Used Mercedes-Benz M-Class review and buying guide (1998 to 2015): which one to buy and what to check

Which used Mercedes-Benz M-Class should you buy?

A W164 (2005 to 2011) with the 3.0 V6 CDI diesel (OM642) is the M-Class to buy, because it's the lesser risk of the engines on offer. Its known fault is an oil leak you can see and smell, whereas the petrol fault ends with the engine timing wrecked. The oil cooler seals in the engine's V fail between 60,000 and 100,000 miles, so look for a receipt showing they've been replaced.

The ML 350 petrol built up to 2008 is the one to be wary of. Its M272 V6 has a soft balance shaft sprocket that wears away between 60,000 and 90,000 miles, the camshaft timing drifts, and the engine's timing is eventually destroyed. The ML 500 V8 from the 2008 model year carries the same risk through its idler gear. A car without Airmatic air suspension also dodges the model's other High fault, which strikes between 80,000 and 120,000 miles.

There's very little on record for the W163 (1998 to 2005) or the W166 (2011 to 2015), and that's a gap in the evidence, not a clean record. The Airmatic fault covers W163 cars from 1999, so the same suspension checks apply to one that has it. On a W166 with the 3.0 V6 CDI, the oil cooler seal fault runs to 2014, so check the engine valley just as you would on a W164.

Which M-Class engines and gearboxes are the safe choices?

The 3.0 V6 CDI diesel is the engine to go for, provided its oil cooler seals have been dealt with. On 2005 to 2014 cars those seals fail in the V between the cylinder banks at 60,000 to 100,000 miles, and the leak runs off the rear of the block onto the undertray. Its timing chain isn't the weak point, though it can stretch once the miles get high.

Don't buy an ML 350 with the M272 V6 petrol, built up to 2008, unless there's an invoice for the balance shaft sprocket. When that soft sprocket wears, camshaft timing slips and the engine's timing is eventually ruined. The M273 V8 in the ML 500 from the 2008 model year has the same weakness on its idler gear. Mercedes issued a technical bulletin about it, so a car that's had the work done should have the paperwork.

A car with the 5-speed automatic (722.6) is billed as sealed for life, but you want one with its fluid and filter changed every 40,000 miles, using MB 236.14 fluid. Whatever the gearbox, test it properly: one owner's ML, bought new in 2006, had its gearbox fail on one of its first trips out, so don't skip a long test drive.

What should you check when viewing a used M-Class?

Oil in the valley between the cylinder banks, or dripping from the rear of the engine onto the undertray, is the first thing to look for; on a diesel it's the oil cooler seals going. A smell of hot oil inside the car on the test drive points the same way. On a petrol V6 or V8, an engine management light, a misfire or rough running can mean the camshaft timing is drifting.

See the car before it's been started that day and check it isn't slumped low over its wheels. A red AIRMATIC warning on the dash, or a compressor you can hear running non-stop, means the air suspension's failing. Ask for receipts for any compressor, relay or strut work, since these parts go between 80,000 and 120,000 miles, and, on a 5-speed automatic, for fluid changes every 40,000 miles.

Listen for wheel bearing noise, as one owner's 2010 ML got through three in 11 months, and make sure the key starts the car every time, since a 2011 ML350 had an ignition switch fault. Check the front tyres on 20-inch wheels: one owner's lasted only 4,000 miles. Try every electrical function too, because a 2006 ML 320 with 65,000 miles on it lost all battery power three times.

Paste the advert into the free check and it runs these against the listing, the MOT history and the DVLA record before you travel.

Is a used Mercedes-Benz M-Class W164 (2005 to 2011) worth buying?

It's worth buying as a 3.0 V6 CDI diesel with proof its oil cooler seals have been done, ideally without Airmatic; an ML 350 petrol built up to 2008 is the one to pass on unless its balance shaft sprocket has been sorted. The W164 is also where nearly every owner's report on record sits, so there's more known about it than about the models either side.

Its big faults arrive with mileage. The diesel's oil cooler seals go between 60,000 and 100,000 miles, the petrol's balance shaft sprocket between 60,000 and 90,000, and Airmatic parts between 80,000 and 120,000. Single owners' reports add more: a 2005 ML350 suffered a second camshaft failure at around 46,000 miles, and a 2006 ML 320 with 65,000 miles lost all electrical power three times and was left immobile twice.

Late cars don't escape. A 2011 ML300 bought used went back under warranty 12 times for brake discs and pads, the rear springs, sensors for ride height and wheel speed, corroded brake pipes, towbar electrics and a steering column fault. Look underneath for rusty brake pipes and make sure the steering feels right on the drive, whatever year you're looking at.

What does a used Mercedes-Benz M-Class cost to run?