Stinking Mercruiser 260

[1981century]

I have had a 1981 Century boat with a 260 Mercruiser (350 GM) motor since 1990. The boat was a project with a frozen motor when I bought it. I completely rebuilt the motor and put all new manifolds on it when I bought the boat. I have thoroughly checked out all the typical things you would typically check in order to make sure there is nothing internally wrong with this motor. The problem is when you run this motor up to any kind of RPM, it stinks. It puts out a smell that is cross between rotten eggs, burning oil, and natural gas. I have checked compression, ran leak down tests with everything coming back pretty much normal. The engine runs strong.
I am beginning to wonder if I am missing something like a pcv valve or something. The motor has a round steel flame arrestor with two hoses that go to the valve covers. The factory hoses fit into the valve covers then go directly up to the flame arrestor. The ends of the hoses basically "direct" the funk toward the flame arrestor, but the carb doesn't really suck it all up. It smells so bad you can almost always smell it when you ride in the boat, although the motor does not leak or burn any oil. Have any of you had similar problems with a stinking motor like this and have a suggested fix?

 

[NHGuy]

Re: Stinking Mercruiser 260

So do you see or suspect that the crankcase vapors aren't getting into the intake air? I'd be surprised. My 260 sucks down any vapors that emerge from the tubes. Maybe you ought to check to see if the valve cover to carb hoses or fittings are leaking crankcase gases. You can clean the spark arrestor with carb by removing it and shooting it with carb cleaner then dry it and reinstall. If it's gunked up by the crankcase vent hose outlets that could be an issue.
If the motor is worn out enough for combustion gases to get by the rings, that can cause more crankcase gas than when new. That blowby could be your smell.
So is this Century an inboard with the motor right in the center? How do the vent tubes look? My 89 stern drive has brittle bilge vent hoses that I am getting ready to change, if yours have any cracks the engine compartment smells might be going in your nose, instead of behind the boat.
When you run your blower do you get a nice strong push of air at the outlet? You should. Run the blower whenever you are going slowly or stationary. Also check that the passive vent system is tight, complete & not blocked.
Burned/hot oil smell makes me think motor oil is getting onto a hot exterior part of the motor. Check that the motor exterior is really clean, that worked for my boat.

 

[NHGuy]

Re: Stinking Mercruiser 260

Here's another thing you can check. Verify that there are no exhaust leaks. In auto repair we use a rag to partially block the outlet, then observe for exhaust hissing out through breaks & cracks. This is not a very likely thing to be your issue but it's worth a look.

 

[Bondo]

Re: Stinking Mercruiser 260

It puts out a smell that is cross between rotten eggs, burning oil, and natural gas.

Ayuh,... Sounds like the alternator is over-charging the battery,....(rotten eggs, 'n natural gas)

The burnin' oil is probably just That....

 

[1981century]

Re: Stinking Mercruiser 260

The motor is clean on the exterior. I have just replaced the intake manifold gasket while I had the motor out of the boat. All other gaskets are dry and free of oil, but the intake gasket appeared to be leaking. I am sure this could be some of it, but I suspect it will still put out the stink after I get it going again. I have a few "extra wires" that I cannot figure out where they go and I will post a separate thread looking for the correct wiring diagram. I will post an update after I get the boat running again. If anyone has any other ideas, don't hesitate to share them with me.

Overcharging the battery? If this were true, wouldn't it be boiling out all the acid?

 

[Bt Doctur]

Re: Stinking Mercruiser 260

Yes, and thats what smells, the battery should feel very warm too.dont suppose those wires could go to the alt? excite with no sense could let it run wild and overcharge the battery.

 

[1981century]

Re: Stinking Mercruiser 260

The wires that are not hooked up are in and around the shift plate. I think they are part of the shift interupter and or trim sender circuit. I don't think the battery is boiling down, but I will look at this with an open mind for sure. I am attaching a picture of the spark arrestor set-up. If there is any oil that you can see on the motor in this picture, what you are seeing is powersteering fluid that burped out of the system while I turned the drive with the lines off of the actuator at the back of the drive. It blew powersteering fluid all over the place. This is just from having taken the motor out. I had the motor out replacing some rusted out freeze plugs and just general maintenance.
I haven't run the motor since I took it out. As I said before the only bad gaskets I found, I replaced. I am still anticipating this same stink after doing this work since this motor has had this problem ever since the first time I rebuilt it.
Would adding a shroud over the flame arrestor help? Does this pcv set-up look right, or did something get taken out. (see additional photos of the end of the hose) As you can see the hoses just plug directly in the hole in the valve cover. There is no pcv valve, just a hollow tube. See attached photos. Thanks

 

[mhouse40]

Re: Stinking Mercruiser 260

Ayuh,... Sounds like the alternator is over-charging the battery,....(rotten eggs, 'n natural gas)

The burnin' oil is probably just That....

No doubt in my mind , Bondo is right Battery. Been there before and always is that, with the smell you are describing

 

[NHGuy]

Re: Stinking Mercruiser 260

Yup, didn't occur to me but I have heard of an overcharging stinky battery a very longtime ago.
Plus Bondo has seen it all, twice. Put a meter on it, prove it yourself.
(I can over analyze anything)

 

[1981century]

Re: Stinking Mercruiser 260

Should I just hook up a voltmeter to the battery and read the volts? How do you know it's overcharging the battery?

 

[NHGuy]

Re: Stinking Mercruiser 260

Volts above around 14.4 while running are too high. 13.5 to 14.2 volts are typical. If the battery acid is below the tops of the plates it has likely boiled off. You top it up just enough to submerge the tops of the plates with distilled water, not regular water. Be very neat adding the distilled water to the battery, spills are acid and dangerous.
Don't look into the battery while the charging system is running, the acid bubbles and can boil in some circumstances. If any gets in your eyes you will be injured. Can't have that.

