Hi guys! I mothballed the boat this week. Sad day. I have a couple of questions though. First of all, the last couple trips I took to the lake when I shut the engine off it wanted to disel. Not bad and not everytime I shut it off but it just kinda sputtered for no more than 3 seconds. It usually only did it once or twice in a weekend and mostly after I ran it cruising speed for awhile. The motor is a 4.3 merc 1995 model. I havent changed the plugs since I have owned it. I have only put about thirty hours on it.

Second issue. My drive is consuming a tiny bit of oil each trip. Depending on how hard and long I run it, my bottle level usually drops 1/8 to a 1/4 of an inch. I cant find any leaks though. The bilge is clean, there is no oil on the sides of the drive, no oil running out the prop, and the bottle hose is tight. No cracks in the bottle either. I pull the drain plug on the drive to check for water each trip out. So far there has not been a drop of water come out and the oil looks and smells the same as it did the day it was put in. I had the drive pressure tested in a tank and it held 15psi for an hour with no air bubbles coming up. There is no evidence of oil on the water at all. We sit at anchor a good bit and I have never noticed a rainbow. Is it normal for drives to consume a little oil? The drive is a 1995 alpha one. Thanks for your help.

Give the motor a few minuites of idling to cool the heads. A long cruise locks a lot of heat in the heads and a quick shutdown just asks for dieseling to occur. See if that works/helps.

Doesn't really sound like an excessive drive lube oil use issue, but guys with I/O's can chime in better than I.

How dirty was the hull? Was it clean or dirty?

What are your WOT rpms? Are you over-propped and lugging the engine?

Lugging builds up a lot of heat. About how many RPMS were you burning?

Last time you replaced the impeller?

the hull was clean, I dont allow any build up of crud on it. I am running about 4700 wot at about 54 mph according to the speedo in the dash. My prop is a 19 pitch 14 dia. I dont think Im lugging it but then again Im relying on the tach in the dash. I have never had it put on a shop tach. Impeller was inspected in the spring looked and felt like new. Temp guage never goes past a 1/4 of the way up on the gauge. It dosent have degrees in it, just normal and overheat ranges. I never thought about idleing for a minute or two before shutting it down. It makes sense.

Not to be an idiot but what causes dieseling?

Run a carbon deposit remover through the engine (GM sells it, I forget the Official Name). Clean the plugs. That should take care of the dieseling.

Dieseling is caused by deposits in the combustion chamber that get very hot and ignite the fuel on their own. That is why the engine keeps running after you turn off the ignition. Removal of those deposits remedies the dieseling.

The consumption of drive lube seems to be normal, not sure why, but it is fairly typical of mercs.

Ever try seafoam? I think it is a deposit remover. I belive they make it in a spray and a bottle you add to your gas tank.

As for the drive lube, have you ever had a problem like that? I thought that ANY variation in drive lube level meant there is a problem. It is comforting to know that mine isnt the only one with this issue.

The 4.3 is especially susceptibla to dieseling if it is shut down after a run without idleing for a minute to cool the heads. I would change the plugs rather than clean them since they are several years old, especially if you plan to de-carb the motor.

My alpha was a genII and never used any lube but some do I guess.

I noticed my drive oil level in the Bayliner is down a bit. Are these things designed to use oil?? Why is it the newer ones use oil, but the older ones, like in the houseboat, never lose oil --if anything, they gain water.

Is THAT the 'design feature'? Better to let the drive push oil OUT rather than let water IN? (can't we just have a drive that that does neither?)

My Alpha was a 2002 genII and it never used any. I had nearly 400 hours on it and the only time the oil level dropped was after an oil change. The first run after changeing lube it would drop about 1/4 inch, probably from an air bubble.

FR, I don't think they could design it to push oil out so as not to let water in. They would have to have positive air pressure pumping into the drive to do that and since the drive lube reservoir has a vented top there is no way for it to hold that pressure.

An engine "uses" oil by leaking some past the rings or valve seats, or from leaking gaskets. A drive should not "use" an appreciable amount of oil. If the propshaft seal is leaking you may not notice a sheen on the water but it may use some oil. The reason for this is that the water pressure at the prop seal will hold the oil in but when you are moveing on plane there is actually a slight vaccuum so it can pull a little lube out.

If it is a few tablespoons of lube for a full day on the water I would just keep my eye on it. If the level goes down 1/4 inch each trip then you have a leak. Let it sit overnight and pull the lower drain to check for water. Drive lube is designed to keep the water in suspension so you don't pump straight water at any time so it takes a few hours for it to settle out.

Basically if you have a leak allowing oil to leak out you have the possibility of water getting in.

Prober, actually the drive's reservoir cap has a 1-way valve on it that keeps the positive pressure in the system. It's the little red button looking thingy on the cap. Run your boat in the driveway for a while in gear, then shut it off and loosen the reservoir cap. You'll hear a hiss as the pressure escapes and you'll see the fluid level rise a bit. The valve only "vents" air into the reservoir when the fluid cools and contracts....this keeps it from sucking lake water in through the various seals.

Interesting, so is it designed to only hold a certain amount of pressure? Seems like it could build up too much otherwise and cause a leak.

The seals on the drive are designed to "weep" a little lube to keep them lubricated. Should be just a little each time, but if you never get any drop in the resevoir, you need to make sure it is working correctly.

T-Squared, thanks for the clear explanation. When you put together your comments about heat generated positive pressure and seals designed to self-lubricate, you begin to see the whole picture. The pressure generated by heating the drive is held in by the 1-way valve and causes the seals to weep. Once the drive cools off, the lube (now slightly reduced due to weeping) contracts and sucks fresh lube from the resevoir. The 1-way valve lets in air to avoid a vacuum that would impede the flow of lube into the drive.

Pretty slick system.

If it was me , I would pull the drive and pressure test . Better to be safe than sorry . if it is a leak its only gonna get worse . cheap insurance !