generation. Owners who do oil changes every 7,500 miles, use approved 5W-30 or 5W-40 full synthetic, and address the timing chain before 80,000 miles regularly hit 150,000-200,000 miles without major powertrain issues.

For the 2017-2019 GLS 450, the 9G-Tronic is a smoother transmission than the 7G-Tronic. A small number of owners report harsh downshifts at light throttle in certain driving conditions. The valve body is typically the culprit, with dealer replacement quotes around $4,500. A transmission fluid service (replacing with fresh ATF 9) often resolves early symptoms before they progress to hardware failure.

M276 summary for buyers: Focus on 2015-2019 GL450/GLS 450 examples, listen from a true cold start, scan for camshaft-correlation faults, and confirm consistent oil service. Do not evaluate a 2013-2014 GL450 using M276 guidance because those model years use the M278 V8.

4.7L M278 V8 Biturbo: 2013-2014 GL450, GL550, and GLS 550

The M278 is the highest-risk powertrain in this generation. It powers every GL550/GLS 550 and the 2013-2014 GL450. Cylinder-wall damage is reported on early production engines, and the consequences can be severe, but forum cases do not establish a precise failure rate.

The cylinder liners in 2013 through early 2014 M278 engines can lose their surface coating between 80,000 and 120,000 miles. The failure sequence: oil consumption increases (one quart per 1,000-1,500 miles), a cold-start knock develops that does not fade at operating temperature, and eventually misfires follow. Engine replacement is typically the only fix. Independent shops can sometimes sleeve the cylinders, but re-scoring is common if the root cause is not addressed. The documented $43,000 dealer estimate for engine replacement on a 2014 GL550 with 95,000 miles is consistent with what owners report across multiple MBWorld threads.

Late 2014 and 2015+ M278 engines appear to have received production changes that substantially reduced bore scoring frequency. Owners of these later examples report the issue is much less common. The M278 cylinder scoring problem on the X166 is concentrated in 2013 and early 2014 build dates.

Secondary M278 concerns include turbo oil line seeps. The small rubber lines feeding the turbos can develop slow leaks over time. These are manageable with regular visual inspections and early replacement, but add to ownership costs.

Owners of 2015+ GL550 and GLS 550 who report long-term reliability share a few practices: they use European-spec 0W-40 or 5W-40 full synthetic (not the 5W-30 common in US markets), they change oil every 5,000 miles rather than waiting for the maintenance indicator, and they avoid long cold-idle periods that concentrate unburned fuel in the cylinders.

M278 summary for buyers: Treat every 2013-2014 GL450 or GL550 as an early M278 candidate requiring a cold-start inspection, cylinder-bore scope, compression or leak-down testing when symptoms justify it, and careful oil-consumption history. On later GL550 and GLS 550 examples, inspect just as carefully and plan for higher per-mile costs than the V6.

5.5L M157 AMG V8: GL63 and GLS 63 AMG

The M157 carries the same bore scoring risk as the M278. It adds AMG-specific complications: service intervals drop to every 5,000 miles, AMG-approved parts cost significantly more, and finding an independent shop with genuine M157 expertise narrows your options considerably.

The GL63/GLS 63 AMG is a remarkable vehicle to drive. It is difficult to justify on financial grounds as a used purchase unless you specifically want the AMG performance. The power advantage over the GL550 is real but moderate in real-world use. The running cost difference is substantial.

One practical note: the AMG Speedshift MCT transmission in the GL63 requires transmission fluid service at shorter intervals than the standard 7G-Tronic. Many CPO examples have this deferred, and catching up on the service adds cost.

AMG summary for buyers: Only consider the GL63/GLS 63 if you want the performance specifically and can absorb AMG-level running costs. The reliability profile is similar to the GL550 with higher maintenance costs and fewer competent independent service options.

3.0L OM642 Diesel: GL350 BlueTEC (2013-2016)

The GL350 BlueTEC was sold in US markets through the 2016 model year before Mercedes withdrew it following the broader diesel emissions controversy. It offers the best fuel economy in the lineup at an EPA-rated 22 mpg combined, a significant operating cost advantage in high-mileage use.

The OM642 diesel's common inspection points include oil leakage in the inner V, emissions-system faults, and intake contamination. Mercedes bulletin LI18.30-N-056338 describes oil traces around the oil cooler and revised sealing parts. The usual symptom is external oil collecting in the engine valley, not spontaneous oil-and-coolant mixing. The bulletin warns that coolant can enter oil passages if a technician removes the cooler without draining the cooling system correctly, which is a repair-procedure risk rather than the original failure mode.

