Mercury 40 1993 running hot.
Submitted by Leighwydra on Tue, 2013-09-17 05:53
I have a mercury 40hp 1993 that I have rebuilt (new piston, rings and reed valves) but now it seems to be running very hot. Its hot enough to keep your hand under for 1 or 2 seconds from idle to half throttle, at WOT its steaming. I have replaced the impeller which increased water flow slightly (old one was worn). It takes a while for the water to get through and heats up quickly even from a cold engine. I have heard some models have thermostats that can get jammed but can't find one in the manual. Or could it be a blockage somewhere in the engine. Any advice would be appreciated, summer is coming fast!
makai
Posts: 459
Date Joined: 28/10/08
Have you tried
Have you tried the really simple things? My 15HP Johnson wasn't pumping water properly and after changing the impeller it was no better. Turned out the outlet hose from the block to the housing (the telltale) was slightly kinked and had some sand in it. I took it off, cleaned it out, made sure it went back on without the kink and all was good again.
Paul H
Posts: 2104
Date Joined: 18/01/07
Telltale itself being blocked
Telltale itself being blocked should be enough to heat it up that much - its only there to let you know the waters flowing through. most of the water flows comes out elsewhere. would be finding out and fixing the problem before other damage is done.
Correct me if I'm wrong But I didn't think they had thermostats??
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inoz
Posts: 18
Date Joined: 12/11/10
merc 40
hi leigh, i have a 45 merc classic fifty and i,m pretty sure it doesn,t have a thermostat. just changed my impeller recently and the telltale i thought wasn,t too bad was instantly 5 times stronger. make sure the tube is clear of crap and seats into the grommet correctly .i am tending to think this may be a case of impeller blades pointing the wrong way. good luck.
Leighwydra
Posts: 5
Date Joined: 29/04/13
narrowed it down
Thanks for the advice guys.
I was pretty sure mine didn't have a thermostat, there are a lot of other similar models that do so I just wanted to be sure.
I took the head & exhaust water jacket covers off to make sure there were no blockages, all was good.
I dropped the leg to check the impeller, which was replaced only about 6 months ago with little use, everything seemed fine. While it was off I put the hose directly on the inlet tube and started it up. I adjusted the water flow to what normally comes out of the tell-tale and the engine started getting hot but I only had to turn the hose up the smallest amount and the temp came down to just warm. I know the engine ran about a minute with the old impeller dry so maybe it damaged the housing so i'll get a new one and try that. Surely its the last thing it can be...
sstevee
Posts: 472
Date Joined: 15/11/11
Might be worth checking that
Might be worth checking that it hasn't chewed out the little 'key' that stops the impeller from spinning on the shaft.
Leighwydra
Posts: 5
Date Joined: 29/04/13
All sorted
Thanks guys. Its all sorted now. I replaced the whole water pump and housing. It had signs of melting but now the tell-tale is running warm through all speeds!