1990 Mercury 175 Overheat warning possible causes?

OldGuy77

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Aug 29, 2020
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28
Put the boat back in the water after being out for nearly a year. Had to put in a new trim and tilt unit. Put the engine on muffs and started it up and was okay - but didn't see it peeing - but water was coming out of the prop - ran it for less than a minute. Next day I put her in the water and the engine started up quickly and idled smoothly but wasn't peeing - friend said accelerate it - I did and got good water flow out of the telltale. Took her on a test run which ended after about 5-10 mins with an overheat warning. Waited for 10 plus minutes till I got the go ahead beep beep when I turned the key on - engine started with no problem and as the boat started moving I accelerated slowly and saw water from the telltale accelerated and ran it at about 3K for 5 mins before overheat warning sounded - turned off the engine and brought her back to the marina on my trolling motor. Once in the dry storage pulled my thermostats and they looked crusted - but think it well could be the poppet valve. Plan is to replace the thermos and get a new poppet valve kit and install it. Put in a complete new lower unit in the late fall of 2022 which included a new water pump and impeller. The engine ran great from Nov 2022 till May 2023 when the trim and tilt went out. Took me 3 months and alot of aggravation to get the tri lobe pin out - think it was the original. After that other factors ie vacation and weather issues kept me from completing the work. So now first time out and Im back under the cover and trying to resolve the cooling system issue. Any advice is appreciated.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,187
The usual causes for overheat are worn water pump parts, bad thermostats, and faulty poppit. If you fix those and she still shows overheat, it could be your head gasket(s) leaking. Put a torque wrench on your head bolts and see if they are loose. Those motors had a tendency for them to loosen up.

Also, trace the telltale back to the engine and see if the thermostats need to open, before you get a telltale. If so, on a flusher at idle, the thermostats may not open, so no telltale.
 

OldGuy77

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Aug 29, 2020
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28
Thanks - appreciate the reply affirms my thoughts - but wasn't aware of the head bolt scenario will check them. Have new thermostats and parts to replace the poppet arrive today. There is a great video on replacing the poppet valve on you tube by Pro Wrenching - to be honest I didn't know there was a poppet valve in the system and its well hidden below the switch boxes etc.
 

Texasmark

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Joined
Dec 20, 2005
Messages
14,568
Thanks - appreciate the reply affirms my thoughts - but wasn't aware of the head bolt scenario will check them. Have new thermostats and parts to replace the poppet arrive today. There is a great video on replacing the poppet valve on you tube by Pro Wrenching - to be honest I didn't know there was a poppet valve in the system and its well hidden below the switch boxes etc.
The stats are for slow speed heating the combustion chamber to prevent fouling and better combustion. The poppet is larger in area than the stats and essentially opens the combustion for full flow of cooling water at the higher speeds. For my 4 cyliinder Mercs it happens at about 2500 RPM......below that speed it would be a stat problem, above the poppet.....BTDT.....launched a new to me used boat, got on plane and after a short time got an overheat warning......slowed down (off plane) putting along and overheat shut off.....repeated a couple of times same thing. Went home pulled the cover off the stat/poppet and the poppet was stuck shut.
 

OldGuy77

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Joined
Aug 29, 2020
Messages
28
The stats are for slow speed heating the combustion chamber to prevent fouling and better combustion. The poppet is larger in area than the stats and essentially opens the combustion for full flow of cooling water at the higher speeds. For my 4 cyliinder Mercs it happens at about 2500 RPM......below that speed it would be a stat problem, above the poppet.....BTDT.....launched a new to me used boat, got on plane and after a short time got an overheat warning......slowed down (off plane) putting along and overheat shut off.....repeated a couple of times same thing. Went home pulled the cover off the stat/poppet and the poppet was stuck shut.
Good info - pretty sure my poppet is stuck shut too - pulled the stats and am replacing them - pulled the cover off the poppet and ran into another problem - no one has probably ever changed the poppet and its the old style to piece cover - top cover comes off easy but now have to figure out how to get the second cover off - drenched it with pro blaster and guess heat is next plus trying to get something into the seam to try and wedge it out - saw some video on a guy doing a Yamaha with a similar problem - wasn't encouraging - but hope springs eternal. Thanks for your help.
 

Dukedog

Captain
Joined
Oct 6, 2009
Messages
3,269

jan... you would be ahead by removing tha lower cowls also.. four 1/4 20 screws (either 3/8 or 5/16 head under neath tha cowl.. a little more work but well worth it for removing and replacing tha poppet.....
 
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