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Clogged water jacket? Merc 30hp EFI, no flow at higher RPMs on plane only?

S

steveg_nh

Regular Contributor
I'm am lost with this engine, as I feel like I am chasing so many weird problems. This is a new problem this season though. First time out, got a high temp alarm and my son (who was using boat) realized there was no water coming out of tell tale when going fast (he was on plane). He let the engine cool and he thought the tell tale was just clogged, so he twisted some weedwacker line we keep in the boat up it, and water flow returned while he was sitting there. Thinking maybe I had a blockage or a spun impeller, I took the boat home, changed the impeller (the one in there was fine though), made sure the keyway was installed, and checked all water passages and pickups for clogs. As far as I could tell, nothing was blocked.

A few observations. With the thermostat out (but engine running with tstat cover on), simulating an open tstat, water flow in neutral at idle was weak. With tstat installed water flow was strong. Maybe normal, but wanted to mention it (seems odd to me). The tstat cavity looked gunked up, so I ran a vinegar wash through it for a while and got some junk out, but not much. I did get some flow from the tell tale after this with the tstat removed and cover on. At this point I put a new thermostat in and brought it back to my slip for testing.

I guess I never paid attention to tell tale water temp before too, but water temp on the tell tale output is 95 degrees, I assume this is correct as the water circulates through the block so it will be warm when the engine is.

Running at the dock in neutral the tell tale is good and strong. Reving the motor in neutral it continues to be strong even in the upper RPMs. But running the boat up on plane, the tell tale stream slows to a trickle around 4700 rpm. If you keep running it I assume it would throw an overheat warning. If you drop down the rpms down and slow the boat the water flow returns in just a few seconds.

What the heck is going on? Is it cavitating and not getting water into the pickups? All of a sudden? Nothing about the motor configuration changed, nor did anything change on the bottom of the boat. I just don't get it.

So in short, 1) is 95 degree water from the tell tale on this motor considered normal (it shouldn't be cold), and 2) why the heck am I apparently losing water flow from the tell tale when running around 4700rpm? As soon as you drop rpms, the water flow returns. Could a blockage cause this, but only when running fast?

(I won't mention the stupid occasional stalling it does when reducing to idle or neutral. Really fun docking in a strong river current).

Thanks!
 
Really hoping someone can help. I ended up buying a complete rebuild kit, so I replaced the wear plate, impeller cut, upper and lower gaskets, and the impeller again. Took it for a ride today, and it did the same thing. The only thing I noticed was that the water pump based had pitting and corrosion along two of it's edges. It's possible it's not sealing well. I can't get one easily, so I put some black RTV to help seal it better. Not sure if this is the problem or not. It has to be cavitating in the water pump, but I just don't understand why it's just started doing it.
 
Still struggling with this. I have replaced the water pump entirely (new lower and upper housing, wear plate, cup, impeller), and it still looses water when running. Water temp coming out of tell tale is around 95 degrees. Could this be an exhaust gasket issue under the powerhead?
 
The water jackets under the powerhead were so clogged, as well as the rubber grommet at the top of the water tube being all squished end expanded restricting flow. Once I pulled the powerhead, replaced the water grommet and cleaned all the water jackets, it was no longer an issue.
 
Thank you. Gonna verify one more time I put it together right before pulling the head 🙃. Odd how it only shows when in gear at high rpm but in neutral high rpm it’s fine.
 
Because at higher RPMs, it's drawing more water (or trying to), but can't pass it. The motor is getting hot and quickly evaporating it or not passing it. Once you slow down, the demand lowers.
 
Doesn’t the drive shaft still spin with the same speed in neutral? I’m not familiar with all that but figured the gears in the lower unit are disengaged but shaft and impeller still increase in speed so it’s odd to me to not see it also occur in neutral at high rpms..

FYI- did post in the evinrude forum to get more detail on the water pump to ensure I did that right! Don’t want to waste your time but appreciate the info regardless.
 
The faster the engine, the faster the driveshaft is spinning (which the impeller is connected to), so yes, you would think if it's spinning faster it would move more water. What I learned and was told (I had the same questions as you), is the water is under much more pressure though, so it actually slows down and the higher engine temps is quickly evaporating some of it. I've also found it only did it under load. I could run it in a 55 gallon drum on the driveway, in neutral, and it didn't really do it. I would see it slow a bit, but never to a trickle or stop altogether and heat up to the point of an alarm. But put it in the water and take it for a rip in gear, the load seemed to exasperate the problem, and boom, overheat and no water flow until I slowed down for a bit and let it cool off. Strange for sure, but on these Merc/Tohatsu 30s, that upper water tube grommet and the passages is the issue. Google that and you'll find many posts on the grommet swelling to the point of restricting water flow.
 
I think you need to consider high RPM in neutral is still a small amount of throttle, so a small amount of fuel is burned. Less fuel burn is less heat created.
In gear you are producing more HP to achieve the same RPM. This requires more throttle, so more fuel burn. That creates more heat to dissipate.
 
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