Inboard thin oil.....and smells like gas ???

[ScottieinNC]

Have a 1996 4.3 MerCruiser LX w/Alpha One drive. When I change the engine oil, it seems thin and smells a bit of gas? Is this normal? Using 10W30 weight oil year round. Oil pressure good (40 psi)on the guage, but fluctuates up to 60 or so as I would expect upon throttle up.<br />Is this normal for an I/O setup?<br />I do some cruising around, rarely WOT, but sometimes, and never for an extended period. I like to settle into cruise speed of 25-29 MPH and enjoy the ride. We do some slow trolling when fishing, I don't do it for hours on end. We tube a lot so theres speed ups and slow downs through turns and straight aways. Should I be using a different oil? <br />I know has been asked before but what about synthetics, would they thin out less?<br /><br />Thanks,<br />Scott

 

[ntheyer]

Re: Inboard thin oil.....and smells like gas ???

It is possible that the rubber diaphragm on your fuel pump has failed and is consequently letting fuel in your oil pan... Have you checked the fuel pump? I would bet that's your issue.<br /><br />Just remove your pump and take it apart and check the diaphragm. This is assuming you aren't fuel injected, which I don't think you are.

 

[ScottieinNC]

Re: Inboard thin oil.....and smells like gas ???

Maybe the FP diaphragm, it seems to take a while before I notice it in the oil. I am planning on changing oil when I think it is getting thinner.<br /> Rather pay $10 in oil than mess up the engine.<br /><br />I will look into that ntheyer, no I am not fuel injected, wish I was!<br />I'm thinking blow by of the piston rings maybe? I always let her warm up real good at the ramp before taking off. It's not too bad, but I am noticing it which is a sign....

 

[Alumarine]

Re: Inboard thin oil.....and smells like gas ???

I'm not familiar with your engine but I would think the oil weight should be 30W or 20W40.<br /><br />I'm sure someone will come along and confirm that.

 

[Haut Medoc]

Re: Inboard thin oil.....and smells like gas ???

30wt or 25-40....<br />10w 30 is too low a viscosity unless you are in Nome....<br />Do you see any gas in the sight tube that goes from the fuel pump to the carb?<br />Blowby will not put gas in your oil, a bad fuel pump will....<br />Oil loses its lubricating qualities when mixed with gas....<br />If this is a relatively new occurance, I would change the fuel pump, there is no rebuilding it....Spend the money & save your engine......JK

 

[ScottieinNC]

Re: Inboard thin oil.....and smells like gas ???

Thanks Haut, I'll be taking off the FP tomorrow due to the responses if the weather after work cooperates. We are in our summer pattern of pop up thunderstorms in the afternoon. We received a 15 minute bout with dime and quarter sized hail a few hours ago.<br /><br />Is this a standard GM type part or dealer/marine only?

 

[ntheyer]

Re: Inboard thin oil.....and smells like gas ???

You can get one from NAPA which basically cross references it with a Sierra part number. It should be around $100. West Marine etc. are more. The NAPA pump is a marine version. Marine is different than auto version. The auto version will overflow onto the ground, which turns out to be your bilge in a boat. The marine version has the overflow outlet which should have a tube running up to your carb to pump and overflow into the carb. I do not recommend the auto version. I know it's more $$$, but what isn't more on a boat? I would bet it's your fuel pump. Opening it up would not be to repair it, only to check to see if the rubber is torn. If it looks cracked etc. it's toast.

 

[ron7000]

Re: Inboard thin oil.....and smells like gas ???

fuel in the oil is common for boats. It shouldn't be considered normal though, because what it really means is the motor isn't tuned properly or there's a mechanical problem.<br /><br />Fuel in the oil thins the oil out, and under certain conditions will cause increased wear and engine damage. The best fix is eliminating the source of fuel in the oil, whether it's from a mechanical fuel pump, bad rings, or an out of tune carb, but that doesn't guarantee there will be zero fuel getting into the oil. The other 2 ways to combat fuel dilution is to run a higher viscosity oil which can tolerate more dilution before the oil is too thin, typically a 15w-40 or 25w-40. And also make sure you engine gets to operating temperature as quickly as possible, preferably at least 160F water temp, so that you oil temp gets hot and run the engine long enough to burn off fuel (and any water) that gets into it. Avoid short engine run times if possible but also realize that letting the engine just idle for long periods can also be a source of fuel dilution.

