Have 2005, 200hp HPDI with 680hrs. Do alot trolling at 600 to 800 RPMs. Have reviewed a number of post about fouling plugs with HPDI mostly at 1100 to 1600 RPM's. I have had problems with this since the beggining and use ring free. If I replace the plugs or clean them they foul after about 12 hours. The 1st sign is a very slight delay in starting, with increased use, the delay gets longer and longer. Its never been hard to start. Last year at the recommendation of the YCM, I increased my ratio of ring free, 1oz to 8g'fuel and went to a lower octane fuel(87) was 89. It made no difference. I am trying Lucas fuel treatment at 3oz to 10g/fuel and have not seen any improvements. Any thoughts?