Help... Metal Shavings on Plugs: Mercury 115 Inline 6

[wcsd106]

Short Version:

• Outboard is a 1986 Mariner 115HP Inline 6 without Autoblend (Premixing 50:1)
• Outboard Overheated in Oct, 2018
• After overheat, Compression Test showed ~125psi compression all cylinders
• Outboard has run fine since overheat
• Yesterday while running, outboard suddenly lost RPMs
• After RPM Loss was hard to plane and WOT RPM dropped by ~1000RPM
• Found metal shavings on #4 spark plug this morning
• Metal Shavings in position to impede spark on surface gap plug
• Compression test STILL shows ~125psi on ALL Cylinders
• Limited visual inspection through #4 spark plug hole shows no visible damage to piston.

So... My gut tells me I wouldn't be finding metal shavings on a plug unless something catastrophic has occurred within the #4 cylinder. But, shouldn't I have lost compression around that time as well?

The metal shavings on the plug could have killed the spark on that cylinder, which would account for the loss of RPM and power.

Am I being over-cautious since the overheat to the point of being paranoid?

 

[Sea Rider]

Have you checked the cylinder wall condition, could be shaved already by piston rings due to overheat, worse if it was a severe one ? Was the overheat due to lack of proper lubrication or water pump failing ?

Happy Boating

 

[wcsd106]

Overheat was due to a failed water pump. I had started the outboard and was letting it warm at idle while I stood on the front deck of the boat throwing a cast net to catch bait. I didn't notice that it had stopped pumping until I went to motor out of the harbor. So it was never ran hard at overheat, just at idle RPM.

I haven't checked the cylinder wall yet. That's my next course of action. I'm going to have to get an inspection camera to check it out.

 

[jimmbo]

The top piston is usually the first to destruct when water starvation occurs, though some will argue it is #3

The piston is going to shed metal first as Aluminum(from the skirts) is softer than Cast Iron, which is what the cylinder is made of.

Pull the transfer cover off and look at the piston skirts, if the skirt on the intake side is scuffed, the exhaust will be much worse. Even if the intake side skirt is perfect, the exhaust side can still be scuffed. The engine is going to have to come apart, so you might as well remove the exhaust cover too and see the mess

 

[wcsd106]

I have a couple of extra powerheads from parts motors. Before I tear into this one, I'll just yank the powerhead and put a different one on after swapping my ignition and intake to it. There's no sense in rebuilding a 30 year old powerhead.

I was hoping that I was being paranoid about the shavings since I was still getting good compression, but looks like I was spot on.

 

[Faztbullet]

That is aluminum throw off from eyebrow of piston. This is usually due to pre-ignition and is caused by : trigger (check cyl #1 as it and 4 are common) switchbox bias or idle stabilizer which need to be trashed anyway.