Wondering what to drive on the Feast Day of the Immaculate Conception?

If you’re the Holy See, only a Mercedes-Benz will do. Specifically, a brilliant pearlescent-white “popemobile,” which started life as an M-Class sport utility before it had a giant aquarium grafted onto its back.

All that bulletproof glass requires a stout foundation, which the first-generation ML possessed with its body-on-frame architecture. The new-for-1998 ML320 was the German automaker’s first SUV tailored for North Americans.

But times change and Mercedes was compelled to adopt unibody construction for its second-generation M-Class, released for 2006. Pope Benedict XVI took delivery of his updated ride in December.

Good things come to those who wait.

CONFIGURATION

Compared to the outgoing model, the 2006 ML350 was 15 cm longer and 7.5 cm wider, riding on a wheelbase stretched 10 cm. While it remained mid-sized, it dropped its previous optional third-row seating due to low demand (the larger GL would pick up the slack).

By switching to a unibody platform, the M-Class gained torsional and bending rigidity while trimming about 40 kg. The revised four-wheel independent suspension featured control arms up front and new four-link geometry in back.

The cabin received more luxury appointments to the delight of returning customers. Designers specified a lighter interior with richer materials and touches of chrome. With the gear selector migrated to the steering column, the centre console was freed up to accommodate vital drink holders.

Space was no longer restrictive, with all five seating positions gaining more room, along with additional cargo capacity behind the split-folding second-row bench.

There were two engines initially; a 268-horsepower 3.5 L V6 replaced the 232-horsepower 3.7 L six in the ML350, while the more muscular ML500 used a 5.0 L V8 good for 302 horsepower. A seven-speed automatic replaced the old five-speed slushbox as the sole transmission.

Engineers adopted full-time four-wheel drive replete with three open differentials coupled with an electronic traction-control system that varied torque front to rear and side to side. There was no low-range gearing for rock hopping.

The M-Class family grew in 2007 with the addition of the 215-horsepower ML320 CDI turbodiesel V6 (398 lb-ft of torque), while the high-performance ML63 packed a big V8 that generated 503 horsepower. In 2008, the ML550’s 382-horsepower 5.5 L V8 displaced the ML500.

The 2009 models received some updated styling. The squeaky clean ML320 BlueTEC used a 210-horsepower 3.0 L turbodiesel V6 that injected ammonia-based urea into the exhaust stream to neutralize toxic nitrogen oxides. All models got the PRE-SAFE system, which cinches seatbelts and optimizes airbag deployment when it senses an impending collision.

ON THE ROAD

Belying its portly 2.2-tonne mass, the ML350 could sprint to highway velocity in 7.1 seconds. The V8-powered ML550 could do it in 5.4 seconds, and the ML63 could scorch the tarmac on its way to 96 km/h in 4.6 seconds – as quick as a Lamborghini Gallardo. Owners lauded the ML’s firm ride and handling balancing act, although body lean was apparent on curvy pavement. The cabin is a serene place and even the diesel V6 generates only a distant thrum.

In a comparo of eight luxury SUVs, the ML350 ranked fourth overall, let down by its poor brake feel and less-than-skinny A-pillars.

At the fuel pump, the ML350 demonstrated reasonably good fuel economy (using premium grade), while the coveted CDI diesel was downright miserly, delivering econobox mileage in a stately wrapper.

WHAT OWNERS SAY

Assembled in a brand-new plant in Alabama, the original 1998-99 ML320 suffered significant teething pains, including ill-fitting doors, electrical gremlins, leaky fuel lines, fragile interior trim and a cacophony of rattles.

Engineers addressed each of the maladies over time, but the second-generation models introduced their own quality headaches.

The automatic transmission may develop a rough shifting condition and eventually fail outright. Harsh downshifts from second to first when braking may require a software upgrade, a revised valve body, or both. It’s important to follow Mercedes’ maintenance regimen.

The camshaft adjuster solenoid can reportedly fail, or balance shaft components may wear, resulting in a lit Check Engine Light and drivability issues. Repairs are dear. The engine may also develop a stalling or no-start condition due to a bad crankshaft position sensor.

Other malfunctions include oil leaks from the camshaft caps, failed air conditioners, drained batteries, sundry fluid leaks, overheated driver’s seats and short-lived tires.

Diesel owners are the happiest bunch. Avoid the inaugural 2006 models.

We would like to know about your ownership experience with these models for future Wheels.ca stories: Ford Fusion Hybrid, Mitsubishi Lancer Evo and Honda Fit. Please email toljagic@ca.inter.net