Mercedes Benz G Class: G 350 CDI, G 350 d and G 63 AMG

The G 350 CDI is where most of the used G Class market in New Zealand sits. It runs the OM642 3.0-litre V6 diesel producing 245 hp and 600 Nm, and on paper that sounds bulletproof. In practice the OM642 has a few well known habits that every G 350 CDI owner should understand before a small issue becomes an expensive one.

The oil cooler and its gasket are the first thing to look at. They sit in the valley of the V6 and when the gasket starts weeping, oil migrates into the surrounding intake area. It is a classic sign on these engines and one we see regularly. Left alone it creates carbon build up that compounds the next issue: the swirl flaps and intake manifold. The OM642 swirl flap system clogs and eventually fails on higher mileage cars, and getting that intake cleaned properly makes a real difference to how the engine breathes and how cleanly it starts when cold.

EGR clogging is next on the list, especially if the vehicle has spent a lot of time in city traffic. The G Class is a heavy machine and short trips mean the DPF never gets a proper regeneration cycle, which accelerates soot loading and puts extra stress on the whole emissions system. If you are buying a used G 350 CDI, find out what kind of driving it has done. A well exercised example that has seen regular open road runs is in a completely different place to one that has spent its life doing short school runs.

When Mercedes replaced the OM642 V6 with the OM656 3.0-litre inline six in the current W463 G 350 d, they did not just swap engines. The OM656 is a significantly more sophisticated unit producing 286 hp and 600 Nm from a smaller displacement, with a higher compression ratio and a completely different architecture. It also brought 48V mild hybrid technology and a more complex emissions system. The result is a genuinely impressive diesel, but the complexity means the faults are different to what CDI owners were used to.

The emissions hardware is where most of the real work sits on the OM656. AdBlue system faults, SCR catalyst problems, and NOx sensor failures are the common complaints, and they tend to arrive as warning lights that owners initially hope will go away on their own. They do not. The EGR system on the inline six also accumulates carbon, particularly on vehicles doing short urban trips, and DPF regeneration issues follow the same pattern as the older V6.

The 48V mild hybrid system is new territory compared to the CDI and worth flagging. The starter generator and its wiring loom can generate faults that look confusing on a generic scan tool but read cleanly on proper factory software. This is exactly why we use Mercedes XENTRY with a genuine interface rather than a generic reader. The guided test capability matters on these vehicles.

Compared to the CDI, the G 350 d is a better car in almost every measurable way. It is quicker, quieter, more fuel efficient and more refined. The trade off is that its emissions system is more involved to maintain, and the 48V architecture adds another layer of electrical complexity. If you buy one, stay on top of the AdBlue system and do not ignore NOx sensor warnings.

Picture a vehicle that started life as a Cold War military transport and came back from AMG with 585 hp, 850 Nm, a nine speed automatic, three locking differentials, and air suspension. The W465 G 63 AMG is genuinely absurd in the best way. It is also the most demanding G Class to maintain correctly, and owners who treat it like a normal SUV service job find out about it eventually.

The M177 is a 4.0-litre twin turbo V8 with a hot vee turbo layout, meaning both turbos sit in the valley between the cylinder banks. That keeps the exhaust path short and spool up fast, but it also means serious heat soak is a real consideration. The intercooler and cooling system condition matter more on this engine than a conventional layout, and the plastic charge air and coolant fittings are a known weak spot. They get brittle with heat cycles and when they fail, they fail suddenly. We see them come in still running on reduced pressure, having been ignored because the driver assumed the slight performance change was just a bad day.

The 9-speed AMG SpeedShift transmission is strong, but the G Class is a heavy machine and the transmission and transfer case work harder than they would in a lighter vehicle. Fluid changes on both are not optional maintenance, they are the reason they last. The solid rear axle and heavy suspension components are robust by design, but the adaptive dampers and active engine mounts are electronics heavy components that wear and can generate confusing faults if diagnosed with the wrong tool.