1994 Merc 225 - 2-stroke oil pressure problem?

[Frank O]

Hi all, I have a 1994 Mercury Offshore 3.0-Liter 225-H.P. carbureted 2-stroke outboard on my 21' Boston Whaler. There is a reservoir for 2-stroke oil under the boat's console; air pressure from the engine's crankcase flows into this reservoir via one hose, causing 2-stroke oil to flow out to the engine through a second hose.

Recently I've noticed a couple of problems that I suspect are related to pressure buildup in this system. First, oil has been leaking from the hose at the top of the reservoir and getting on the floor of the console, even though the hose appears to be fairly tightly connected and is in good shape. Second, I found 2-stroke oil had gotten into the plumbing around the primer bulb and water-fuel separator filter. This isn't coming from the gas tank (the gas there is clean), so the only way I can imagine it's getting there is that pressure in the 2-stroke oil system is pushing the oil through the oil injection pump within the engine and then it's exiting out the fuel inlet. Both of these symptoms suggest to me that there must be fairly significant pressure remaining in the 2-stroke system after we turn the engine off for the day.

As a temporary fix, I can loosen one of the filler caps on the oil reservoir under the console when we finish running in order to let off pressure. However, I'd like to figure out what's going on and fix it. Does this scenario suggest a failure of a particular component? Maybe a bad check valve somewhere? Thanks for any ideas.

 

[gss036]

Re: 1994 Merc 225 - 2-stroke oil pressure problem?

There are 4 or 5, 2psi check valves in the system. If you do a search here you should be able to find an oil schematic. One was posted within the past couple of weeks, so should show up in the search funtion.
I would suspect the one on the lower starboard side of the engine block. This is were the engine injects pressure into the oil tank. After the engine is shut down it is supposed to relieve the pressure after a short time, it fails more than the others. There also is one under the engine oil tank, (ant-siphon) two in the fuel lines leading from the oil/fuel pump. They keep the oil from flowing backwards in the system, one keeps the fuel oil seperated until it is pushed into the carb feed lines.
Hopefuly you can figure those out. The valves are under $20 each and easy to replace.
It would be nice if you had an OEM manual to follow.

http://forums.iboats.com/showthread.php?p=1572267&highlight=oil+check+valve#post1572267

Read this post on the above link and then click on the Whaler link the Kiener posted, it will take you to the diagraham.

 

[Frank O]

Re: 1994 Merc 225 - 2-stroke oil pressure problem?

Thanks much for the links. That does seem to point to the crankcase check valve (the one that supplies air pressure to the oil system's remote tank) as the likely culprit. I was particularly interested in this comment by hkeiner discussing his problem of about a year ago:

"[A Mercury mechanic] explained that the oil pump (when stopped) will sufficiently prevent any meaningful seepage of oil towards the fuel pump or fuel tank as long as the oil tank's air pressure is not higher than design and the pressure does not persist longer than design. If the crankcase valve is defective, however, then the pressure may be too high or persist too long and could cause too much oil to seep through the oil pump after motor shutdown."

I actually do have a Mercury Service Manual for my engine. It has an oil system diagram similar to the one on continuouswave.com that the thread you mentioned linked to. This diagram just labels it as "Crankcase pressure with one-way check valve" and has no further discussion of its location, how to change it, etc. I haven't picked my way through the section on the powerhead yet to see if there is more about it there. You mention that it's on the lower starboard side of the engine block. Does the valve screw into the engine block, or is it an inline valve in a hose? Thanks again.

 

[hkeiner]

Re: 1994 Merc 225 - 2-stroke oil pressure problem?

I haven't picked my way through the section on the powerhead yet to see if there is more about it there. You mention that it's on the lower starboard side of the engine block. Does the valve screw into the engine block, or is it an inline valve in a hose?

I found that the Mercury shop manual is surprisingly quite silent on the function of the crankcase check valve. I found the that the best source for information is right here on this forum. Do a search on "check valve" and you will find several posts with good information.

