1998 Mercury OptiMax 135; Low Compression on One Cylinder

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Author Topic:   1998 Mercury OptiMax 135; Low Compression on One Cylinder

blepisto posted 10-25-2008 02:06 PM ET (US)
I recently bought a 1998 Dauntless with a Mercury OptiMax 135. I had starting problems and replaced both fuel pumps. Now my mechanic indicates that one cylinder has low compression. As this just happened and the engine still starts easily and has very low hours (250), he suggests we simply remove the cylinder head on the one side and check or repair that piston with rings to get the compression back up. What normally causes only one piston to lose good compression when are others are perfect?
Any thoughts on doing this or should I simply toss in the towel and re-power with another engine? Thank you!
jimh posted 10-26-2008 08:22 AM ET (US)
If the engine has a removable cylinder head, the loss of compression could be due to a failure of the gasket between the block and the cylinder head.
Cylinders usually lose compression due to damage to the piston rings and the cylinder walls. The damage is usually caused by lack of lubrication. In a two-cycle motor the lubrication is often carried with the fuel. If a cylinder is starved for fuel it is also starved for lubrication. A cylinder running lean on fuel also tends to become hotter due to the lean fuel-air ratio. The lean condition creates a need for more lubrication while at the same time providing less.
The decision to purchase a new engine instead of repairing an existing one is hard to assess and make a recommendation. Generally repairing an existing engine is less expensive than buying a new engine, but it is difficult to know at what point repairs won't be cost effective.
Tohsgib posted 10-27-2008 11:04 AM ET (US)
I don't know of any modern outboard that you can repair the rings without a total teardown. You would be looking at roughly $3k to rebuild that engine and in my opinion is not worth it. Place by me has a new 2007 Suzuki 150 4 stroke for $7595.
Blackduck posted 10-27-2008 11:31 AM ET (US)
How low is the compression in this bad cylinder? With only 250 hours, I might be tempted to do my own compression test or get another repair shop's opinion. If the cylinder is bad, I would replace the engine, not rebuild.
phatwhaler posted 10-27-2008 01:08 PM ET (US)
My Mercury factory repair manual indicates that it's normal procedure to only repair a single cylinder if the damage isn't too bad. Evidently a larger piston can be installed that weighs the same as the old smaller piston and that the engine will run fine. If your mechanic intends to only repair that one cylinder I think it can be done for much less than 3gs. If the engine is otherwise in nice condition, I would repair the motor. An Optimax has the main feature that most people are repowering for these days, fuel economy. Sure a new four stroke would be quiet and smoother, however just the tax on a repower will pay for a large chunk of your repair.
I think your mechanic could probably repair one cylinder with a new piston and rings, gaskets for maybe 1200 bucks.
Tohsgib posted 10-27-2008 01:48 PM ET (US)
It is not the parts, it is the labor, especially if you have to bore or sleave a cylinder.
pglein posted 11-03-2008 07:25 PM ET (US)
Low compression in one cylinder is generally caused by one of three things:
1. Damaged or blown head gasket. This would usually lead to zero compression on that cylinder. But it's also the cheapest and easiest to assess and, if necessary, repair. It can generally be done by your average Joe in his driveway in about 30 minutes, unless the head or block surface needs to be machined, which can get more complicated.
2. Scored cylinder wall. This can be assessed when the head is removed at no additional expense or effort. However, repair is much more costly. It is not, however, anywhere near as expensive as replacing the engine. I spent about $2,000 on a complete rebuild. Aside from machining, I did all the labor myself. It took me about three man-days of work to tear it down and rebuild it. It's not something just anyone can do, but if you've got a buddy who's done some engine work, you might be able to enlist his help. Otherwise, figure another $1,000 for your neighborhood shade-tree mechanic to do the whole thing. The machinist who did my heads and block (and through whom I ordered my parts) informed me after the fact that he would have assembled the block for me for $300. Although I was able to do it myself, I wished I'd paid him. For that price, it simply would have been worth it, both for my time and peace of mind.
