Question - what engine would you rather have in a high mileage van?
Obviously, as a Merc owner I am biased, but here is my attempt to be as least biased as possible!
I don't think it's too unfair to say that when it comes to very high mileage - 250k and above for example - Mercs are probably a better bet. The old Mercs are famous for their longevity, not just the engines but the drivetrain - the manual gearboxes seem to go for ever. It is no co-incidence that the reason you never see pre 95 mercs on our roads is that they have mostly been exported to Africa, where they are still running. In Morocco, almost every van is a 5 cylinder pre 95 Merc - hugely overloaded, looking completely wrecked, but just going on and on and on. I know guys in the UK and EU who make a living from searching out old Mercs and shipping them out to Africa. They don't do that with any other make that I know of! Another reason is that they are relatively easy to fix - they are the good old fashioned traditional design - in-line engine, gearbox, propshaft rear axle driven wheels. Front wheel drive vans tend to be harder to fix because everything is crammed under the bonnet. Old Mercs also have a reputation for taking abuse. If badly maintained, they tend to moan and groan and whine but they rarely explode or fail catastrophically (although it can and does happen - it just seems to happen less than with others).
It is also worth considering that every old make has its "features" and reputation for common faults. Here are one or two - based on what I have read and picked up over the years. Mercs - over the years the fuel system gets clogged, leading to loss of power and sometimes complete breakdown. Usually fixed by changing filters and cleaning the fuel tank. Mercs will also overheat, especially on long mountain passes. This is usually dues to radiators clogged with muck and or faulty fan. Mercs often lose gears, leading to owners thinking the box is broken, but in reality it is the selector mechanism, which hardly ever gets maintained. It's an easy fix. Fiat family - the commonest and most famous problem is with fifth gear - it seems to wear out and break easily. It is a very well documented problem. It is critical to maintain the correct gearbox oil level. The fix is easy, and quite cheap, but only to a mechanic who is familiar with the problem. Whole gearboxes have been replaced at great expense, when all that was needed was a new gear and some knowledge. Fiat family also can overheat on long hills - they have electric fans which can fail, but this is also an easy fix. There are others, but I think those are the main ones.
The other big thing is that Mercs are RWD and Fiats are FWD. FWD vans get stuck in the mud and wet grass far more easily than RWD vans. RWD is also better in the snow. That's a fact, and nothing to do with make - it's horses for courses. The other thing is that Fiat family vans have quite a wide range of engines, turbo and non turbo, and are generally faster, and more economical. For many people this is the clincher. The Mercs only have one engine - the 5 cylinder 95 HP non turbo, they are not fast, much happier at 55 than 70, and thirsty - 25 mpg max.
However in motorhomes, the duty cycle is a lot less than in their commercial counterparts. So it's not just about the engine and transmission. A commercial van will hit the road from new, and work hard for maybe 5 years - ten years max, and then will be worn out and scrapped. But during that time it will have been regularly serviced. Motorhomes are the opposite. They spend most of their time doing nothing, and then several weeks of intense activity. A 25 year old van with 100k on the clock has only done 4k a year - a commercial van will do over 50k or more a year - it's a huge difference! Commercial vans and trucks can and do clock up a million in 10 years, and this is what they are designed for. So the problem with motorhome use is not so much mechanical wear and tear, but age related, and standing around. Rubber gets perished, fluids get old, wiring harnesses get corroded - in fact it's a whole host of little problems, and some of them can be truly frustrating. Old vans of any make need quite a lot of TLC - you can't just treat them like modern cars, which hardly ever go wrong.
So service history becomes really important - any van, any make, over 20 yrs old, over 100k or 200k on the clock, with service history, is a great find. Unfortunately most service histories tend to end after a couple of owners, or after 10 years. This is one of the reasons why some people buy German vans from Germany - the Germans tend to keep their service histories bang up to date, and their TUV (MOT) system is much more rigorous than ours.
Finally, another thing to consider is - suitability. How handy are you? If you have experience of a particular make, or have friends who are, or a local garage who knows a particular make well, then it's daft not to take advantage of that. Similarly - what do you want a van for? If you want a van for UK holidays, weekends and festivals, and generally just a few weeks a year of traditional use, then again, it doesn't really matter what you have - cover yourself with a good breakdown policy to get you home, and just get on with enjoying your van. But if you intend to go on a big road trip around Europe and N Africa, and really clock up some miles and abuse, then it's a different equation. Both Mercedes and the Fiat family have extensive dealer and parts cover, so at the end of the day you just have go with your gut feelings.
My gut feeling is for Merc, and I have clocked up over 350k over 16 years in 3 separate vans and in some really heavy and rough use. Not without problems, but in all that time I have only ever actually broken down and had to be recovered twice. Once was a broken oil pump that was directly caused by a mechanics mistake during service, and the other was a broken alternator - I could have carried on to a garage if needed, but I didn't know that at the time, so chose recovery, as I was covered. I have had many other problems, including a new flywheel and clutch, over heating, gear selectors, brakes overheating, glow plugs and relay, but most of these have been fair wear and tear or bad maintenance, and I have limped on, sometimes for hundreds of miles.
In the spirit of fairness, I also know know many Fiat owners with a similar story ...
As they say on TV - the final choice is up to you.
