Brand new water pump only works at low speed and neutral

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Jun 21, 2015
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I have a 1984 50 hp Mercury that wasn't pumping water at high speed. I recently bought and installed a new water pump kit and am still having the same problem. It works fine at low speed, starts to sputter at medium speed and stops working at high speed. It also works fine at high rpm when in neutral. I did notice that the exhaust seal on the lower unit has some damage. Maybe a pressure issue?
 
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Joined
Jun 21, 2015
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Disrupted water flow to the intake vents?

How would that happen? Semi-plugged intake? Motor position? The problem seemed to show up towards the end of the season last year.

One thing I noticed when putting the lower unit back on was one of the exhaust seals is damaged. Could it be a pressure issue?
 
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Fed

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How would that happen? Semi-plugged intake? Motor position?
Could be, I don't know as I wasn't there & can't see it. Maybe trimmed too high?
It won't be the exhaust seal.
 
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Could be, I don't know as I wasn't there & can't see it. Maybe trimmed too high?
It won't be the exhaust seal.

Manual trim. It's in the lowest position as far as I can tell. I'll have a closer look tomorrow. The issue came about towards the end of last season. I was sure it had to have been a pump issue..
 

minuteman62-64

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Maybe semantics, but, to me, if it is in the lowest trim position, it means at speed you will push the bow down and transom up. Go fast enough under that setup and the water intake will ride right up out of the water - result, no cooling water flow.
 
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Pretty sure it's fixed. Someone suggested the water line might be plugged and to blow air through the outlet. I filled up my portable air tank and gave it a shot. I got it up to full speed and seemed to work for a bit then started to sputter. I shut it off, then started it back up and it worked! I must have dislodged whatever was in there and restarting kicked it out. Thanks to those with suggestions.

Update: Just got back from a couple mile trip and the intermittent stream showed up a few times. Restarting seems to fix the problem. Looks like I didn't get everything out of the water passage. If air doesn't work, I'm planning to try some mechanic wire tomorrow. Learning lots with this issue!
 
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ehenry

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Not sure if an 84' model has a thermostat but you may be seeing the telltale running strong then weak because of the thermostat opening and closing.
 

gm280

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How about the impellor not keyed to the drive shaft properly? It would work at lower RPMs but as you go faster the shaft is spinning and not grabbing properly. IDK just an idea...:noidea:
 
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Not sure if an 84' model has a thermostat but you may be seeing the telltale running strong then weak because of the thermostat opening and closing.

Good thought. I'll check my aftermarket manual out and see what I can dig up!
 
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How about the impellor not keyed to the drive shaft properly? It would work at lower RPMs but as you go faster the shaft is spinning and not grabbing properly. IDK just an idea...:noidea:

I'm going to look into the thermostat idea before I take the lower unit off again. The impeller not being keyed properly would definitely make sense.
 

emckelvy

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No thermostat on a 50hp 4-cyl.

Is the powerhead getting hot when you see indication of no water pumping? The exhaust plate should stay cool or lukewarm at most. The telltale could be intermittant due to debris lodged in the block, or issues with leaking exhaust plate/gaskets. In that case, the exhaust would blow into the cooling passages and blow out the water.

When underway, you should see a steady spray of water coming out the exhaust relief ports at the back of the engine, under the lower cowling. This and the exhaust plate temperature are better indications of water flow/cooling than the telltale.

If the impeller key were missing, it wouldn't pump water at any speed.

Maybe what you need to do is pull the lower unit and run water up the copper water supply tube. Find a rubber hose of size that'll slip snugly over the end of the copper tube, then flow water from your garden hose. You'll be able to see what kind of water flow you get out the telltale then.

And if you're extremely careful, you can fire up the engine, at idle speeds only, without the L/U installed. With a positive supply of water, there is no way exhaust can blow it out. Instead, you'll be accomplishing a pressure test of the exhaust system; any leaks will cause water to be sucked into the exhaust ports and you'll see droplets of water on the spark plugs, and probably notice erratic idle as well.

Be prepared to shut the thing down fast, 'cause without any load or backpressure it's gonna wanna rev like a Mad Dog if you give it any throttle at all!

HTH & let us know what you find...........ed

p.s. don't forget to replace that bad exhaust tube gasket, you don't want exhaust blowing around where it shouldn't.
 

steveg_nh

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Jun 8, 2021
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I'm going to look into the thermostat idea before I take the lower unit off again. The impeller not being keyed properly would definitely make sense.
I know this is a few years old, but I am having the exact same issue on my 2010 Merc 30 hp EFI. Did you ever figure it out? Just started happening this season. Changed the impeller, the tstat, etc. Works fine until you really get going, and will pump water fun in neutral at the same higher RPMs. I just don't get it. Thanks.
 
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