It is not dirt, it is carbon build up. It can be removed with a high quality fuel and sustained higher combustion temps. Putting the engine under a good load. It doesn't have to be high RPM. We don't have those types of issues here. Not sure why you are unless the previous owner putted around and did lots of short trips. It's not a common issue with port injected fuel injection. It is more of a problem with direct injected engines. Catch Cans that I stated will help with this issue. A 3 to 4hr trip should fix it. We don't drive short distances here. That is not the norm.
|