Big update: So…I decided to wait a week or two before taking the car in for the thermostat replacement and carbon cleaning, since the problem seemed minor…for now. I cleared the check-engine light using Carista, and over the next several days, the temp reached and remained at normal operating temperature, and the engine check light did not return. (Even the hidden, permanent code, within Carista, was gone.)
Then, Friday the 13th happened. The first half of my drive to work was awesome, as usual. Then, I downshifted to (aggressively) pass a slow car, and as soon as I gunned it, there was absolutely no response from the engine. It was like the pedal was disconnected from the car, at least when it was over 3,000 or 4,000 RPMs. I took it easy for the rest of the drive, and if the RPMs were kept low, there was no sign of any issue, and no check-engine light.
While at work, my Google research using GolfMK6.com and VWVortex appeared to indicate that my (new) issue was the High Pressure Fuel Pump. I was nervous about starting the car, as several posters warned that it may be difficult to start…it was. Took six long cranks, but it finally started, and the check-engine light appeared, with the engine running very rough while in park. Made it home, ran a Carista scan, and had the following codes:
P2293 – Fuel Pressure Regulator 2 Performance
P053F -
P2177 – System Too Lean Off Idle Bank 1
P0087 – Fuel Rail/System Pressure, Too Low
I took a chance, based on all available information (and the fact that the Humble Mechanic has a great HPFP DIY video with Deutsche Auto Parts), I ordered the HPFP from ECS Tuning. I even ordered the cheapest one, for $160, since it was Bosch, and appeared to be exactly the same part as the VW version for $260. (Other places appear to have the Bosch part for even cheaper, FYI.)
Last night, I removed the old HPFP, installed the new one, and cleared the fault codes. After several test drives, all is well so far. The only DIYs that I’ve done on the GTI have been oil changes and full brake jobs, so I was very nervous and anxious about this working out. I couldn’t have done it without the awesome GTI community! Thanks!