7.5 Merc. Cooling system issues? What is this tube?

[xjfish]

Recently purchased small 14' aluminum boat with an old 7.5 Mercury. '79 model if I'm correct. Previous owner stated no known issues with recent carb kit and impeller. I took said unit on the river and it overheated badly. Had ran briefly with no water and may have destroyed impeller at that time. Immediately pulled lower unit and found impeller destroyed and housing compromised. Replaced water pump housing and impeller. Copper water tube has been previously damaged and had a 45 degree angle cut at water pump end. Verified water pump working with a drill on lower unit. Still bad telltale from pee tube, smoke/steam and a few drops of water only. As water tube suspect ("NLA", found one in Canada and ordered. Purchased related stuff locally... Pulled powerhead and expected to find issue, no smoking gun. Melted hose found in water jacket?? Seeking advise and hopefully attaching a pic or two...

 

[tpenfield]

The copper tube feeds the water from the water pump to the powerhead. There should be a phenolic washer where the arrow points. Maybe it came up with the powerhead, check to see if it is there. Also be sure the water pump has the larger plastic tube that mates with the copper tube during installation of the lower unit.



See if you've got good water flow into and through the powerhead. There should be a plug on top of the powerhead that you can remove to check to see if you are getting water up to the top cylinder.

You'll need a new gasket (or 2) to put the engine back together. Make sure all of the water passageways where the powerhead meets the shaft housing are clear of debris/corrosion.

I have several threads on my 1969 Merc 7.5 HP cooling issues. Do a search to find them.

 

[xjfish]

Thanks for reply tpenfield! Sorry my post is confusing. I am aware what the water tube is and will be replacing it as the bottom is beat up. Any tips on replacing appreciated. Assuming I will need to remove lower unit and it just press fits into place? The phenolic washer that is supposed to be on top of it is missing. I have one along with base gasket.

My question about "tube" is referring to the partially melted horizontal hose with notches cut into it, as shown, located just below the prop shaft. Does it belong there?

 

[tpenfield]

I was not sure what that was, but I do not see that on any parts diagram for the motor. That passageway feeds water to the exhaust (not the only water feeds to the exhaust - there are more). So, that 'tube' . . . probably not supposed to be there.

 

[xjfish]

I could also not find that on any parts diagram. It almost seems like someone was trying to create a restriction for some reason? Still waiting for my NOS water tube to show up. I guess I'll just clean it up and put it back together at this point? Also there is no thermostat!! Looks like there should be one? Pretty sure I saw one in a diagram... I will look again when I get a chance. Maybe I should pull side cover/plate off at this point?

 

[xjfish]

Yup, there is supposed to be a 'stat per diagram I looked at. Obviously, this will affect water flow but can confirm was not stuck closed. Do you absolutely need one on this engine? I suspect you do?

 

[tpenfield]

I am not sure if there is a t-stat on these engines, but mine is 10 years older than your 1979 model.

 

[racerone]

A few years did use thermostats.----Found at the top of exhaust cover.----A spring thingy on the thermostat cover.---Was for easy maintenance and inspection.

 

[xjfish]

A few years did use thermostats.----Found at the top of exhaust cover.----A spring thingy on the thermostat cover.---Was for easy maintenance and inspection.

This one shows a 'stat. Has the springy cover. I'm going to attempt to reseal water tube (along with replacing it) and see what water flow looks like. Will be blowing out ports with compressed air before reinstalling powerhead. If it cools OK without a thermostat, I may not bother installing. If not, maybe I should. Some vehicles with a closed cooling system will NOT cool properly with a thermostat removed...

 

[xjfish]

Update: Fixed

I cleaned and reassembled everything previously described and blew out powerhead passages with compressed air. Good telltale now with no overheating. I did not install a thermostat, and may never? I believe the root of the problem was the beat up and poor fitting copper water tube. In hindsight, a guy could probably make one easily with a piece of brake line or copper tube.

I hope this post can help someone in the future.