Good ideas above. I’m guessing a fuel trim test has already been done, suggest to ask your shop for that data and post it here. It would be hard to believe a shop would remove the cylinder head and clean the tops of the pistons without first doing a fuel trim test.
If the fuel trim tests says the fuel mixture is too rich, likely causes are
- Engine coolant temp sensor is faulty (perhaps this is actually the “water pressure sensor” referenced by OP)
- Fuel rail pressure is too high
- Fuel injector(s) leaking
- MAF sensor is faulty or dirty
- Engine air filter clogged
- Evap system problem, especially purge valve
- O2 sensor faulty
- Exhaust system not flowing freely
- Engine cylinder compression is too low (internal engine problem)
Higher octane fuel is unlikely to help. Suggest to give up on that idea. A fuel injector cleaning procedure or treatment might help though. It doesn’t pertain to the exactly same problem, but it might still be helpful to read through the following thread.