I just bought a 2004 Supra Launch in March. A few weeks ago the engine started to run rough and it sounded like it was running uneven, if that makes sense. I took it to a shop and they said that two of the eight pistons are running at 4 and 5 pounds of pressure instead of 170. Anybody have any idea what the problem could be?
I would bet you have a blown head gasket. Are the two cylinders with low compression side by side? If thats the case (and i bet money that it is). You will need to pull the intake and then the head. Not that big of a deal. Just a pain.
The engine only has 92 hours on it and I know for a fact that the previous owner took immaculate care of the boat/engine with regular tune-ups etc. Plus, I had it surveyed before I bought it and the surveyor said it was in pristine condition. It had been running just fine this whole summer and then this. I am seriously at a loss...
Like Big E has stated, chances are it is a blown head gasket. This is certainly less than needing a rebuild but still not cheap. Take it to a traditional auto mechanic, my experience has been that they are a little less expensive then boat dealerships and there is nothing much different about the engine.
If you plan on tackling it yourself, even with an experienced persons help, get the shop manual. Seems to me you need to find the cause to prevent a second blown gasket. Did the warning horn ever come on, telling you that it overheated?