99 ox66 250 spark plug issue

[jf123]

I'm fighting this spark plug issue and have run out of ideas what it could possibly be. I enclosed a picture of the plugs. I think it is water but I may be wrong. Cylinder #3 and #4. I had the heads exhaust and intake off with no signs of leaks. Compression is good. Any ideas what could be causing this ?

[boats.net]

Have you checked fuel filters? Look for water or a thick residue in bottom of filter. Does boat have a fiberglass fuel tank?

[jf123]

Aluminum tank. Filters were changed and I never saw anything unusual with the fuel. LP pumps were changed as PM. I have been through the whole fuel system twice. Injectors were sent out to be tested/cleaned. What strikes me as odd is that it started on #3 and now takes out #4 also. Do the plugs look like saltwaterwater intrusion to you or something else? Maybe bad fuel grade but it doesn't explain why it takes the center two cylinders every time.

[jb123]

u sure exhaust plate was ok ???

[jf123]

Yes exhaust plate was off. The 250 does not have an inner plate the exhaust runners are part of the block. From everything I can see and the overall condition under the plate no signs of corrosion. Unless there is a crack on the back side where you can't see it. Just very strange since I have never heard of these blocks having this issue.

[boats.net]

When you had heads off did the pistons have clean spots(no carbon at all) on top next to intake ports? Do the plugs have water on them if removed when first shut engine off? Are you using a good TCW-3 certified oil? After you replace plugs does it run on those cylinders?

[jf123]

Pistons are completely covered over in carbon. I enclosed a picture with Cyl #4. When I remove the plugs is appears that there is water on them and a coating of a tan/white substance like they were dipped in something. I use only Yamaha 2M by the drum. I can get @ 60 gallons of fuel thru it before it starts acting up. Spin new plugs in and go again. Last year it started on #4 only now I also get it on #3.

[boats.net]

Check the fuel pump diagrams to see if ruptured.

[jf123]

LP pumps were replaced. I'll check them again just in case I have a bad one out of the box. Would it help if I swapped positions on the pumps and see if it follows to the top or bottom cylinders?