Thread: What caused this 2000 25hp mercury

Originally Posted by Inline6

Is the severely damaged part of the piston the bottom facing side?

Yes sir it is the bottom facing side!

Makes sense what archer said and that being the bottom side confirming lack of lube on the side that sees the most piston pressure. Always learning something on this sight.

That's a common failure among the mercosil block 25 mercs. A failed gasket in the exhaust manifold has allowed water to be pulled into the cylinder. Although that is caused by a bad gasket, I've seen others that suffered a similar fate due to running ethanol fuel that had alot of water absorption. If you will do a search of Mercosil, it will provide lots of reading material describing the problem.

Originally Posted by sliderman

That's a common failure among the mercosil block 25 mercs. A failed gasket in the exhaust manifold has allowed water to be pulled into the cylinder. Although that is caused by a bad gasket, I've seen others that suffered a similar fate due to running ethanol fuel that had alot of water absorption. If you will do a search of Mercosil, it will provide lots of reading material describing the problem.

Lots of Mercury motors of this vintage have had this problem. I work with 2 guys that bought that same motor at the same time. One of the guys motor did the same thing, the other guys did not. A friend at the local FD had the same model motor and his did the same thing.

I agree, air leak. Possible lower crank seal. Definitely lean mixture on that cylinder causing major overheat.
The water intrusion thing is also a strong possibility. It's amazing how they can run with a LOT of water in the cylinder. Usually you will notice a big change in idle when it's a bad water leak.
JBJ

Originally Posted by bayoumonster

I agree, air leak. Possible lower crank seal. Definitely lean mixture on that cylinder causing major overheat.
The water intrusion thing is also a strong possibility. It's amazing how they can run with a LOT of water in the cylinder. Usually you will notice a big change in idle when it's a bad water leak.
JBJ

Nope... very common. Failed exhaust gasket allows water to enter exhaust flow stream. At varying rpms, the cylinder pulls escaped exhaust gases back into cylinder before port closes. It's a rebounding pressure wave that causes this. As close to the port as the water leak is, the returning gases carry water back into the #2 cylinder. (#1 cylinder too far from leak to catch water) At first, the amount of water is minimal and turns to steam, effectively steaming the cylinder. As the water leak increases, the water will nullify combustion in #2 cylinder and water may begin to appear in #1 cyl in small amounts. The engine will begin to idle smoother. (It being a loop charged engine, it idles smoother on one cylinder than two) It's at this point that most folks begin to consider having it looked at. Others will simply throw ideas at it and continue to run it till the bottom cylinder is completely dead.
If you run one of these, check the spark plug from time to time immediately after running it. If water is visible on the #2 plug, change the exhaust gaskets before running it again. Once the problem is allowed to progress to the point that the amount of water is nullifying combustion on the bottom cylinder, it's too late to correct. As the aluminum piston and steel rings continue to dig into the aluminum sidewalls, the compression goes down as damage increases. The end result is a dead #2 piston with bad damage to piston and cylinder in the same clock area of the exhaust port.
Another growing problem is ethanol blended fuels with a high concentration of water in suspension. The damage looks very similar but both top and bottom cylinders will show equal amounts of damage. LOTS and LOTs of info on the web about the mercosil block issues.
Mercury did have a switch box that had full control of the timing on some motors. Those are the most prone to damage because the box allows timing to run radical and create EGTs that melt the piston and cylinder surface. The owner will not be given a symptom of a problem until the damage is done and the block is toast.
Having said that. The EGTs of the newer motors are running high due to the main jet being abit smaller than older models. If you will increase the main jet by .002 or so, it will lower the EGTs and not flood the engine and create other problems associated with washed out cylinders. (The lower EGTs will lessen the chance of a burnt and damaged gasket in the exhaust manifold)

It's not my intention to sound argumentative on this. But this is a widely known and documented problem. There have even been attempts to bring suit against mercury because of the problem. Having 25yrs and counting experience with them has put me in the 'know' as to the issue.
As for your comment on the lower crank seal, that is a rarity among the 20/25 mercs. HOWEVER, for those running the 9.9/15 mercs, the lower crank seal is a one of a kind rubber armored seal. The rubber is very sensitive to it's exposure to naptha. Naptha is a key ingredient in alot of fuel additives. (It makes up to >95% of the ingredients in some). Being on the bottom, the seal is exposed to pooled fuel containing the naptha. The rubber will swell and become soft. One compression stroke of the bottom cylinder will blow the seal wide open. If you have a 9.9/15 that no amount of carb cleaning will correct an issue of poor idle / running, it's a safe bet that the seal is blown. I've repaired several this year and diagnosed several others for folks working on their own. (water may or may not begin to appear in the bottom cylinder of the 9.9/15 with a blown seal. The incoming coolant water circuit is very close to the bottom seal and I have seen afew that were sucking water into the #2 cyl by way of the blown seal.)

Here's an example of the 9.9/15 lower crank seal. My advice is for folks to get away from ethanol blended fuels and use a Non ethanol without any additives.


And another of the cylinder of a toasted 25 merc with a bad exhaust gasket. (Taken with a bore scope)

Very informative post. I am just a hobbyist and have nowhere near the experience of a professional. Glad you're posting on the site--- I may have some questions for you myself in the future.
JBJ

Thanks. And again, I hope my post didn't come across with an aggressive slant. I'm not here to step on toes. I'm merely trying to clarify the answer for the OP. He needs to be sure to replace those gaskets or his repair will fail. As for the rest of the readers who own a like motor, I hope I've given some clarification on the subject. Despite the number of years this problem has existed, there remains a good number of shops who do not understand what is happening inside the motor.