# '95 30hp merc cooling system question



## Swamp (Jul 12, 2010)

I'm having an issue with my 30hp 2 stroke Merc ('95). It has a new impeller and pump housing (last week) etc. It will pee like a champion race horse when I flush the engine with muffs or stick it in a barrel (no hose pressure). It is also just fine when I first launch. After about 20min on plane it will start to dribble and continue to do so for the rest of the day. Off plane there is a continuous stream but it basically just drops straight down, on plane it dribbles. I believe it has about the same water flow on and off plane, the wind just breaks up the stream at speed. Yup it beeps at me if I keep running at WOT (@30mph), however if I run just below 20mph or below it does not care. Certainly not ideal and not my concept of a good idea of things to do. So I'm thinking that it takes a while (about 20min) for the thermostat to open and there is a problem with either the thermostat itself or in the water passages in the block. I sure hope it is just the thermostat and/or associated system. I plan on pulling the thermostat, do a visual inspection, and clean any crud out of it that may present. This is the first time I've ever had anything other water pump issues since I started running my own boats 25yrs ago. I'm pretty good at dealing with the majority of common OB issues but I'm in somewhat uncharted waters here. Any advice or things to look for? Maybe I should be looking elsewhere?

Thanks

Edit: I live on a very bumpy dirt road so it is possible that the thermostat is jarred on the way home, thus resetting and allowing full water flow when I flush the engine. That might explain why it won't go back to normal on the water despite extended cool down periods(no jarring dirt road). Again new situation for me I might be very wrong.


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## HaMm3r (Dec 11, 2006)

Sounds like you're on the right track already. Thermostats are easy to replace and do corrode/build up deposits until they won't operate smoothly. Something else you might consider doing is back-flushing the water passages while you have the thermostat off, just in case some pieces of debris or bits of impeller are jammed in there somewhere. Using a 50/50 vinegar/water solution poured through the open thermostat housing is also a good, mild DIY way to dissolve some of the deposits that can form in the passages too.


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## DuckNut (Apr 3, 2009)

Not possible the thermo moved...unless you have the bolts out of the housing as the housing clamps it in place.

I work with water pumps and I am certain you have an obstruction. Could be a small shell, rock, or similar.

What is happening is: at slow speeds the obstruction is not severe enough to cause issues but when you get up in the rpm range the water is being forced past the obstruction and causing turbulence and at wot the turbulence is great enough to alter the flow.

Clean out all of the water passages.

I had a very similar issue with one of my motors (25 Merc). The intake tube attaches to the block using a rubber oring. Corrosion started growing behind it and eventually grew large enough to nearly pinch off the passage. At slow speeds it appeared everything was normal. At high speeds it barely spit. Had to clean out this fitting and all has been well.


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## Swamp (Jul 12, 2010)

Well the thermostat was a bit crusty so I'm replacing it and the diaphragm (might as well).

HaMm3r: I was considering Saltx etc, but the acetic acid mix sounds cheaper as a start! <grin>

Ducknut: That makes sense, and it crossed my mind that something is blocking the plumbing to the block. The one thing that makes me wonder is that it discharges just fine at WOT for about 20 min, then it starts to dribble. There is little change to outflow regardless of RPM (okay there is a LITTLE once you get above idle). I think I will take a look at the riser between the pump and block though. It could be that if there is loose debris in there that it gets knocked back out on the trailer ride back home. 

Thanks for the suggestions.


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## Swamp (Jul 12, 2010)

Update: I replaced the thermostat, gaskets, and diaphragm. I had to let it run for 40min before anything happened and then it went back to dribbling. I turned the motor off and upon restarting it started peeing full force again. I have been unable to make it mess up again. So I took it down to Choko and it performed great all day. Well not really, I dropped a cylinder around Deer Island. Luckily I managed to limp back on the second cylinder at 5 knots. I did a quick compression check and both cylinders are identical at 105 psi. I'm pretty sure it is electrical in nature, I found two wire connections leading to the voltage regulator badly corroded. While I did not like the hick up after the thermostat install (I'll be watching it like a hawk JIK), at least for now the cooling issue seems to be solved.

Thanks for the help.


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