Yanmar 6lp sudden overheat at cruise

[gfuller]

Hi all,

As the topic states, I was running back home last night from a day off offshore fishing when I heard the engine get slightly louder, it ran louder for maybe 5 seconds and I immediately backed off the throttle. The temp got up to 90c and the alarms (engine temp & exhaust temp) sounded. I shut the engine down and pulled the cover. I saw that there was a leak by at the raw water pump, pretty significant even with the engine shut down. I checked the strainer etc... and found no blockage. To me, it seems like a raw water pump failure or sudden blockage post-water pump (due to the leaking). Upon squeezing the hoses there did not seem to be much if any raw water circulating in them and I could hear an air leak at the raw water pump. I did swap out the impeller and water seals last spring but have a good 400+ hours on the engine since then. I also inspected and removed zincs from intercoolers & used rydlyme. Other thoughts could be a bad thermostat maybe? I'm not sure where this is located for the raw water pump or if it even has one on the raw water side. It's probably worth mentioning that a few runs ago I had the exhaust temp light flicker but turn off on the way home, no change in engine temp, just the alarm came on and off briefly.

Any thoughts are greatly appreciated as usual!

[Norseman]

How many hours on the engine?
All maintenance performed as per the factory manual?

[gfuller]

1400 hrs. I called Yanmar yesterday and they said it's hard to tell without cracking into the pump but it sounds like something may have gotten sucked up into it, destroyed the impeller from running dry and melted the lip seals. I'm going to crack into it tomorrow after work and see what I'm working with.

Yes all maint has been performed per factory manual to this day.

[Tree]

The lip seals do fail - i rebuilt a few pumps last year. One of the pumps failed so catastrophically it ruined the helical gear on the end - all due to the lip seals failing. Once they fail, you will be pulling in air.

You may be lucky and find the impeller is intact but i suspect it will be chewed up if its had no lubrication.

Rebuilding the pumps are pretty simple and will only require two bearings, two lip seals and a cir clip - I've got one here that i rebuilt last year ready to rebuild again as i noticed it dripping slightly while under power.

[Paddy]

My first one started dripping bad after 500 hours so I bit the bullet and bought a new unit from the dealer for $525. That one just started leaking last week at another 500 hours. I luckily scored a 3rd unit from ebay for $180, so that unit is now on the boat. I bought the kits and plan on rebuilding both spares in the coming weeks. Does look very straight forward minus the pulling / pressing of the bearings. My local auto shop should be able to handle that.

[gfuller]

Tree nailed it. Bearings blew out busting the seals and damaging the helical gear as the shaft was crooked in the housing. Rebuild is simple, $100 kit. Use a $40 bearing separator from harbor freight to pull the bearings. Put the new bearings in the oven at 250 degrees and the shaft in the freezer. They slide right on, no tools needed.

In my case the impeller still looks brand new as the bearings were the culprit.

[Paddy]

I love the oven / freezer tip, will be trying that soon!

[RobS]

I love the oven / freezer tip, will be trying that soon!

Same reason you never leave a prop baking in the sun before you install it.

[Tree]

Tree nailed it. Bearings blew out busting the seals and damaging the helical gear as the shaft was crooked in the housing. Rebuild is simple, $100 kit. Use a $40 bearing separator from harbor freight to pull the bearings. Put the new bearings in the oven at 250 degrees and the shaft in the freezer. They slide right on, no tools needed.

In my case the impeller still looks brand new as the bearings were the culprit.

Just be cautious, there is a circlip between the two bearings so don't try to remove them together

[gfuller]

That's a good note.. I had to do a double-take on what was going on between those bearings before throwing the separator on there. Also, do you guys pack the bearings with grease? I know there is oil lubing up the gears themselves but wasn't sure... I put some grease on them anyway.