About four weeks ago, our generator suddenly went from normal output to having far less power. Instead of the 10+ kW it normally produced, it could only generate 7 kW. We worked through the Northern Lights diagnostic procedure and concluded we had a valve or valve seat problem on #3 cylinder exhaust. The steps we took and the conclusions we reached are documented in the video below, Diagnosing Low Power Output on a 12kW Generator.
We concluded that the engine problem was very likely a failed exhaust valve seat on #3 cylinder. We were fairly confident we understood the problem, but you never know with certainty until the engine is apart. We’d been holding off taking the generator apart since it’s still our backup power source even though it’s suffering from internal mechanical problems.
Last Friday night the new cylinder head and other seals and gaskets needed for this job arrived here in Stornoway. Within minutes of the ferry arriving, a Stornoway Port Authority worker had transported the fairly heavy parts all the way to our boat, since they knew we were anxious to get this generator back online. Our thanks to everyone who works at the port. They have all been very helpful to us and we really appreciate it.
Saturday morning, the old head came off. The #3 exhaust valve, at the far left in the photo above, was visibly much higher in the cylinder head than the rest of the valves (click image for a larger view). For context, the clearance in the valve train, also known as valve lash, is adjusted in thousands of an inch. But this problem is visible from a meter away without accurate measuring tools.
In the picture above we’ve turned the old cylinder head over to have a more detailed look at the problem. The combustion chamber looks normal, where it’s all black with light carbon deposits. Here we’ve scrapped the carbon off and rust is visible beneath, which suggests there may have been water intrusion sometime in the distant past.
We have carefully checked the exhaust against the manufacturer’s specifications and this installation meets their specifications. We’ve also checked for excessive water flow back into the water lift muffler on engine shut-down. If excessive water is in the muffler, it can splash back up into the engine in rough water. That doesn’t appear to be a problem either. Nonetheless, it looks like there has been some water in this engine sometime in the past. We need to dig deeper into this issue.
We don’t have the right tools to remove the valves, but the valve springs are fairly light on this small engine so we were able to remove them and have a more detailed look at the valves.
In the picture above, we’ve removed the exhaust valves from cylinders #2 and #3. The #3 exhaust valve seat has recessed a long way into the cylinder head. Sufficiently so that this cylinder head no longer is serviceable, since these heads don’t use removable valve seats. Both of the removed valves have very poor sealing surfaces. A normal valve would have a polished section where the valve contacts the seat. These valves have been sealing poorly and leaking.
We also checked the intake valves and saw the same thing. They also have been leaking. All the valves and seats are in poor condition. Looking at the removed valve heads in the pictures above, we see that there is a section in each that is heavily corroded (click image for a larger view). But most of the valve is not corroded at all. Valves rotate in the cylinder head in normal operation, so seeing valves with corrosion on just one side suggests they were in sea water, but probably only once. And for damage to occur, they would have had to had sat like that for days, if not weeks.
As the valve lash reduces, it has little impact on engine operation, until it starts to hold the valve open. At that point, an entire cylinder stops producing power. That’s why the output suddenly dropped from 10+ kW to 7 kW. And it’s also why the generator can sustain 7 kW even though an entire cylinder is not operating. Unsurprisingly, it’s running at roughly two-thirds of its normal output, since it has two out of three cylinders operating.
During our North Atlantic crossing back in 2017, we saw some very rough water (see video North Atlantic Gales). Our hypothesis is some seawater splashed into the engine from the muffler during those storms. It may even be the case extreme heel angle or large wave pressures forced water in from outside through the exhaust system and all the way into the engine. And since we landed at a marina with good power in Kinsale, Ireland, the generator didn’t run for weeks after the trip.
We suspect some sea water was in this engine for a few weeks after the North Atlantic crossing. This caused only minor corrosion in the valve and seat area, but is probably why the engine needs valves and seats at 6,700 hours rather than 15,000 to 20,000 hours.
The lesson we take away from this inspection is, whenever the boat sees extreme weather or is knocked down, we will run all engines and the furnace to ensure all are clear and no moisture is anywhere in the system. It’s a good lesson and one we’ll follow going forward.