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Fuel slick when I shut the engine down.

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briang0

Regular Contributor
I have started noticing that after I shut the engine down I see what appears to be a small fuel slick behind my boat. It looks like it is coming form the top of the out drive. The engine is a 1983 Mercruiser 305 with a 2 barrel carburetor. I have put new plugs, rotor, distributor Cap, wires, fuel separator, and fuel filter. I set the timing to 8 BTDC like it says in the tag on the engine. The engine starts and runs great. I will say that it does take a few minutes to warm up if I try to leave the dock to quickly it will not let me because it will not rev about 1500 RPMs but after about 5 minutes or so its fine. I have verified that the choke is working properly and that it is opening fully and not sticking. I have checked for a vacuum leak and couldn't find any. When the boat is running it is smooth and has good power. The WOT is a little low at about 3700 RPMs but I believe that is due to the prop but I'm not worried about that right now. I really hate seeing the fuel entering the water after I shut it off. As I have said before this boat is new to me so I don't have a lot of back ground on it. I did spray clean the carburetor in the spring. When I changed the fuel filer I found a lot of varnish in it. Am I looking at a carburetor rebuild even though it is running well?
 
You sure it's fuel ?

pressure test you drive first drain it if there's water in the drive the oil will look like milkshake post back.

we will go from there.
 
I'm assuming it's fuel because I see the slick whether or not I put the drive in gear or raise or lower it. If I start the engine and run it for a few minutes then shut it off I will see a slick on the water coming out of the upper exhaust ports. Tonight I went down to go for a ride and it took longer for it to warm up and when I tried to go out it started to stumble at 2000 rpm I came back to the dock because I had my grand kids in the boat. Somethings up!
 
The engine is a 1983 Mercruiser 305 with a 2 barrel carburetor.
Click to expand...
The carb is likely seeping from a 30 yr old gasket of from a casting plug. If it is a 3 piece carb buy a reman. newer model. Otherwise rebuild it.
The WOT is a little low at about 3700 RPMs but I believe that is due to the prop but I'm not worried about that right now.
Click to expand...
It could be a tired engine. Compression test is in order.
 
Oil will look like what on the water ?

Do the compression test on a warm engine, all the plugs out and the throttle fully down, use a screw in type gauge. Write down the numbers.

Keep track of what plug came out of what hole.

Then see what pressure the fuel pump is giving you and look for fuel in the yellow hose coming from the pump to the carb.

Pressure test the drive.

Post back the info. We will go from there.
 
This is my first out drive how do I do a pressure test on the out drive? When I pull the boat out if I check the oil and it looks good will I still need to do a pressure test?
 
Best way to tell if the slick is from fuel or oil is to use the nosetest....yes, sniffing .... if you can see the drops from the exhaust ports, its likely the carb...probably running rich...

BTW, when the engine is running, the gears in the outdrive are spinning so oil is being moved within the drive's cavities. Since it is new to you, it wouldn't be a bad idea to pressure test it but you may decide to defer it if you know the slick is from fuel.

Whether the rebuild is on order is driven by your wallet and your brain...leaving a slick on the water is a federal offense - is it worth going thru the hassle??
 
I was doing some investigation tonight I now don't think it's fuel. Yes, I do get a very small amount after I shut the engine off but I don't think that is abnormal. What I think is happening is I am seeing trim oil coming from the trim cylinders. I ran several experiments and after every time I raised or lower the out drive I saw the plume in the water. If the engine is running it gets masked because of the turbulence. I think the wiper washers are messed up. When I got a close look at the trim cylinders I could see that they had quite a bit of algae growing on them I think it is maybe grown in the end cap and causing the wiper washer not to seat right This is speculation right now. I have to wait until the weekend to prove my theory. I'll have to put on a bathing suit and go for a dip and the water doesn't get very warm up here. Anyone seen anything like this before?
 
Trim cylinders have been known to leak but I wouldn't call if a common occurrence, at least down here in the mid atlantic...you may want to check with a couple local dealers to see if there's something unique with your location...if they are original, they would likely benefit from an overhaul - a good local hydraulic shop should be able to get you new soft parts very economically...
 
I had the same thing with the same symptoms. A stainless trim cylinder hose was splitting. When up on the trailer it lost pressure over night and dropped the drive right down. Replaced all 4 stainless lines just because. All is good since.

Dennis
 
My oil leak turned out to be a loose fitting on the down hose to the starboard trim cylinder where it connects to the bottom of the out drive. The nut was not loose but the steel tube could wiggle. I got about a 1/4 turn on the it snugged the tube down tight. I did remove the end to make sure that the flange was not cracked. Mystery solved. Thanks for the suggestions. While I had the boat out I did check the oil in the lower unit it was full and I took a little out to see what color it was. Everything looks looked good I topped it off and planning on going back in tomorrow if the weather holds. Thanks again for the suggestions.
 
Can anyone tell me if I drop the hydraulic manifold off the gimbal housing will it drop down enough to get the lines from the trim pump off? I want to put a new zinc on but one of the bolts is broken I stink at drilling out broken bolts so I'm thinking about replacing the manifold. I looked in the service manual but it just shows the two lines but doesn't mention anything about removing them. I'm thinking I'll put the zinc on with one bolt and replace the manifold next spring.
 
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