fuel or oil film on water behind boat

[onelefty]

89 mercruiser Bravo One 7.4L leaving a slick behind boat at idle. not sure if gas or oil. suspecting gas. No smoke or ill engine performance
Here is what I have done to idenitify or correct the problem;
checked plugs-clean, dry no fouling,
compression test 125 to 135PSI,
timing 8 deg BTDC,
engine oil clean & clear, No streaks or foaming
outdrive oil replace-no streaks or water in old oil,
no noticeable consumption in oil reservoir
rebuilt the quadrajet & set jets with vacuum gauge.
Fuel pump vent tube dry

This is driving me crazy. Any suggestions are appreciated.

 

[QC]

Re: fuel or oil film on water behind boat

What about a trim fluid leak from the rams or lines? You should be able to smell gasoline . . .

 

[haulnazz15]

Re: fuel or oil film on water behind boat

What about a trim fluid leak from the rams or lines? You should be able to smell gasoline . . .

I would agree, check the trim lines. Many trim systems use SAE 30 or similar oil as the hydraulic fluid. Could be seeping past a ram seal, or a hose fitting.

 

[onelefty]

Re: fuel or oil film on water behind boat

rams & lines tight & dry. trim plate cylinders also dry with no leaks.
the film on the water occours only with the motor running.
Not sure it's oil due to the lack of buildup on the transom?

 

[bigskiohio]

Re: fuel or oil film on water behind boat

trim rams , maybe when you turn only. mine did that and left big slickof purple.

 

[bamadave]

Re: fuel or oil film on water behind boat

If it is running rich, it could cause the exhaust to leave traces of petroleum on the water. My boat was doing the same until I adjusted the carb.

 

[a70eliminator]

Re: fuel or oil film on water behind boat

If it slicks up only when the engine is running then I would think the exhaust is where it is comming from, It doesn't take but just a drop of fuel or oil to make pretty colors.

 

[onelefty]

Re: fuel or oil film on water behind boat

thanks for the ideas. I just rebuilt & adjusted the carb as I thought the same thing, rich = unburned fuel in exhaust. unfortunately it continues.
the boat is stored on a lift at the lake. This makes looking for outdrive oil leaks easy. When on the lift above the water there is no indication of oil seeping or dripping from the drive, trim, tabs, anywhere.
Engine oil is not likely? since the compression test & plugs look OK?
Question, is there a potential that exhaust pressure is causing lube oil to be forced from an outdrive seal? then no leak when engine not running?

Fustrating

 

[Don S]

Re: fuel or oil film on water behind boat

The 7.4's have an oil cooler. Might want to remove it, drain it and pressure check it.

 

[onelefty]

Re: fuel or oil film on water behind boat

Thanks Don. I did not know about the oil cooler. I also understand there is a power steering cooler?
Where are these coolers & is there a procedure to removing them?