In brief, the check valve is screwed into the crankcase near the bottom of the starter. An air hose is connected to the valve, which then goes to the remote oil tank. You can't miss the valve if you follow the air hose from the remote oil tank towards the motor. The air hose connects to the valve via a barb on the valve and the hose is then secured to the valve with a tie strap. Use the special Mercury tie straps, not regular tie straps you might find at an auto parts store or similar. The threads on the new replacement valve will have a dry coating of thread locker on it. So all you need to do is screw the valve in and tighten. No additional sealant or thread locker is needed.

 

[Frank O]

Re: 1994 Merc 225 - 2-stroke oil pressure problem?

Thanks much -- I was going to ask whether it took Loctite, but you even answered that. Sounds like it should be a straightforward fix.

 

[hkeiner]

Re: 1994 Merc 225 - 2-stroke oil pressure problem?

If replacing the crankcase check valve does not solve your problem, there is another check valve on the oil injection system that, if stuck open, can allow oil to be pushed/siphoned into the fuel line towards the carbs and/or the primer bulb. This check valve is located (in-line) between the oil pump and the fuel pump. This valve is less likely to be the cause of your symptoms, but it may be worth replacing if replacing the crankcase check valve didn't do the trick. This part is not expensive, it is easy to replace, and it is not very easy or practical to bench test for proper operation. It is labeled as #6 on the diagram linked below.

http://continuouswave.com/whaler/reference/mercuryOilInjection.html

 

[Frank O]

Re: 1994 Merc 225 - 2-stroke oil pressure problem?

Thanks. I can certainly check out the other check valve, though I'm pretty sure that the crankcase check valve must be suspect because I specifically noticed the pressure in the remote oil tank persisting long after the engine was shut down.

Also I took a look through some photos I took of the outboard when I had it opened up once, and could pretty easily locate what must be the crankcase check valve:

http://www.inkbox.net/whaler/225/crankcase-checkvalve.jpg

 

[hkeiner]

Re: 1994 Merc 225 - 2-stroke oil pressure problem?

Yes, the valve with the red arrow pointing to it is the crankcase check valve. Please note that the new replacement valve you buy will be smaller in size than the one you are replacing. When I bought a replacement valve for my motor, I saw that it was smaller than the original and the parts guy explained that Mercury redesigned the valve since the 1990's. I only mention this because you might think you got the wrong part when you are replacing the valve and see the difference is size.

http://forums.iboats.com/showthread.php?t=224687

 

[Frank O]

Re: 1994 Merc 225 - 2-stroke oil pressure problem?

I got the check valves changed out today, which hopefully resolves the problem. In addition to the valve that was the probable failure point (screw-in valve on the crankcase where the hose to the remote oil tank attaches), Mercury also suggested to my local dealer that I also replace the in-line check valve between the oil pump and the fuel pump, as well as a T-valve on the hose hanging off the oil reservoir located on the outboard. These were all easy to do.

Use the special Mercury tie straps, not regular tie straps you might find at an auto parts store or similar.

My local dealer didn't have any of the Merc straps, and just provided me with some small ziptie-type straps that look undersized. Do you know a part number for the Merc tie straps? I couldn't find them on the Mercury Parts Express website searching under "tie straps," "straps," "tie," etc.

 

[gss036]

Re: 1994 Merc 225 - 2-stroke oil pressure problem?

That should fix your problem, BUT!!!!!!!!!!!! make sure you squeeze the ties down tight until you can get the right ones. Other wise use small metal clamps. There is not much pressure the so the regular ties might work ok. You might try pulling the line off after you put the clamps on. The real problem here is that there is no alarm to tell you if the lines start leaking or pop off. hen of coure you will get an alarm then, but way toooooo late.

 

[Robj]

Re: 1994 Merc 225 - 2-stroke oil pressure problem?

Just go to your local auto parts supplier and get bigger tie wraps. That should be an easy fix.

Have a great day,

Rob.