3. Broken or worn piston ring. Usually a worn piston ring goes hand in hand with a scored cylinder wall. It's a sort of chicken/egg thing. But rings can break without damaging the cylinder wall, causing a small loss of compression. Count yourself very luck if this is the case, and get it repaired right away. You will only have to remove the block and pull the one damaged piston. Still a lot of work, but a lot less than a complete rebuild.
Repairing just one piston/cylinder can be done, however, these failures usually only happen on older motors, or motors that have experienced a lack of lubrication or cooling. So, knowing that, most people opt to rebuild the entire engine, since it's not a whole lot of extra expense and zero extra effort once you've got the block disassembled for machining. And, truth be told, since you need to remove all pistons for machining, it's too risky to attempt to put the old pistons, rings and bearings back in. If you swap one part for another on accident, you can have a problem, since not all parts will wear evenly, and you open up the opportunity for a compression leak or increased wear on a bearing. It can be VERY difficult to keep track of where each and every part went, and which way it was oriented. They all look the same, but the microscopic wear patterns are very different indeed.
If you indeed find a scored cylinder, on that new of a motor I would rebuild. But I would rebuild the whole thing, not just one; for the reasons above, plus the possibility that one or all of the other cylinders are also worn, and are just barely still making compression.
You should not need to re-sleeve unless the engine has been rebuilt and bored out multiple times before. If you do, you will then want to weigh the cost of a new powerhead vs. a whole new motor.
pglein posted 11-03-2008 07:30 PM ET (US)
Word of caution. It takes special skills to machine a 2 stroke block. Machinists with these skills are becoming very hard to come by. But they are out there. When finding someone to do this, make sure they have experience machining outboard motors. The guy I found was an insurance agent who, in a previous life, had been the primary engine rebuilder for a number of local dealerships, but due to the switch to 4 strokes, had found that it no longer made ends meet. However, he was more than happy to take on the task of machining my engine for a few extra bucks on the side. His work was top notch and he gave me much valuable guidance.

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Author	Topic: 1998 Mercury OptiMax 135; Low Compression on One Cylinder
blepisto	posted 10-25-2008 02:06 PM ET (US) I recently bought a 1998 Dauntless with a Mercury OptiMax 135. I had starting problems and replaced both fuel pumps. Now my mechanic indicates that one cylinder has low compression. As this just happened and the engine still starts easily and has very low hours (250), he suggests we simply remove the cylinder head on the one side and check or repair that piston with rings to get the compression back up. What normally causes only one piston to lose good compression when are others are perfect? Any thoughts on doing this or should I simply toss in the towel and re-power with another engine? Thank you!
jimh	posted 10-26-2008 08:22 AM ET (US) If the engine has a removable cylinder head, the loss of compression could be due to a failure of the gasket between the block and the cylinder head. Cylinders usually lose compression due to damage to the piston rings and the cylinder walls. The damage is usually caused by lack of lubrication. In a two-cycle motor the lubrication is often carried with the fuel. If a cylinder is starved for fuel it is also starved for lubrication. A cylinder running lean on fuel also tends to become hotter due to the lean fuel-air ratio. The lean condition creates a need for more lubrication while at the same time providing less. The decision to purchase a new engine instead of repairing an existing one is hard to assess and make a recommendation. Generally repairing an existing engine is less expensive than buying a new engine, but it is difficult to know at what point repairs won't be cost effective.
Tohsgib	posted 10-27-2008 11:04 AM ET (US) I don't know of any modern outboard that you can repair the rings without a total teardown. You would be looking at roughly $3k to rebuild that engine and in my opinion is not worth it. Place by me has a new 2007 Suzuki 150 4 stroke for $7595.
