1990 mercury longshaft cooling issue

Hey guys have a 1990 merc and noticed a few weeks ago I had no water comming out the motor cowling, thought maybe impeller was shot took the hose off the block fitting noticed it was plugged solid about 4'' of green crap. Replaced the hose took it for a run no issues. Was at the cottage this week motor ran fine for 4 days then on the 5 day comming back from fishing at full throttle looked back no running water, slowed to half throttle and water started comming out again. Heres whats happening at idle to half throttle the motor runs great give it gas I loose water till i cut back the throttle to half or less. Any thoughts and whats the green crap that was in the old hose? I had the boat out a week earlier no problem not like it sat for a season. If its the impeller would I not have issues at idle up to half throttle? I talked to one guy that said it was the T stat behind the fitting that screws into the block where you attach the hose, took it apart no stat there. Should there be ? Sorry for the long post thanks Paul. Sorry forgot to mention its a 25 hp

I have 98 Merc short shaft that had the same problem. I removed the impeller and it looked OK,but,the cogs were very weak and worn. I think at high speed,they simply flatten out and stop pushing water. I changed it and it seemed to fix the problem. The green crap is weed and mud crud that gets sucked up from the lake bed into the water cooling system. Usually,it'll flush itself out,but,it can't if your impeller is worn. I hope this helps.

Thanks for the reply trimmer 21 is this a fairly easy job to replace the impeller?

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Originally Posted by cuda

Thanks for the reply trimmer 21 is this a fairly easy job to replace the impeller?

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It's not hard,but,it's time consuming. There's lots of YouTube videos to show you how it's done.

Impeller should be changed every few years (some say every year!). I have seen the situation with weak blades "bending" at high throttle causing loss of pressure, but not complete zero. I also recently had a problem with a worn key way, the impeller would basically start to slip on the shaft at high RPM. In both cases replacement impeller does the trick.

BUT

I think you also have a lot of crap in your engine block, this happens and I suspect that you are seeing high water resistance facing the pump. I have had good luck with muffs on the engine, but to get a really good flush you may have to remove some plates to open up the water jacket. Not a hard job, but I wouldn't tackle it without a service manual.

Not sure what the green stuff is, but your intake screen might be lose, cracked, or even missing! It is located in the lower unit, most of the time you can see it through the water intake holes.

Most of my work is on Johnson / evinrude.

If you haven't done the impeller yet on a 1990 then look no further. I just had mjne done this week on my 40hp. Still had warm water flow but the impeller had bent over cogs when it came out. It's a wearable part that needs periodic replacement.