engine stat

[andreworiordan]

Hi, I recently bought a 10hp 2003 Mercury 2stroke engine, she is a long shaft tiller controlled model, I have ran the engine and found that the stat isnt working and the water is coming out boiling, i removed the stat to confirm it was not the impeller, sure enough it few through and runs fine now, Can i leave the stat out or am i better off replacing it? I have a 15hp engine that i have for spare parts, I went to take the stat out of that one and found there was not one there, so again can i run the 10hp without a stat? thanks!

[Boats&Outboards]

People often remove them as with that model it does not have a constant flow of water, rather it pumps when the stat opens as required then stops. The problem with that is your forever worried when you take a look and see nothing coming out.

Not sure of the risks running without but I'd suggest replacing it with a restrictor plate. Basically that can be a washer with it's hole drilled out to allow a constant flow but not too much to allow it to cool too much.

Peter ~ Boatsandoutboards4sale ~ askboatsandoutboards4sale@sky.com ~ 07930 421007

[Fenlander]

I have the 15hp Mariner from 2001 which is a very similar model to yours. The advice B & O gives will get you up and running cheaply and will be fine. I'm a bit of a by-the-book guy so in the same circumstances I bought a new stat. Cost about £40 inc post though.

[Boats&Outboards]

Originally Posted by Fenlander

I have the 15hp Mariner from 2001 which is a very similar model to yours. The advice B & O gives will get you up and running cheaply and will be fine. I'm a bit of a by-the-book guy so in the same circumstances I bought a new stat. Cost about £40 inc post though.

It's not advice. It's just experience of how they come in to me. I'd always suggest running as per manufacturer specifications but I know people do prefer a constant tell-tale rather than not

Like you say cheap enough to replace. Each to their own I guess

Peter ~ Boatsandoutboards4sale ~ askboatsandoutboards4sale@sky.com ~ 07930 421007

[Fenlander]

Hope you didn't think I was saying you were wrong Peter... just giving the alternative option. No doubt the variable tell tale via the stat on this model is a pain to keep an eye on.

[Boats&Outboards]

I know don't worry just did not want people thinking I'm suggesting to "bodge" things up that's all

Peter ~ Boatsandoutboards4sale ~ askboatsandoutboards4sale@sky.com ~ 07930 421007

[uncle al]

your obm will run cold with a risk of glazing the bores because the fuel mixture hasnt been completely burned,it will not run as eficiently either

[Courageous]

As Al says - FWIW I advise with thermostat or nothing at all. If you start playing with orifice plates how will you "guess" the size of the hole? Before you know it you'll have a cracked block!

[Searider]

I couldn't believe the cost of outboard thermostats.

Just bought a secondhand 4hp Yamaha 4 stroke and thought that it the thermostat was cheap i'd replace it as a precaution - Nearly £50 though!
Tested the old one and it was fine, so happy.

[9D280]

The 50 / 60 clamshells seemed to get it removed as part of the factory build! The thermostat dissappears from the microfiche for the last three or so variations!

I would agree with Uncle Al ref. the running cold, but I reckoned that if Merc removed it, then they presumably must have tested before doing so, and as my boat spends most of it's time either moving... If I am helping out as a rescue boat I give the starter quite a lot of exersise, which also saves a bucketload of fuel.

The only real difference I've noticed to operation since I removed it is that I need to use the fast idle lever for 1 min or so on a cold start & 10 - 15 seconds on a warm start as opposed to not needing it at all.

[Courageous]

Interesting observation 9D. Is it possible that when they were removed something else was adjusted at the same time? On a cold start initially the engine (and water) will be cold anyhow. The engine wouldn't start to warm through for a minute or so in that case. The fact that there is an effect on a warm start implies some other change to me.

[Fenlander]

You can tell when the thermostat is open or not on my Mariner 15 2-stroke as it changes the way the telltale flows. At idle and with very gentle trolling it does shut the stat on and off a fair bit but once on the plane it's open all the while so I'm guessing leaving it out with perhaps a mild restrictor fitted will not effect any outboard used under load.

Having said that I replace the stat in every used outboard I buy... just in case.

[9D280]

Originally Posted by Courageous

Interesting observation 9D. Is it possible that when they were removed something else was adjusted at the same time?

The poppet valve weas also deleted. I got fed up puddling & threw it at a repair shop for final tuning, but I would guess it's running slightly richer / with microscopically more throttle opening at idle than it used to.

Originally Posted by Courageous

On a cold start initially the engine (and water) will be cold anyhow. The engine wouldn't start to warm through for a minute or so in that case. The fact that there is an effect on a warm start implies some other change to me.

Yeah - nothing to sieze closed! In all seriousness, the poppet valve is also removed by the change, and it usually only opens when the engine hits (I think) 1500ish RPM. The hole left by lack of it makes a huge increase in area at idle water flow. The other interesting (but not surprising) thing is that the telltale flow isn't quite as strong at idle - probably because it is tapped from the "always open" passage, but of course now at idle there is a far higher proportion of the water going straight to the block cavities. It still clears the toob by 3' when running!

As for longevity effects, I'll post back in 10 years!


Note for any clamshell owners planning on following suit - there is a vent hole in the cover behind the poppet diapragm that will need plugged! (it's the one that leaks if your diagpragm has burst)