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Author Topic:   Reliability of Mercury Oil Injection

AZdave posted 03-04-2010 11:48 PM ET (US)
I am faced with a decision on whether to repair the oil injection system on my Mercury 135-HP two-cycle to the tune of some hundreds of dollars, or to block off the system and go to pre-mixing oil in the gasoline. I realize that those with an unhappy history are more likely to write. I need good data points, and several of them. If I get this fixed, am I likely to have several years of uneventful oil injection?
I know that this has been discussed before, and I have used the search function. Thanks. Dave
jimh posted 03-05-2010 08:06 AM ET (US)
What component in your Mercury two-cycle outboard has failed and needs replacement?
AZdave posted 03-05-2010 10:41 AM ET (US)
The sensor that sits on top of the oil metering unit is missing, and my tech tells me that the alarm module is defective as well.
L H G posted 03-05-2010 04:11 PM ET (US)
A missing sensor, removed by someone, is not a failure, just stupidity.
On the older 2.0 liter Mercs they seem to have had a bad run of "black box" alarm modules in the late 80's. Get it replaced and your problems will be over. I have a 2.0 liter, a 1988 model, and had to replace my module also, at about 700 hours. At the same time I had them replace all oil and fuel hoses, cheap insurance in this day of ethanol.
I presently own 3 V-6 Mercs, (including the one mentioned) and previously owned 2 other V-6 200HP Mercs, 1988-1997 model years, representing a total of 9500 hours of running time. Other than that one alarm module problem, all five work and oil perfectly. The alarm module malfunction does not affect engine oiling, incidentally. It's only a sound nusiance problem. Mercury's oil injection system is excellent, one of the best, unless you are consistently running the engine over 6500 RPM in racing conditions. The recreational engine drive gear was not designed for high RPM racing use.
Diabling any engine's oil injections system tells a prespective buyer of previous problems, some that could have even damaged the powerhead. I would NEVER buy a used engine where oil has been disconnected, particularly an OMC. So doing it, will kill the value of your engine/rig. Don't do it! Why do it, and use oil at 50 to 1, when the injection system uses it between 100 to 1 at idle, and never greater than 50 to 1 wide open?
sosmerc posted 03-07-2010 08:13 PM ET (US)
Merc's mechanical oil injection system has been used on the V-6 engines since 1984. I have been repairing Mercs since 1979. I have seen enough failures on this system to warrant the usual discussion of "removal" with my customers that have it. Many of the problems that I have seen with the system could have been prevented if annual service and inspection were part of the owner's routine.
The clear plastic hoses that carry oil from the engine mounted reserve tank to the oil pump can crack with age, along with the clear line than transport the oil from the oil pump to the fuel pump. I routinely replace the lines when I am servicing a V6.
Oil warning modules fail from time to time as well as warning horns.
The infamous nylon oil pump drive gears will sometimes shred or strip out.
The oil pump assembly itself has a seal that needs to be replaced periodically.
So, yes, the system often needs attention to keep it going properly. Oil injection on these engines is a convenience feature. Who wants to measure out and pour oil into a very large built in fuel tank? Some folks consider that a BIG hassle. But if your boat has a smaller fuel tank and you don't really consider it a problem to mix your own oil/gas then I say I'd rather have the peace of mind knowing the oil is in the gas.
On the later model V6 engines with this mechanical system, Mercury removed the motion sensor. My guess is they did this because the plastic bearing that contained the magnet that the motion sensor monitored was creating some problems on its own. In some cases the nylon "bearing" would swell, and this put an additional load on the nylon drive gear that is on the crank. The nylon bearing was replaced with a brass bearing.
When I visually inspect the nylon drive gear on the crank (by removing the oil pump so I can get an inspection light down the hole), if I seen any visual indication of the drive gear starting to show signs of wear or trauma I recommend removing the oil injection system and mixing manually to prevent a potential catastrophic problem.
The V6 Mercs now use an electronic oil pump that is controlled and monitored by the engine computer. They seem to be VERY reliable and a much superior system.
Newtauk1 posted 03-07-2010 10:22 PM ET (US)
Do these problems exist in the mid-range motors such as 75 and 90-HP two-cycle engines?
sosmerc posted 03-08-2010 12:52 AM ET (US)
Even though the other carburetor two-cycle Mercury engines with oil injection have a similar mechanical design, it has proven to be somewhat more reliable. You do still need to inspect and replace the clear oil lines when they start to get old, and the oil tank caps tend to leak from being over-tightened.
The oil warning modules seem to fail far too often, but rarely do the occasional problems cause engine damage. I have not felt the need to remove these systems, though I did on a pair of 90hp L3's that I owned years back.
jimh posted 03-09-2010 12:31 AM ET (US)
Again, many thanks to sosmerc for his contribution of first-hand experience with the repair and service of the oil mixing system on Mercury two-cycle engines. With over thirty years of hands-on experience in working on the oil system, I am confident you have become well acquainted with its tendencies.
Also, thanks for the mention that the old plastic gear driven pump used to mix the oil has now been abandoned by Mercury, and the newer Mercury motors have changed to an electrically operated pump. That is a significant change in the system design. Do you know about what model year the change to an electric pump for oil mixing was made?
jimh posted 03-09-2010 12:43 AM ET (US)
Problems with the alarm module on older Mercury engines seem to be quite frequently reported. I don't have an older Mercury engine myself, but among three friends that do, all three have had to replace the alarm module.
