The advice to change oil frequently is solid. Not too sure about the carboned up intake valves though. If anything, it's the 5.0 that has issues there. Its valves wear paper thin until such a point that the holes (usually 4 and 8) drop compression.
I know VW/Audi/Lamborghini has had major growing pains with direct injection. The lack of fuel spray over the valves prevents the back of the valves from getting washed. All the same, I've yet to see a 3.5 GTDI come in with coked up heads. Why Ford is avoiding this across all of its direct injection motors is beyond me.
Common 3.5 ecoboost issues include
- defective turbochargers (most often the wastegates seize, but once in a while the impeller wheels poop the bed)
- building oil (high pressure fuel pump, but if you ask Ford it needs a PCM reflash and a new block heater)
- catalysts (see, building oil)
- turbocharger control solenoid (P0234 overboost code)
- carbon tracking on the spark plugs (change the plugs every 50 000 - 60k)
- faulty primary and secondary timing chain tensioners and stretched chains (runs like a dog, P0011/12, P0021/22 cam and crank out of relation)
2003 GMC Sierra 3500 -- dually, 4x4, hasn't blown up yet and that's what i look for in a truck
Ford Doctor -- They come in broken and leave fixed. Ford of Canada Master Tech