I have a 2005 50 hp Merc on a 21' Sun Tracker Fishing Barge. I went out today and noticed that at wot, 5200, the water pressure gauge was reading 10psi. That seems low. Also, it is using more fuel than last season and seems to have more vibration as well. It starts on the first turn of the key with no problem and does not bog down. The impeller was replaced last year. What should I be looking at? I was thinking of pulling the plugs this weekend to see if they tell me anything. The motor has only been in fresh water.