AZdave posted 03-09-2010 09:27 AM ET (US)
Yes, thanks to you all. There is no resource to compare with first hand experience. I will also link back to http://continuouswave.com/ubb/Forum3/HTML/010414.html which has a very large number of anecdotal data points. Dave

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Author	Topic: Reliability of Mercury Oil Injection
AZdave	posted 03-04-2010 11:48 PM ET (US) I am faced with a decision on whether to repair the oil injection system on my Mercury 135-HP two-cycle to the tune of some hundreds of dollars, or to block off the system and go to pre-mixing oil in the gasoline. I realize that those with an unhappy history are more likely to write. I need good data points, and several of them. If I get this fixed, am I likely to have several years of uneventful oil injection? I know that this has been discussed before, and I have used the search function. Thanks. Dave
jimh	posted 03-05-2010 08:06 AM ET (US) What component in your Mercury two-cycle outboard has failed and needs replacement?
AZdave	posted 03-05-2010 10:41 AM ET (US) The sensor that sits on top of the oil metering unit is missing, and my tech tells me that the alarm module is defective as well.
L H G	posted 03-05-2010 04:11 PM ET (US) A missing sensor, removed by someone, is not a failure, just stupidity. On the older 2.0 liter Mercs they seem to have had a bad run of "black box" alarm modules in the late 80's. Get it replaced and your problems will be over. I have a 2.0 liter, a 1988 model, and had to replace my module also, at about 700 hours. At the same time I had them replace all oil and fuel hoses, cheap insurance in this day of ethanol. I presently own 3 V-6 Mercs, (including the one mentioned) and previously owned 2 other V-6 200HP Mercs, 1988-1997 model years, representing a total of 9500 hours of running time. Other than that one alarm module problem, all five work and oil perfectly. The alarm module malfunction does not affect engine oiling, incidentally. It's only a sound nusiance problem. Mercury's oil injection system is excellent, one of the best, unless you are consistently running the engine over 6500 RPM in racing conditions. The recreational engine drive gear was not designed for high RPM racing use. Diabling any engine's oil injections system tells a prespective buyer of previous problems, some that could have even damaged the powerhead. I would NEVER buy a used engine where oil has been disconnected, particularly an OMC. So doing it, will kill the value of your engine/rig. Don't do it! Why do it, and use oil at 50 to 1, when the injection system uses it between 100 to 1 at idle, and never greater than 50 to 1 wide open?
sosmerc	posted 03-07-2010 08:13 PM ET (US) Merc's mechanical oil injection system has been used on the V-6 engines since 1984. I have been repairing Mercs since 1979. I have seen enough failures on this system to warrant the usual discussion of "removal" with my customers that have it. Many of the problems that I have seen with the system could have been prevented if annual service and inspection were part of the owner's routine. The clear plastic hoses that carry oil from the engine mounted reserve tank to the oil pump can crack with age, along with the clear line than transport the oil from the oil pump to the fuel pump. I routinely replace the lines when I am servicing a V6. Oil warning modules fail from time to time as well as warning horns. The infamous nylon oil pump drive gears will sometimes shred or strip out. The oil pump assembly itself has a seal that needs to be replaced periodically. So, yes, the system often needs attention to keep it going properly. Oil injection on these engines is a convenience feature. Who wants to measure out and pour oil into a very large built in fuel tank? Some folks consider that a BIG hassle. But if your boat has a smaller fuel tank and you don't really consider it a problem to mix your own oil/gas then I say I'd rather have the peace of mind knowing the oil is in the gas. On the later model V6 engines with this mechanical system, Mercury removed the motion sensor. My guess is they did this because the plastic bearing that contained the magnet that the motion sensor monitored was creating some problems on its own. In some cases the nylon "bearing" would swell, and this put an additional load on the nylon drive gear that is on the crank. The nylon bearing was replaced with a brass bearing. When I visually inspect the nylon drive gear on the crank (by removing the oil pump so I can get an inspection light down the hole), if I seen any visual indication of the drive gear starting to show signs of wear or trauma I recommend removing the oil injection system and mixing manually to prevent a potential catastrophic problem. The V6 Mercs now use an electronic oil pump that is controlled and monitored by the engine computer. They seem to be VERY reliable and a much superior system.
Newtauk1	posted 03-07-2010 10:22 PM ET (US) Do these problems exist in the mid-range motors such as 75 and 90-HP two-cycle engines?
sosmerc	posted 03-08-2010 12:52 AM ET (US) Even though the other carburetor two-cycle Mercury engines with oil injection have a similar mechanical design, it has proven to be somewhat more reliable. You do still need to inspect and replace the clear oil lines when they start to get old, and the oil tank caps tend to leak from being over-tightened. The oil warning modules seem to fail far too often, but rarely do the occasional problems cause engine damage. I have not felt the need to remove these systems, though I did on a pair of 90hp L3's that I owned years back.
jimh	posted 03-09-2010 12:31 AM ET (US) Again, many thanks to sosmerc for his contribution of first-hand experience with the repair and service of the oil mixing system on Mercury two-cycle engines. With over thirty years of hands-on experience in working on the oil system, I am confident you have become well acquainted with its tendencies. Also, thanks for the mention that the old plastic gear driven pump used to mix the oil has now been abandoned by Mercury, and the newer Mercury motors have changed to an electrically operated pump. That is a significant change in the system design. Do you know about what model year the change to an electric pump for oil mixing was made?
jimh	posted 03-09-2010 12:43 AM ET (US) Problems with the alarm module on older Mercury engines seem to be quite frequently reported. I don't have an older Mercury engine myself, but among three friends that do, all three have had to replace the alarm module.
AZdave	posted 03-09-2010 09:27 AM ET (US) Yes, thanks to you all. There is no resource to compare with first hand experience. I will also link back to http://continuouswave.com/ubb/Forum3/HTML/010414.html which has a very large number of anecdotal data points. Dave