The 1.8T in the Mk4 GTI (codes AWP / AWD and relatives) is strong but has two signature issues: coil packs and oil sludge.
Coil packs: the 1.8T is the engine that made VW coil-pack failure famous — misfires, flashing CEL. Diagnose and fix as on any turbo four (see Coil Packs Misfire) and fit the revised coils.
Oil sludge — the important one:
- The 1.8T's small oil galleries and oil-pump pickup screen clog with sludge if oil changes were stretched or the wrong oil used. Sludge starves the turbo and bearings and kills the engine.
- Prevent it: oil changes every ~5k miles with a proper VW 502 00 full-synthetic. On a used one, drop the sump and inspect the pickup screen — a sludged screen is a red flag.
Also common: the diverter valve, the cam-chain tensioner at the back of the head, and the breather/N249 plumbing.
Bottom line: buy a 1.8T on its service history — a sludge-free, regularly-oiled one is bulletproof; a neglected one is a time bomb. See GTI Service Intervals.