Blackduck	posted 10-27-2008 11:31 AM ET (US) How low is the compression in this bad cylinder? With only 250 hours, I might be tempted to do my own compression test or get another repair shop's opinion. If the cylinder is bad, I would replace the engine, not rebuild.
phatwhaler	posted 10-27-2008 01:08 PM ET (US) My Mercury factory repair manual indicates that it's normal procedure to only repair a single cylinder if the damage isn't too bad. Evidently a larger piston can be installed that weighs the same as the old smaller piston and that the engine will run fine. If your mechanic intends to only repair that one cylinder I think it can be done for much less than 3gs. If the engine is otherwise in nice condition, I would repair the motor. An Optimax has the main feature that most people are repowering for these days, fuel economy. Sure a new four stroke would be quiet and smoother, however just the tax on a repower will pay for a large chunk of your repair. I think your mechanic could probably repair one cylinder with a new piston and rings, gaskets for maybe 1200 bucks.
Tohsgib	posted 10-27-2008 01:48 PM ET (US) It is not the parts, it is the labor, especially if you have to bore or sleave a cylinder.
pglein	posted 11-03-2008 07:25 PM ET (US) Low compression in one cylinder is generally caused by one of three things: 1. Damaged or blown head gasket. This would usually lead to zero compression on that cylinder. But it's also the cheapest and easiest to assess and, if necessary, repair. It can generally be done by your average Joe in his driveway in about 30 minutes, unless the head or block surface needs to be machined, which can get more complicated. 2. Scored cylinder wall. This can be assessed when the head is removed at no additional expense or effort. However, repair is much more costly. It is not, however, anywhere near as expensive as replacing the engine. I spent about $2,000 on a complete rebuild. Aside from machining, I did all the labor myself. It took me about three man-days of work to tear it down and rebuild it. It's not something just anyone can do, but if you've got a buddy who's done some engine work, you might be able to enlist his help. Otherwise, figure another $1,000 for your neighborhood shade-tree mechanic to do the whole thing. The machinist who did my heads and block (and through whom I ordered my parts) informed me after the fact that he would have assembled the block for me for $300. Although I was able to do it myself, I wished I'd paid him. For that price, it simply would have been worth it, both for my time and peace of mind. 3. Broken or worn piston ring. Usually a worn piston ring goes hand in hand with a scored cylinder wall. It's a sort of chicken/egg thing. But rings can break without damaging the cylinder wall, causing a small loss of compression. Count yourself very luck if this is the case, and get it repaired right away. You will only have to remove the block and pull the one damaged piston. Still a lot of work, but a lot less than a complete rebuild. Repairing just one piston/cylinder can be done, however, these failures usually only happen on older motors, or motors that have experienced a lack of lubrication or cooling. So, knowing that, most people opt to rebuild the entire engine, since it's not a whole lot of extra expense and zero extra effort once you've got the block disassembled for machining. And, truth be told, since you need to remove all pistons for machining, it's too risky to attempt to put the old pistons, rings and bearings back in. If you swap one part for another on accident, you can have a problem, since not all parts will wear evenly, and you open up the opportunity for a compression leak or increased wear on a bearing. It can be VERY difficult to keep track of where each and every part went, and which way it was oriented. They all look the same, but the microscopic wear patterns are very different indeed. If you indeed find a scored cylinder, on that new of a motor I would rebuild. But I would rebuild the whole thing, not just one; for the reasons above, plus the possibility that one or all of the other cylinders are also worn, and are just barely still making compression. You should not need to re-sleeve unless the engine has been rebuilt and bored out multiple times before. If you do, you will then want to weigh the cost of a new powerhead vs. a whole new motor.
pglein	posted 11-03-2008 07:30 PM ET (US) Word of caution. It takes special skills to machine a 2 stroke block. Machinists with these skills are becoming very hard to come by. But they are out there. When finding someone to do this, make sure they have experience machining outboard motors. The guy I found was an insurance agent who, in a previous life, had been the primary engine rebuilder for a number of local dealerships, but due to the switch to 4 strokes, had found that it no longer made ends meet. However, he was more than happy to take on the task of machining my engine for a few extra bucks on the side. His work was top notch and he gave me much valuable guidance.