Having had 3 VW GTIs (2x Mk5 and 1x Mk6) over the past few years- I can shed some light on these cars. Both engines are great, very tunable and pretty reliable for the most part (though there are some things to watch out for). With that being said, I’ve never had an issue with either the FSI or TSI engines.
The most common things to watch out for include:
FSI
-potentially high oil consumption (the culprit is usually the PCV system). Easy fix
-Dodgy diverter valve. Many upgrade to the revised revision D diverter valve or aftermarket. Easy fix
-Carbon build up. Consider walnut blasting the valves around 120,000km
-Coil packs and spark plugs fail (tuned cars only). Upgrade to Audi R8 CoP and NGK plugs
-Cam follower wear. Check cam follower every 2-3 years and replace when required. Easy fix
-Still uses a cambelt. Replace as required (including ancillaries)
-Stage 3 (i.e. turbo upgrade – k04 etc) needs an upgraded fuel pump
TSI
-Dodgy cam chain tensioner (early models up to 09/2012 build dates). Catastrophic engine failure if this goes bad. Not a cheap fix due to labour.
-Coil packs and spark plugs fail (tuned cars only). Upgrade to Audi R8 CoP and NGK plugs
-Carbon build up. Consider walnut blasting the valves around 120,000km
There are other issues that I failed to list such as intake manifolds going bad, cracked intake pipes etc but these aren’t too common…
The DSGs are another matter altogether. The Mk6 DSG is better programmed than the Mk5 (blip downshifts, a bit smoother etc) but both can be very temperamental if not serviced correctly. VW requires servicing every 60,000km. If there’s no record of this being done I’d walk away. Make sure you drive a few vehicles to get a feel for the transmission as some might have clutch wear (particularly the imports sitting in Tokyo traffic). If you get a good one- you’ll be sorted as long as you take care of it. They’re great gearboxes overall just a bit hit or miss; shopping around and test drives are key…
Finally, 04-08 Mk5 GTIs have hydraulic steering which I prefer over the slightly numb electric racks of the later models.
With all that being said, they’re pretty solid cars and I enjoyed my ownership with them.
If you have any other questions just ask- happy to help