25hp Mercury Cooling - Thermostat Help

[jakeboomer]

Hello,<br /><br />I have read everything I can find on 25hp motors and cooling systems, but I still have a few concerns. The motor is a 25hp 2005 2-stroke with electric start. I've run 2-3 tanks through it, in all water conditions... shallow, deep etc... <br /><br />The telltale stream only dribbles at times, then goes full stream, then back to dribbling. I figure this is the thermostat, but I've owned other outboards, and don't ever recall the stream going to just a dribble. Is this normal operation for this thermostat motor?<br /><br />Also, the top cylinder seems to run a little warmer than the bottom one. It this normal? During my test, the top cylinder was never too warm to prevent me from touching it. <br /><br />The motor may have been doing this from the start, but I didn't notice. I ran into some millfoil and mud on the last trip out, causing me to pay extra attention. I forced compressed air up the telltale, then ran the unit in a garage can, and nothing changed on the stream. In the past, I owned a 1988 25hp 2-stroke, but it wasn't electric start. I never noticed the stream goto a dribble on that motor. My johnson also streamed consistently.<br /><br />Thanks everyone for the shared knowledge. This site is excellent. I tried to make the Subject line "Search" friendly so others with this same issue can get all the answers in one stop. <br /><br />Thanks again.<br />- jboomer

 

[andy6374]

Re: 25hp Mercury Cooling - Thermostat Help

First make sure that your model has a thermostat. Enter you serial number at www.mercurypartsexpress.com<br /><br />The water that sprays out the tell tale on a mercury is after it passes through the water jacket and through the tstats. So unless the tstat is fully open, which it might not be at idle, you won't get that strong of a stream. On a Johnson the water out the tell tale is just coming from the water tube, so it is always cool and strong.<br /><br />If your nervous though, drop the L/U and inspect the impeller. Check to see if it is damaged or if it has a spun hub. With the L/U off flush some water down the t-stat hole and see if anything falls out the water tube.<br /><br />Have you ever replaced the water pump and or tstat. You should. And like Rodbolt always says "it's better to replace the water pump and tstat, than water pump,tstat, and piston". Preventative maintainance is the name of the game. And these two are crucial.<br /><br />Good luck-<br />andy

 

[ehenry]

Re: 25hp Mercury Cooling - Thermostat Help

I have the same engine only mine is a couple years older than yours. What you are seeing is normal for that engine. The thermostat is opening till the engine cools thus you're seeing a full tell tale stream. Once the engine cools the thermostat closes and the tell tale reduces flow. You will notice it more running in the winter time in cold water than in the summer time. If you were to pull the thermostat out you will see that there is a small hole in it to allow water to flow through so water will still flow out the tell tale. If you runt he engine with the thermostat out you will have a full tell tale stream all the time BUT the engine wont reach normal operating temprature. <br /><br />I use my engine year round both hunting and fishing and it has quite a number hours on it. I replace the impeller every year. I think this year I'll replace the thermostat along with the impeller just to be on the safe side.

 

[jakeboomer]

Re: 25hp Mercury Cooling - Thermostat Help

andy6374 - The motor is a month old, so I am pretty sure it doesn't need an impellor. I was paying more attention to the stream after running through some millfoil, and with the colder water the t-stat must be staying closed longer. <br /><br />I just wanted to make sure this was normal for this motor. I've had 2 other outboards, and I replaced the impellers on both of them. My 1988 Mercury 25hp must not have had a t-stat, because it always had a strong stream. <br /><br />I am guessing that the top cylinder runs hotter due to the cooling stream getting there after it passes through the bottom one...? <br /><br />Also, do these smaller HP motors have an overheat alarm? I checked the manual a little, but didn't notice anything about one.<br /><br />Thanks,<br />- j.boomer

 

[andy6374]

Re: 25hp Mercury Cooling - Thermostat Help

What's your serial number?<br /><br />If you have one, the overheat switch is located under the #1 spark plug. It is usually tan/blu but sometimes it is just tan. It could also be under the #2 plug. This wire should eventually run to the alarm.<br /><br />If you are worried about a clog in the cooling system, like I said run some water thru the system with your garden hose. But it is probably just the tstat not fully opening at idle. I bet if you watch the tell tale stream at WOT it is a full hard stream.

 

[jakeboomer]

Re: 25hp Mercury Cooling - Thermostat Help

Serial number is 1B194478.<br /><br />I looked it up on mercurypartsexpress.com and it shows a t-stat. I didn't see anything for an overheat alarm though.<br /><br />- j.boomer

 

[andy6374]

Re: 25hp Mercury Cooling - Thermostat Help

Yep, no overheat switch. I bet the stop and go of the tell tale stream at idle is normal. If you are worried take out the tstat and test it. Or check for clogs in the cooling system

 

[Bigjohn1]

Re: 25hp Mercury Cooling - Thermostat Help

Owned a 1998 model and it did the same thing. I think a dribble at times is quite the norm on this engine.