The diesel also carries AdBlue/DEF system requirements. The system adds a periodic fluid cost (about $10-$30 per fill) and an injection nozzle that can fail with deferred maintenance.

Diesel summary for buyers: The GL350 BlueTEC is a legitimate option for buyers who want the fuel economy advantage and can verify a clean service history on the OM642. Do not skip the oil-coolant mixing check at inspection.

Trim Levels

The GL/GLS did not have traditional trim levels. The powertrain defined your tier. Within each engine variant, the optional packages that matter most to used buyers are these.

Active Curve System (ACS): Optional active roll stabilization that replaces the conventional sway bar with hydraulic actuators. The ride improvement is genuine. The ACS is also the subject of NHTSA Recall 18V272000, which involves the system's oil reservoir venting foamy oil in a way that can cause a fire. The fix is free at any Mercedes dealer and takes under an hour. Verify completion before you buy any ACS-equipped example. If the recall is open, get it done before you take delivery.

Panoramic Sunroof: The panoramic roof covers the first and second rows. It adds the cabin light and air that many buyers want. It also creates sunroof drain maintenance. Clogged panoramic sunroof drains are a documented water intrusion source on the X166. The water bypasses the drain channel and routes directly to the footwell, where the Front Signal Acquisition Module (front SAM) sits. A failed front SAM takes out multiple electrical systems simultaneously. Before buying any panoramic sunroof example, check the driver and passenger footwell carpet for any trace of dampness.

20-inch wheels: Factory 20-inch wheels paired with 275/50R20 Pirelli Scorpion tires produce steering wheel vibration at highway speeds (typically 65-75 mph). This is widely documented and specific to the tire compound, not a structural issue. Owners who switch to 19-inch wheels (275/55R19) consistently report the vibration disappears. Budget $1,200-$1,600 for a quality set of replacement tires if the car has the 20-inch fitment.

Driver Assistance Package: Adaptive cruise, blind spot monitoring, rear cross-traffic alert. These systems are well-integrated on the X166 and hold up reliably long-term. Worth paying up for on a 2017-2019 GLS if you will use them.

Which Year Should You Buy?

The 2013-2014 GL550 is the problem vehicle in this generation. The bore scoring risk on early M278 production is the single largest financial hazard in the X166 lineup. A clean CarFax on a 2013 GL550 does not tell you anything about bore scoring history. The symptoms can be subtle until misfires appear.

The 2017-2018 GLS 450 is the target. The 9G-Tronic transmission is meaningfully better than the old 7G-Tronic. Apple CarPlay means the infotainment has not yet become frustrating to use daily. The M276 improvements are fully in place. In the market snapshot used for this guide, 2017 GLS 450 examples run $14,000-$29,000 depending on mileage, which puts a clean 70,000-mile example within a $22,000-$26,000 budget.

Decode the VIN before relying on the rear badge. The 2013-2014 GL450 and the 2015-2016 GL450 share a model name while using different engine families. A specialist can also confirm the engine from the build data and underhood layout. That single check prevents a buyer from applying V6 service advice to an early V8 truck.

A 2016 GL450 (not the 550) under $18,000 is also a legitimate option if you verify the ACS recall was completed and a cold-start inspection and scan show no timing-system faults.

Pre-Purchase Inspection Checklist

Organize your inspection around the specific failure modes of this generation.

Before you see the car:

• Run the VIN through the recall lookup. Confirm Recall 18V272000 (ACS oil leak) is marked complete, and clear any other open campaigns the VIN check surfaces.
• Ask the seller for service records going back at least 40,000 miles. Short-interval oil changes on a V8 (every 5,000 miles) are a meaningful positive signal.

Cold start (do this before the car warms up):

• Start the engine from dead cold. GL450 and GLS 450: listen for a timing chain rattle. A few seconds of rattle that disappears is borderline; rattle that continues past 60 seconds warrants a shop inspection.
• GL550 and GLS 550: listen for a cold-start knock or tick in the engine. A tick that fades as the engine warms up to operating temperature is less concerning. A tick that persists at operating temperature, or oil consumption above half a quart per 1,000 miles, are symptoms of bore scoring. Walk away.

AIRMATIC inspection:

• With the engine running, check the instrument cluster for any AIRMATIC MALFUNCTION warning.
• Park on a level surface. All four corners should sit level. A sagging rear corner indicates a failed air spring, typically a $600-$900 part plus 2-3 hours of labor.
• Ask how long the car sat before you arrived. An X166 that sat 2+ months without starting frequently develops air spring leaks as the rubber seals dry out.