 

[Don S]

Re: Inboard thin oil.....and smells like gas ???

Forget about the fuel pump, you have an electric pump with no diaphram to leak, and it's not attached to the oil galley like the old mechanical pumps.<br />I would look more for too tight a choke, or rich mixture if you do a lot of idling/low speed operation.

 

[ScottieinNC]

Re: Inboard thin oil.....and smells like gas ???

Hi Ron,<br /> I forgot to mention I am raw water cooled so I think it takes a bit longer to warm up, but I do warm it at the ramp first, or take off very slowly before I get on it. <br /><br />I knew there was a proper term and fuel dilution sounds right. <br /><br />Would a synthetic 15w-40 be a good oil? Have an Excursion diesel and that's the same oil I use in it.<br /><br />Hey Don S! Thanks for the post, It's electrical huh?<br />I do some slow trolling and this may be the cause. Any suggestions? Change oil frequently?

 

[man-of-war]

Re: Inboard thin oil.....and smells like gas ???

Scott k, ron7000 hit the nail on the head. The engine HAS to warm up in a hurry. How I fixed my problem was before I take the boat out I hook the hose up to the laundry room sink and pump some luke warm water through the drive. Then fire it up and in less than 2 min she's up to temperature and away you go. The problem is the rings and cylinders need to expand to make a good seal.<br />Last year I went through 4 engine rebuilds @ $4000 a pop on one of my boats. My new boat with a new mag mpi engine had the same problem the long standing still warm ups at idle.<br />I'm using 20w50 GTX oil. Forget the synthetic you'll just be wasting money. Change your oil every 20 hours or if you see a rise in oil level. GOOD LUCK

 

[ron7000]

Re: Inboard thin oil.....and smells like gas ???

fuel dilution is fuel dilution, a synthetic oil will thin out just the same and offer no better protection. So in this case it would be more economical to run a good "non-synthetic" oil and change it more often, the idea is keeping the dilution to a minimum.<br /><br />with the choke, as soon as the engine is running you can almost always manually open the choke by hand. The motor won't need any choke (a richer mixture) once it's running, the electric choke usually takes a minute for it to open the choke flap fully but if you help it and get it open as soon as possible, all the better because the mixture will be better (leaner) and the engine will run smoother and be less chance of getting fuel in the oil. And with cold idle speed once it's started, a faster idle is better. not only will you get better oil flow and lubrication, but the rings will heat up and seal faster. so don't start your motor and idle it at 600-800 rpm to warm up, get it up around 1500 rpm right away.<br /><br />what syn is 15w-40? most syns are 5w-40. I've been running mobil 5w-40 in my 3.0L and have been real happy with it but it's no longer sold so I'll probably go with one of the 15w-40's if the rotella 5w40 doesn't stay available. If you can find delvac 5w40 that would be a very good oil to use.<br /><br /> be careful running a 20w-50. temps around 60F and lower can result in less than ideal oil flow on startup because of the oil being too thick, and can be damaging. 50 weights arent really necessary unless you plan on running really high oil temps, or see lots of fuel dilution, so unless you know what oil temps you are running the 40 weights will protect just as well especially if you have an oil cooler and provide the startup lubrication you want.

 

[ScottieinNC]

Re: Inboard thin oil.....and smells like gas ???

Thanks everyone! Been raining all day here, can't get out to the boat, maybe tomorrow, but then Alberto is coming up our way too.<br /><br />I'll just kick back and read posts on iboats!