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Thermostat Question

N

NSDON

Advanced Contributor
I know it’s not a Honda but you fellows have lot of experience with outboards and this is a generic sort of question. Friend has a 1996 mercury 50 hp 4 stroke, which is one merc sourced from Yamaha while they developed their own 4 strokes. It a 4 cylinder carbed model, a real nice running little outboard.

It starts perfect, runs like a Honda, but it started to stumble and miss after it warmed up and seemed to be overheating, he got no alarm. It’s putting out a steady stream on the telltale. Merc dealer thinks it has a blocked water passage somewhere and needed the power head removed to fix it.

I suggested he remove the thermostat and run it in a barrel of vinegar while monitoring the temp at the thermostat housing. He went to remove the thermostat and it had none.

What does the no thermostat do to how it runs, it has a manual choke on the key- push in.

It’s a bit of a mongrel, some things are merc but the block is built by Yamaha, they had the deal to build these for numerous years, the 1996 may be the first year. The outboard under the cover looks brand new, the guy who worked on it last was not the merc dealer, he is no longer running his outboard repair shop, sounds like a good idea. Plugs look like it running rich, very sooty black.

Thanks for your advice/info.

Don
 
Hi Don,
Hmmm.
A 4 cylinder 50....wow! Those must be some tiny pistons.

The thought occurs to me that he may be having a slowly worsening carburetor problem. If she's starving for fuel under load, that might cause increased combustion temps.

Running without the stat would typically cause the combustion quality to suffer and might cause a good deal of carbon to deposit on the valves and pistons.

I doubt, in this case, that it would cause sudden overheating to begin occurring.
Especially since it sounds like he's been running it that way for a time now.

I would pursue the possibility of overheat first, followed by a de-carb procedure.

I have seen good tell tale streams produced by waterpump impellers that were falling apart so I wouldn't rely on that to necessarily diagnose the cooling system.

I think he has no choice here but to pull the pump and verify that it's delivering enough water.
While it's off he can backflush from the top to see if he can dislodge anything that way.

I can tell you that thermostats are USUALLY left out in an attempt to solve overheating issues and you may want to query him about when that might have been happening before.

If problems persist, I would always pull the head to look for and clean blockages because that's where they are more likely to occur rather than pulling the entire powerhead . Removing the head is typically not a terrible job. I don't know about this Merc/Yami though.

Wish him luck.
 
Thanks for that advice, about what I was thinking about the no thermostat. Since he was getting no alarm, he only noticed a problem when the performance issue popped up. Crapped up pistons and valves could certainly lead to issues running smoothly.
Further research says this motor was only sold 1995-1998, now hard to get parts, merc doesn’t offer a full impeller kit, discontinued. Sierra has a kit for the impeller and gaskets, not for the stainless housing.
 
Yeah, that lack of parts thing is only going to get worse for EVERYONE as time goes by. These manufacturers don't want a relationship with anyone that isn't buying a new product it would seem.
I wonder when was the last time that outboard got a new impeller? Hopefully the housing is still serviceable.
Finding a replacement tstat might be impossible as well.
 
He has the impeller out, it has some cracks starting so being replaced. This is/was a sweet running 4 stroke for most of 27 years.
He inherited a building full of antique outboards, Evinrude, Mercury, Chrysler, a Bearcat 55 4stroke, and a clarke Troller (look that one up), Also a River jet engine and a stack of Make & Break one lungers. May have to clean something up. :)
 
Oh! That's interesting. What's his phone number?

kidding!

I sure wouldn't give up on that outboard. Someone, someplace has some parts for it and it's just a matter of finding them.

And now you say he's got parts to barter with so that could work out well for both parties.
 
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