Water intrusion check:

• Press the floor mats in the driver and passenger footwells. Dry is the only acceptable result. Any dampness suggests a sunroof drain blockage or taillight/liftgate gasket failure.
• Inspect the rear liftgate rubber seal and the edges of both tail lights. Cracks or separation in these seals are where water enters the cabin.
• Ask if the panoramic sunroof drains have ever been cleaned. The procedure takes 20 minutes with a compressed air line and is basic preventive maintenance on any high-mileage panoramic roof example.

Test drive:

• Cycle through all three AIRMATIC height settings (low, standard, high). The car should visibly raise and lower between settings.
• Run through all gears with paddle shifters. Harsh or jerky downshifts on the GLS 450 suggest early transmission valve body wear.
• Check all electronics: COMAND, navigation, all seat adjustments, panoramic roof operation.

Running Costs

Annual scheduled maintenance averages $1,286-$2,000 for V6 models and $1,500-$2,500 for V8 models, not counting unscheduled repairs. Budget a separate $6,000-$12,000 for a full AIRMATIC rebuild in your ownership window. Most owners do this incrementally over two service visits: front struts one year, rear struts the next, compressor when it fails.

Oil changes at a Mercedes dealer run $150-$300. At a competent independent, $80-$130. Change every 7,500 miles on the V6, every 5,000 miles on the V8 to reduce wear risk.

Tires typically run 275/55R19 or 275/50R20 depending on wheel size. A set of four Michelin Pilot Sport or Continental CrossContact in these sizes runs $1,200-$1,600 installed. Rear tires wear faster than fronts on 4MATIC, so budget for rear-biased replacements on higher-mileage examples.

Towing capacity is 7,700 lbs on the GL550/GLS 550 and GLS 63 AMG with a properly equipped hitch. The GL450/GLS 450 is rated to 7,500 lbs. These are genuine numbers; the platform handles trailer weight well when the hitch is factory-installed or dealer-fitted.

FAQ

Is the Mercedes GL450 reliable for long-term ownership? The GL450 with the M276 V6 biturbo is the most reliable powertrain in the X166 generation. Owners who maintain short oil change intervals and address timing chain service before 80,000 miles consistently report 150,000-200,000 miles without major mechanical failures. The AIRMATIC air suspension requires a rebuild around 100,000-150,000 miles regardless of powertrain, which is the predictable large cost in the ownership cycle.

Which year GL450 or GLS 450 should I buy? The 2017-2018 GLS 450 is the best combination of reliability, features, and used market value. The 9G-Tronic transmission arrived in 2017 along with Apple CarPlay and improved suspension software. The 2018 has the same hardware with one additional year of production refinement. Both years typically run $18,000-$32,000 depending on mileage and condition.

Should I buy the GL550 or GLS 550? Only if the car is a 2015 or later and you have verified the engine is not burning oil. The M278 V8 bore scoring issue is concentrated in 2013 through early 2014 production. A compression test and oil consumption log (if available from service records) are the only useful pre-purchase data points. A 2015+ GL550 with documented short oil change intervals is manageable. A 2013 GL550 with unknown history is a financial risk.

How much does AIRMATIC suspension cost to repair? A single air spring (strut) replacement averages $1,910 at a dealer, $600-$900 in parts plus 2 hours of labor at a competent independent. The compressor runs $1,000-$1,500 to replace. When one spring fails, others typically follow within 20,000-30,000 miles. A full four-corner rebuild plus compressor runs $6,000-$12,000. Some owners replace AIRMATIC components with conventional spring suspension kits for $1,500-$3,000, which permanently eliminates the air suspension failure risk.

What does the GL/GLS third row actually seat? Adults fit. Comfortably. This is what separates the GL/GLS from the GLE or BMW X5 with optional third rows that realistically fit children only. The X166 third row has genuine legroom and headroom for six-foot passengers, which is the main reason buyers choose it over smaller three-row competitors.

The Bottom Line

Buy the 2017-2018 GLS 450. The V6 avoids the M278 bore scoring issue entirely, the 9G-Tronic is the right transmission, and CarPlay keeps the infotainment usable. Plan for an AIRMATIC rebuild somewhere in the 100,000-150,000 mile window and factor that into your purchase price. Run every VIN through a recall check first and confirm the ACS recall 18V272000 is complete.

CarScout members can track price drops on specific GLS 450 model years, with plans at $5/week, $15/month, or $99/year.

Sources: Mercedes-Benz USA X166 brochures, service information, and LI18.30-N-056338; NHTSA campaigns including 18V272; EPA fuel economy data; MBWorld.org, BenzWorld.org, and PeachParts owner reports; Fuelly owner fuel-economy tracking.