Mercury 7.5HP Outboard Waterpump/overheating issue

[highamperage]

Hello,
I am having some issues with my 1975 (serial: 4091234) 7.5hp Mercury outboard... I am not exactly sure what the issue is.

When first started I notice it takes 15-30 seconds for water to start coming out the exit port. From idle to maybe 40% power the water seems to come out ok, but when i really get going (the faster the worse) it seems to start spurting sporadically and then steaming... so I cut it off at that point.

I took the lower unit off and examined the water pump, and it appears to be in good working order. Keep in mind I have no idea what I am doing, of course. lol. But, all the impellers were there, and still soft/flexible - and the little "nubbins" at the end were present.

I would still assume it's the water pump, but would anyone have any ideas? I thought perhaps thermostat, but I don't see that this model has one.

Anyway... I appreciate any help!

 

[kingsnakejr]

Re: Mercury 7.5HP Outboard Waterpump/overheating issue

Make sure when outboard is in the water that the water inlets are completely coverered with water. Less cooling water at wide open throttle could mean the engine is trimmed too far back. Also make sure that the inlet screen is not blocked. And you could blow compressed air up the outlet to make sure the inside passages are clear and water can circulate inside the engine.
Hope this helps.
ks

 

[joed]

Re: Mercury 7.5HP Outboard Waterpump/overheating issue

I don't know about 1975 but my my 1995 does similar as the thermostat opens and close.

 

[dajohnson53]

Re: Mercury 7.5HP Outboard Waterpump/overheating issue

I don't know about 1975 but my my 1995 does similar as the thermostat opens and close.

I don't think the '75 has a thermostat.

To OP: as a start, you should just rebuild the water pump, or at least replace the impeller. That is a basic maintenance item, and if you had it apart far enough to look at it, you'd already done most of the work. You often can't tell if the impeller has taken a set (and therefore less effective) by just looking at it. There also might be wear on the metal parts.

After you do that, see how the water pump reacts. It takes mine ('74 9.8 - very similar to 7.5) a few seconds to get water out of the tell tale, so that's normal.

If you're still getting spitting/steaming at higher speeds, your inner water passages might be blocked up with silt, etc. That can be a pretty easy fix too, but see what the new water pump parts do first.

You might also just see if the engine is cooling. The block shouldn't get too hot to keep your fingers on it for several seconds.

 

[FunFishin]

Not sure if this helps - but this is what I figured out. I have a 84 7.5 HP and water quit pumping out the teltale. Replaced the impeller, seals, and plastic tube out top of pump. Still no luck, could get some drips at first then just some steam. When I put it back together, I noticed the tube out the top of the water pump had quite a bit larger inside diameter than the outside diameter of the copper tube. After a few other tries, I got a sleeve out of a copper compression fitting and glued it into the top of the output plastic tube so most of the water would go up the tube instead of half of it just running out between the plastic tube and the copper tube. Now it works great - better than I have ever seen. Not sure if this a common problem but worth looking at! Good luck!

 

[racerone]

This was 10 years ago.-----I am sure the problem was fixed.------BOOM , BOOM for this thread.----Plastic tube out the top of the pump is nothing but a guide for assembly.----Has nothing to do with water flow.