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BF50A - Intake Valves Corrosion Mystery ?

P

pcaker

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My BF50A is 11 years old with about 500 hours on it. In June I had a totally normal outboard starting and running experience. However, 2 weeks ago in August, I could not get engine to start. Investigation revealed that intake valves were badly corroded and leaking - i.e. very poor compression, and insufficient vacuum to pump the fuel. There was a mild ring of rust at top of cylinders, above the highest ring position. Internal examination of the water galleries, intakes and exhaust showed virtually no corrosion. All gaskets, including head gasket, were in very good condition with no breaches. Generally, the engine looked in excellent shape internally and externally - except for the valves. I did discover that the 2 drain tubes to the intake air plenum were completely blocked, but it is hard to see how water could accumulate in quantity in the plenum, even with blocked drain tubes. My marine mechanic was at a loss to diagnose where the water might have come from. All water sources in his previous experience of water getting into the cylinders could be eliminated with what could be seen in this engine. My only theory left is that 11 years of damp winters with engine not run for 6 months each time has taken its toll. I will confess that I did not put a tablespoon of oil into each cylinder before storing, as recommended by Honda, because this would not have protected the valves, and I used sticky synthetic oil in the sump which I judged would protect the cylinders (which were essentially unworn, the original factory honing marks could still be seen), and any oil added through the plug holes would turn to black carbon deposited in the cylinders when the engine next run - not a good thing, as I saw it.

If anyone has ideas as to where the corroding water might have come from, I would be glad to hear.
 
I can't tell you (at least from HERE anyway) where the water came from but I do know that engines stored in damp climes have been said to benefit from a process called "fogging" when put away for the season. It involves spinning the engine over, with plugs removed, ignition off and fuel system dry, while fogging the intake with a water repellent lube like WD-40.

Some folks swear by Marvel Mystery Oil for doing this but WD-40 is already aerosol'd...something I think you would have to fabricate for (squirt bottle) with the Mystery Oil.

I'm also not saying this would have prevented your problem but it is just something to think about.

The intake plenum does receive crankcase ventilation gasses that CAN be corrosive in nature but that has been considered and engineered for when putting the engine all together. The drains are there primarily to alleviate any build up of condensed oil vapor that tends to reform in pools of liquid so I would agree with you that the clogged drains were probably not the culprits.

That's all I can think of on the subject except....don't forget to reinstall the plugs!
 
UPDATE: It now looks much more probable that the timing belt slipped, misaligning valve timing to piston position, then bending some of the valves, and that corrosion was not a significant factor. I spoke with the technician who re-built my head, and this was his opinion. So now I have to figure out why belt slipped. I was already replacing timing belt, now I will also replace bely pulley tensioner spring. Not sure what else to replace or how to confirm diagnosis - head is off, timing belt removed.
 
Well, if the head is off and it was in fact the piston contacting the valve that bent the valve, then there WILL be witness marks on the piston crown. They might be hard to see but they will be there if you look carefully enough.

Another possibility might be that the your mechanic is seeing damage caused by detonation. Detonation can do some strange things inside the combustion chamber because of the localized pressures that are created. Detonation is basically just what it sounds like....an explosion in the cylinder. It is typically caused by low octane fuel, engine overheat. burning lean fuel mixtures...or a combination of any of the three.

Your decision to replace the belt tensioner and spring along with the belt is a wise one. You might also want to measure crankshaft end play and thrust while the head is off and the end bearings and thrust bearing are more readily accessible.

Good luck
 
Thanks jgmo for your further advice. I had not noticed piston crown marks, but then I was not really looking closely for them. This I will now carefully do. Head technician was clear on these points: some valve stems were bent (at least 2 out of 3 intakes), valves & seats were not significantly corroded, guides were undamaged & little worn, valves slid freely in guides (except for stem bend effects). I always use mid-grade gas from the same marina. I have never heard detonation in this engine, which I would recognize. Thanks again
 
Bf50's through 2003 did have issues with detonation if the motor was not rigged to throttle up to 6000 rpms. If it only got to 5500 rpm with a heavy load, detonation could occur. If there was no damage to the motor, the ICM was replaced and the engine was rigged to get to 6000 rpm.

Mike
 
Thanks Hondadude - very interesting comment. The OB was bought in spring 2004, but could have been a 2003 model. I very rarely run flat out, and not in the past 2 years, mostly I run at 3/4 throttle or less. RIB boat typical total loaded weight is 1,700 lbs, and typical cruising speed about 15-18mph. I just examined the piston crowns. Unfortunately for diagnosis, there are semi- circular valve accomodating recesses cast into the crowns, i.e. a lightly impacting valve would hit the recess squarely and might not leave a mark. This was in pursuit of the timing belt slippage explanation. Perhaps a slippage of just one or two teeth could have been enough to bend valve without damaging or marking piston. On the other hand, it could have the detonation referred to by yourself and jgmo. It is still unclear which.
 
Well, I've seen many engines that have jumped time and bent valves and I don't recall ever having seen one that didn't leave a mark on the piston. Usually the mark or "impact zone" is significant enough to make it necessary to consider piston replacement. The little ridges and scratches that can be left after piston to valve contact can become "hot spots" on the piston and can lead to pre-ignition. Also, that sort of impact (valves don't bend easily) can set up "stress risers" in the metal that can lead to subsequent piston breakage. It has been my experience that finding the contact "evidence" was never difficult but with the big topic being "should we replace the piston too?"

If yours is a case of the piston striking the valves and causing them to bend and leave NO damage to the piston whatsoever....then I would say you are VERY lucky indeed.
 
You were right JGMO about there always being a visible impact mark. However, I had to clean the crown valve recesses very carefully with Q-tips, then examine them with a high power lens under a strong light, to see the impact marks. These took the form of a line of smoothing along the top edge of the recess which contrasted with the “roughness” of the original casting. By comparison, the raised numerals on the crown were relatively like mountains. Therefore, if the numerals do not cause detonation, I can’t imagine how this smoothing could be worse. As for possible internal damage to the pistons, it looks to me unlikely from the slightness of the observed smoothing, and in any case I cannot afford to rebuild the bottom end as well. So, I will proceed with re-assembly. One more thing: the marine mechanic returned from time off, and I was able to bring him up to date. He confirmed that when he originally checked the engine, the timing was way off - maybe 10-15 teeth. He had reset the timing, but of course it made no difference. I am not sure why he did not emphasize this finding more. Perhaps he thought that “corroded” stuck valves caused the belt to slip, rather than the slipped belt causing valve damage. It was news to him that some of the valve stems were bent. I understand also that this belt tensioner is preset by the spring then locked in position. Therefore, if the belt does stretch, the tensioner does not take up the slack, making slip more likely. If I had understood this before, I would have checked the timing belt tension at what I now find out was a recommended 200 hour interval, which I had not done. I had assumed outboard timing belts were like car engine’s, i.e. good for at least 1,000 hours without adjustment or replacement - an expensive lesson.
 
Well, the description you give of the pistons makes me think that you will be just fine and the pistons are serviceable. I have reused many pistons that have struck valves with probably much worse impact stress than what you describe here.

Sounds like you got unlucky with the belt and valves but can count your blessings with the pistons. AND...now you know how to keep it from happening again.

It's ONLY MONEY...and it only HURTS for a little while....

ALSO...due to your diligence and follow through....we all know EXACTLY what took place.

MYSTERY SOLVED!
 
Many thanks JGMO and Hondadude for your continuous support input and well informed comments. These were very helpful. The lesson seems to be that the BF50A timing belt MUST be inspected and re-tensioned at about 200 hours (about half my engine's hours to belt failure).
 
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