Merc 7.5HP outboard, no cooling water

[irbrothers341]

I have an older (~15yrs) 7.5HP Merc outboard. The cooling water stopped coming out and when I disassembled, found the impeller in pieces. I replaced the impeller but something else must be wrong as I still get no water out of the discharge hole.

I have cleaned every tube and connection, poured water back through everything to ensure no blockages and have also noted hot air comes out of the discharge hole when it's running. The impeller is spinning when I turn the shaft without the pump cover on.

No visible cracks in the water pump or lines...

I would appreciate any thoughts you may have.

Thanks!

 

[emckelvy]

Re: Merc 7.5HP outboard, no cooling water

You may have a chunk of impeller up the water tube and/or in the powerhead.

With the lower unit off, you might try a strong blast of compressed air up the copper water supply tube to see if it blows readily thru the powerhead.

For a final circulation check, connect a water supply to the tube and see if you can get water freely flowing thru the powerhead and the telltale. If you're unable to push air or water into the upper end, there's a blockage and at minimum the back cover over the spark plugs (cylinder block water jacket cover) will have to come off for inspection. Worst case, time to pull the powerhead!

If these checks are successful, make sure that the impeller cup and impeller plate are in good condition; replace if they look worn.

Check the rubber grommet in the upper water pump body; if it's in poor condition you'll get a bad seal around the water tube.

It's real hard to get the water supply tube to engage the pump body when you're installing the lower unit, unless you have the plastic guide tube inserted into the top of the pump body.

Make sure this is in place and you're getting the water tube into the guide while the lower unit is on its way up.

Anyway, give those suggestions a shot and let us know how it goes............ed

 

[billybones]

Re: Merc 7.5HP outboard, no cooling water

I own nearly the same motor. last year I replaced the impeller reassembled 2 times till I realizeed that the guide belonged on the bottom and it did the same thing till I moved it there before reassembly. It just won't be a tight fit till it is moved to the bottom. mine was stuck to the copper tube. thought it belonged up there. now I own a book and 17 dollars a year to not have to chase broken impeller parts. priceless. well nearly

 

[emckelvy]

Re: Merc 7.5HP outboard, no cooling water

I've seen a lot of the original guide tubes swollen up so much that they're barely recognizable and some won't even fit back into the pump body. Time to renew!

And yeah, seen them get stuck up there on the water tube where the 'layman' might not even notice that it exists!

Sometimes on the bigger motors, the grommet 'glues' itself to the water tube then the whole mess comes out when you drop the lower unit. You have to pry the doggone thing off the end of the water tube with a set of pliers. Unless you've worked on them before, you might even be led to believe it's supposed to be that way. DOH! Always good to have a manual with decent illustrations in it.

Cheers..........ed

 

[irbrothers341]

Re: Merc 7.5HP outboard, no cooling water

Ed,
Thanks

 

[irbrothers341]

Re: Merc 7.5HP outboard, no cooling water

Ed,
Thanks for the reply...I am just now getting back to the motor. When I first disassembled I poured water in the telltale tubing and got water out the intake so I figured there were no blockages. Today, I tried to force water up the intake and only got a trickle out of the telltale. I also saw water coming out from inside the prop. I'm not sure how much water I should see out the telltale?

Is is possible that the impeller I got is the wrong size? The parts store listed two impellers and they gave me the larger one. It seems to me that it sticks up about a 1/16th of an inch above the pump body, and when tightening the pump cover on, it looks to me like it would be too tight? But I'm a rookie and I went with it.

Does the fact that hot air comes out the telltale hole mean anything?

 

[emckelvy]

Re: Merc 7.5HP outboard, no cooling water

The impeller blades are a bit tall so that they will press firmly against the upper pump plate, and the bottom of the pump cartridge.

The difference in impellers is not the height of the rubber blades, but the Inner Diameter (i.d.) of the impeller hole. There are 2 different sizes; you wouldn't be able to fit the smaller-i.d. impeller on the larger-o.d. driveshaft, but if you had the smaller-o.d. driveshaft, it'd be possible to put the larger-i.d. impeller on and I suppose it'd be real sloppy.

The impeller should fit snugly around the driveshaft, if there's a lot of slop it's the wrong one. I'd definitely pull the lower unit down and remove the top of the pump. Don't take the impeller out yet; turn the driveshaft and make sure the impeller is spinning. Check with a strong lite to see if there's any gap between the impeller i.d. and the driveshaft.

I'm not sure how much water you can actually force up the intake screen on the lower unit, you likely can't get a perfect seal there, and the water has to get past the pump before going into the powerhead.

The way to be absolutely sure there are no obstructions in the upper parts of the motor is to apply water to the copper pickup tube (with the lower unit removed, as described earlier). If water goes up the tube, out the telltale and down the middle of the "exhaust tower" (center section), you should get good flow with the lower unit installed, provided the pump is in good shape and the water tube is properly inserted/sealed into the pump body.

The fact that you're getting hot air out the telltale means little or no water is getting to the powerhead, and exhaust exiting down the center section is being blown up into the water discharge passages of the block and out the telltale.

Anyway, best to yank 'er down, check out the pump, flow water thru the motor, see what ya got. BTW if the pump plate or pump insert (cartridge) have deep ridges worn into them, you won't get much pumping. The ribs on the impeller seal against the top and bottom to make the pump more efficient. Time to put all the parts under close scrutiny; you may find a combination of worn parts is adding up to no pumping.

Keep Us Posted...........ed

 

[irbrothers341]

Re: Merc 7.5HP outboard, no cooling water

Ed,
I took the lower unit off and sprayed water up the copper tube as you said. I got plenty of water out! So I took the water pump apart again and inspected everything...it looked to be in order. I reassembled and tried to run it again...and this time I got cooling water when the motor started! Not sure what I did differently if anything, but I'm not going to complain. The only problem was in order to get the lower unit back on I had to turn the "clutch" keyed shaft. The motor had to be put in "F" to actually be in Neutral. After some monkeying around, I got everything in order. One last trial and I should be good. THANKS FOR YOUR HELP!