posted 07-16-2011 10:12 PM ET (US)        

So at this point I am aware of 2 options...pull the oil pump, install the plug and convert to premix or the electric oil pump retrofit. The electric retrofit is $400-$450. I understand that premixing is inconvenient (on average I use 150 gallons of fuel a year, most of which is purchased in 5 gallon carboys at the regular gas station...so I don't see mixing as a big deal). I also understand that premixing 50:1 is for full throttle so I will be using more oil vs the electric pump which will vary the oil based on speed. If we decide to premix, how do I convert. The tank is 87 gallons and is half full. Do I run off a premix remote tank, add the correct oil to get 87 gallons of gas to 50:1, take the boat out on the remote premix tank to slosh the oil around in the main tank and then keep adding gas to the tank until it is full...my issue is that I don't know exactly how many gallons are currently in the tank...I only know its capacity.

Any advice would be appreciated!

Thanks,

posted 07-16-2011 10:38 PM ET (US)            

I'd do a compression test anyway - especially after a major lubrication failure - but I bet it's fine. The Mercury system was engineered very well - so much so that if it's like the one on my old Black Max, the only failure mode (barring passage blockage or broken oil lines) is that if the pump stops working, the oil in the powerhead mounted tank will "gravity feed" into the fuel and provide a measure of protection for as long as that tank has oil.

I'm not as familiar with the 175 Offshore, but I assume that the engineers didn't change too much in terms of redundant/fail safe features.

posted 07-16-2011 11:03 PM ET (US)            

I disagree with Dave's analysis about the failsafe mode. The oil mixing pump is driven by the plastic gear. If there is no drive shaft input motion to the oil mixing pump, I don't think you get much oil mixing. Dave is correct that there is a gravity feed of oil to the pump. I don't know how much oil can flow through the mixing pump from the gravity feed when there is no motion of the pump drive shaft.

I think the engine was saved by the low speed operation following the warning horn. Limiting engine speed to 1,200-RPM was a good idea.

Did you ask your relative what engine speed was being used prior to the failure? I have heard that the plastic drive gear in the Mercury oil mixing system tends to fail when the engine speeds are quite high. For old-timers, who like to loaf along at 2,500-RPM on their Mercury engines, the life of the plastic gear can be extended. But revving to 6,000-RPM might cause the plastic gear to to fail.

posted 07-16-2011 11:13 PM ET (US)            

I do not know if the 175 Offshore motor has this powerhead-mounted reservoir or not. If it does, it is very likely that the engine never really suffered a "no oil" situation.

posted 07-17-2011 08:30 AM ET (US)            

If the plastic drive gear on the input shaft to the oil mixing pump fails, the oil mixing pump stops operating. The pictorial-schematic diagram that Mercury shows for the system illustrates that the oil mixing pump inlet is lower than the oil mixing pump outlet. This is reasonable, because the system is designed to allow the oil mixing pump to meter the flow of oil from the under-cowling reservoir tank to the fuel system. The output of the oil mixing pump is just Tee-ed into the fuel hose upstream of the fuel pump inlet.

I don't see how gravity would be able to push the oil through the oil mixing pump if it was not being rotated by its input shaft driven from the crankshaft via the plastic gear. Perhaps the pump is ingeniously designed so that when it stops working it still allows some oil to pass through it and go on to the fuel pump.

posted 07-17-2011 07:26 PM ET (US)            

Now what to do premix or electric oil pump. Once a year we have a Fresh Air Kid from NY City and he loves the boat...he gets here on Thursday...so If I am going the pump route I have to order it Monday without the benefit of doing a compression test. What do you think I should do???

Thanks,

posted 07-17-2011 08:12 PM ET (US)            

Jim is correct about how the oil injection system works; the reserve reservoir will supply oil to the motor only *if* the oil injection pump is still working.

Regardless of how seavard's motor survived the failure of the injection pump (and two stroke's are much more robust and tolerant of abuse than most folks give them credit for), it is what it is. Given that only 150 gallons of fuel are used in a season, premixing is the obvious and not very burdensome solution to the dilemma.

To add oil to an existing quantity of fuel in the tank, premix the oil with a small amount of gas, say five gallons, then add the mixture to the tank.

posted 07-18-2011 07:52 AM ET (US)            

L H G tells a story of his fuel system becoming clogged with oil in the fuel lines after a marina gas dock refused to allow him to be on the boat during fueling, and he could not mix the oil with the gasoline as it was being added to the tank.

posted 07-18-2011 10:22 AM ET (US)            

There is no question that adding gasoline and oil simultaneously to a funnel with a fine screen is the best way to ensure proper mixing, but with a 140 gallon tank, I will never got to that extreme. Oil mixes with gasoline very easily and adding all the oil to five or ten gallons of gas and mixing that will do a very thorough job of creating a tank of premix.

As to the dilemma of how to calculate the proper amount of oil to add to a partially full tank, that is easy for anybody who has a fuel flow meter. My FloScan tells me exactly how much fuel my fuel tank can take be keeping track of how much has been used since the last fill up.

I filled my tank a couple weeks ago and the FloScan indicated I had used 124.7 gallons since I reset it. The gas station fuel pump indicated I put 125.01 gallons in the tank. In six years of ownership, that FloScan has never been off by as much as even one percent.

If I had had to premix, I would have needed 2.5 gallons of oil added to the gasoline. In that case, I would have added 1.25 gallons to two five gallon gas cans, filled them with gas, agitated them and then added those to the boat's tank half way though the fueling process.

I would also not care too much if I ended up with a 55:1 ratio or a 45:1 ratio. I might lament any wasted expense on extra oil used, but neither of my motors would care one iota. Given seaverd's description of his use, he will be just fine with premixing.

posted 07-18-2011 02:27 PM ET (US)            

Your engine, a later model 2.5-liter, should have a motion detector for the gear-driven oil pump for detecting failures of the plastic gear. This would give you instant information of the failure of the gear.

By limiting your RPM you most likely did no harm to the engine, as the fuel lubricates the cylinders more than oil at lower RPM. I doubt much if any oil got pass the oil driven pump during your failure.

posted 07-24-2011 02:21 PM ET (US)            

"I have a much more modern two-cycle motor that actually has an oil injection system; it has a pulsing oil pump and a oil distribution manifold that injects oil in about a dozen locations on the engine block."

The engine is a naturally air pump, there is no external air pump or a second pump. The Mercury system takes advantage of the natural characteristic of the engine. It is the best design on the market for transfering oil from the main tank to the engine mounted tank. There is not two pumps. What would you count as the parts of the pump? A check valve? The whole pump is made up a check valve?

You have to give credit to the design.

Just trying to keep everything fair and pass correct information. There is only one pump. ;)

posted 07-24-2011 04:10 PM ET (US)            

In more modern motors that actually have an oil injection system, not the simple little Tee-connector on the fuel hose, the oil is actually injected at various places around the engine.

As I said, Mercury calling their simple oil mixing system an oil injection system was a poor choice of words.

Fuel injection is also misleading, at least in the sense that Mercury and other have used it. They really do not inject fuel in a cylinder at all, they just spray fuel into the air path, just like a carburetor would do. I suppose they have some sort of injector doing the spraying, but it just sprays the fuel into the air path upstream of the cylinder. Not much injection going on.

We do seem to have a lot of interjection going on in this discussion.

I wrote a long article that compares the Mercury oil mixing system with the Evinrude oil mixing system. If you want to see my opinion of each system and the components in each system, just read that article.

Automatic Oiling System: OMC/Bombardier versus Mercury
http://continuouswave.com/whaler/reference/OMCvMERC.html