 

[Scott Danforth]

Re: Stinking Mercruiser 260

I second the alternator boiling the batteries. however on your post you also mention that you have rusted out freeze plugs. this would indicate that steel was used vs Brass. when you did the engine in 1990, did you use a marine block, or automotive crate motor. if you went automotive route, the head gaskets are not SS, and the freeze plugs are not brass.

 

[1981century]

Re: Stinking Mercruiser 260

That's right the freeze plugs are steel. They lasted 20 years, so I don't feel bad about that, as for the head gaskets, my guess is they were automotive too, but I don't know for sure. I don't seem to have any head gasket issues, although I guess I could. Since all the compression and leak down tests are good, I think it's still solid. I am running it in freshwater. Do you think the head gaskets could be partially blown and causing a problem somehow?

 

[dollarten]

Re: Stinking Mercruiser 260

Volts above around 14.4 while running are too high. 13.5 to 14.2 volts are typical. If the battery acid is below the tops of the plates it has likely boiled off. You top it up just enough to submerge the tops of the plates with distilled water, not regular water. Be very neat adding the distilled water to the battery, spills are acid and dangerous.
Don't look into the battery while the charging system is running, the acid bubbles and can boil in some circumstances. If any gets in your eyes you will be injured. Can't have that.

Even if your battery voltage is within spec , you could have a cell in the battery that is starting to short out . This will make a bad smell also . Test the batter with one those load testors that puts an actual load on the battery to determinine if is any good . A bit off topic here but needed to help you out . I once helped a relative moving out of state and was motoring along and the check engine light came on and went back off along with a few misfires of the engine and everything was fine from then on . To make a long story short The battery shorted out when this all happened . Found this out during the next fuel stop and was smelling quite sulfery and would not start the engine . It also took out the alternator as well . Rare story ? Probably . But these things happen

 

[1981century]

Re: Stinking Mercruiser 260

I am going to check out the battery as soon as I get it back running, but since this problem has persisted through several seasons and even an alternator failure and successful rebuild, I believe it is more of a blow-by or oil problem. My "rotten eggs" description is probably a little bit off, and the smell is more common to burnt oil or the smell you get from sticking your nose in the oil filler hole in the valve cover. Once I rule out the battery with the right tests, I would like to see if there is any other tests I could do on the crankcase pressure. I am also concerned about a potential head gasket issue since Scott Danforth mentioned automotive gaskets. After he mentioned it I do recall seeing some difference in the exhaust ports on one side of the motor while I had the exhaust manifold off. When I looked inside the exhaust port at the exhaust valves, they looked a little like they had had water on them at one time. I just replaced an exhaust manifold on one side of the motor while I had it out. The old one had some cracks starting to form in the center, so I changed it while I was in there, but I don't think it had started to leak internally or anything, just dripping water on the outside of the manifold.

NHGUY said "Maybe you ought to check to see if the valve cover to carb hoses or fittings are leaking crankcase gases." I believe this is the problem. I failed to mention this is an Inboard Outboard if it matters, I have also posted some pictures of the flame arrestor and pcv set-up or lack of one. Does this look like everything is there that is supposed to be, or should there be a shroud over the top of the flame arrestor. The later models have an additional shroud.

Also I am wondering if using Castrol motor oil 10W40 in it could be any part of the probem. Thanks for everyone's input. I expect to dump it in the lake tomorrow if I can get it going. I'll know more then. Thanks again.

 

[littlebookworm]

Re: Stinking Mercruiser 260

Are you working with a service manual for your engine? There are a number of websites which will give you a free download of the correct manual. I think you need to get it. It will give you a wiring diagram for the engine and the fuel and vent set-up. The PCV hoses have no valve, just open hoses to the flame arrestor. I'm almost positive
10W40 is the wrong oil for that engine. I believe you should be running straight 30 oil. Check the manual. If you're running in fresh water you shouldn't have a problem with automotive head gaskets. Just be certain your electricals and fuel components are all marine rated. The odor might be coming from the steering fluid burning off. It can take quite a while for that to finish. I definitely agree that you should check your battery and alternator. Just bring them into an auto parts place to check them out. It's usually done for free. Draw a diagram or mark the alternator wires before you remove them. Check your oil filler cap. Make sure you have the correct one for your engine. Some are vented and some are not. Hope this helps. Hy

 

[1981century]

Re: Stinking Mercruiser 260

Yeah the manual I have, may not be the best it is a "Clymer" manual for mercruiser and it covers several years. It kind of paints it with a broad brush. I bought the boat and the manual back in 1990...way before I had access to the internet. Who's manual do you recommend? Remember this is a 1981 model. You're right about the straight 30 oil. I will change it out. The steering fluid was not there prior to this motor pull, that was some of my handywork while having the hoses off and moving the gimbal housing back and forth while I painted it. I will have to gunk it up and flush out the hull real good before I crank it up again. In fact it will take some time for everything to kind of get burnt back in since I have a brand new exhaust manifold too. The last time I got a new one of those it smelled of paint until it got "broke in". Thanks all for your help. I'll be back with feedback soon!

 

[fmalott]

Re: Stinking Mercruiser 260

it might be the battery on my 470 merc i bought the boat with the old voltage regulator still being used the owner told me that the regulator was not working it was over charging the battery when he fired up the boat after about 30 seconds man did the battery smell just like you smelt so i would look at battery
p.s. when it was being over charged it was not spilling over but it was bubbling so don't go by that