mercury 4.5 HP approx. 1983 cooling problem

[lenb12]

Hi :

I have a 4.5 mercury ourboard motor and it outputs water when it first starts, but when it heats up the water just trickles out and the engine overheats.
I have checked the impeller and it is fine. I have taken the side plate off the head as well as the cylinder head cover. There was no debris (eg. water impleller pieces from previous impleller "explosions") in the side plate canals and no debris in the cylinder head cavity. I have yet to take an air compressor and blow it up the lower end shaft to see if air will come out the cylinder head cavity. I also have to check the flow of air through the two holes in the side plate as well as through the top plastic piece which guides the output hose.

Does anyone have any clues as to what I should do ?

Len

 

[emckelvy]

Re: mercury 4.5 HP approx. 1983 cooling problem

Blow air and/or run water up the copper supply tube to verify flow thru the powerhead water passages.

I'm guessing you'll find it's inadequate. Most likely (since you've already pulled and inspected under the side and back covers) it's the rubber water tube grommet and/or plastic grommet washer under the powerhead.

The gromment and washer will squeeze shut from heat or salt buildup and this restricts the flow of water to the powerhead. Same thing is very common in the 7.5 and 9.8's and isn't limited to Mercs either; 9.9 and 15HP Johnson/Evinrudes have the same problem.

The only solution is to pull the powerhead and remove the offending grommet/washer along with the water tube. Then clean out any deposits and reinstall with fresh soft goods. I like to coat the outside of the grommet and also the exhaust plate recess where the grommet seats with Permatex No. 3 gasket dressing. This ensures that the grommet will never salt up again and squeeze shut.

Anyway, at least there's not a lot of nuts holding the powerhead in place! Once the powerhead's unbolted, undo all the interfering linkages, fuel line, wiring, etc and the powerhead should pull right off.

When you have the powerhead off, be sure to flip it upside down and check out the area where water discharges at the bottom center of the block. If these holes are plugged up, water comes in but can't go out. I had a 4.5 with a strange water flow problem, and I found the discharge holes were completely blocked.

HTH and let us know what you find.

 

[lenb12]

Re: mercury 4.5 HP approx. 1983 cooling problem

Thanks very much for the info. I will proceed with blowing air up the tube from the lower end. I believe that I will have to pull the power head off and perform the tasks that you have mentioned.

The good thing about my situation is that I have a working 7.5 HP and I was using the 4.5 as a backup. I have the time to perform the work that you were describing because I won't be fishing until May. I am also buying a electric trolling motor (cheap - $200.00) this year because I find that the noise from trolling with my 7.5 affects the number of walleye that I catch. The only problem with my electric motor purchase is that I am not sure if it will push my 12 foot deep hull aluminum boat.

Anyhow, there is no way that I am bringing the 4.5 to a shop because I got burned once for a large sum of money. The labour charges alone do not justify the repair.

Len