1979 1980 mercruiser 260hp 350 elbow & exhaust manifold gaskets & metal plate gasket

knightowl

Petty Officer 2nd Class
Joined
Sep 20, 2008
Messages
111
1979 1980 mercruiser 260hp 350 elbow & exhaust manifold gaskets & metal plate gasket

I want to figure this out once and for all. Last winter my engine ran very high temp around 180 to 200 degrees and higher if i went faster. I knew i had an overheating problem so i tried to keep it around 180. The brand new rebuilt engine ran hot to begin with :(

Anyways, this year i replaced the impeller and replaced my old welded manifolds because last year no matter how i placed my gasket's between the "raisers elbows and log style manifold" one of the two was overheating, either engine or manifolds.

With the new parts changed, everything is running nice and cooled this year. However, Ive notice my boat doesn't go past 140 degrees with a 142 degree thermostat. When my engine idles, it stays around 100 degrees. At cruising speeds its around 115 to 125. Full speed barely around 140.

I personally think my cooling system is dumping my water out just a little too fast. I DON'T have the metal gasket plates between the manifold and the raisers elbows. The reason is this. 1 dealership says you need them in fact most of them say i need them. But those guys are just "by the book" employees of the dealership and are very hard to trust. However the specialist that work strictly on boat engines dealership say my engine does need the plate gasket.

One thing is for sure, when last year i had overheating problems, the plates did keep my engine hotter but removing them caused my elbows to get hotter.

I know if i put the plates in, my engine will no longer be at 140 and below. I am sure it would be around 142 degrees to possibly 180? ? ?

So my question is do i really need the plates? Is it better to run your engine cooler or run it hotter and save some gas? How much gas does it save if you run around 160 degrees instead of 110 for example? I know the standered mercruiser manual says i need the plate for that engine but why do i keep getting two points of views?? The manual also says engine must have a 142 degree thermostat so this means the engine should be around 140-150 at it hottest point??

What do you think? Should i try with the plates now since i got a new impeller and manifolds and elbows? THANKS!
 

sarantis

Chief Petty Officer
Joined
Feb 4, 2007
Messages
436
Re: 1979 1980 mercruiser 260hp 350 elbow & exhaust manifold gaskets & metal plate gas

Re: 1979 1980 mercruiser 260hp 350 elbow & exhaust manifold gaskets & metal plate gas

i have similar problems.If you search my threads Don s had told me that at this cooling system no water should flow from exhaust manifolds to the risers.There must be the block off plates with one small drain hole (which also lets any air bubbles to exit the cooling circuit).Personally in the past i run without these plates,the temp indeed was low but i had overheatings to the
intake manifold at the crossover point.Anyway i put the plates on,i swaped from 305 to350,therefore i ' m not sure i ve solved my temp problems.as about economy ,i prefer to make my boat work properly and then look for economy.
 

knightowl

Petty Officer 2nd Class
Joined
Sep 20, 2008
Messages
111
Re: 1979 1980 mercruiser 260hp 350 elbow & exhaust manifold gaskets & metal plate gas

Re: 1979 1980 mercruiser 260hp 350 elbow & exhaust manifold gaskets & metal plate gas

so your saying add the plates... what is the normal temp for the merc 350 260hp? 100 to 142 or 142 to 180?
 

sarantis

Chief Petty Officer
Joined
Feb 4, 2007
Messages
436
Re: 1979 1980 mercruiser 260hp 350 elbow & exhaust manifold gaskets & metal plate gas

Re: 1979 1980 mercruiser 260hp 350 elbow & exhaust manifold gaskets & metal plate gas

putting in a boiler the thermostat ,the sender and a thermometer and counting the resistance of the sender with an ohmmeter (that later i applied on the boat to get sure about the engine's temperature) i find out that :
the thermostat starts open at about 146 and if fully opened at 165 so i would say that up to 175 is a safe temperature.I learned in an older threead that above 194 there is definetely a problem (this is also the alarm valve temp).Personally my boat tends to go easily to to 176 running at about 3000 rpm for 2-3 minutes (i don't know if the seawater temp which here is 68 has to do with it) so i intend first to return to its original 17 propeler (now 19).If i have no significant results i intend to enlarge a little the hole of the plates .Yes i think you have to use them.Anyway i don't know if this old cooling system is can provides with sufficient cooling the engine especially at worm climates and if this is the reason that later Merc changed it and put the risers at the center of the manifolds with more direct flow of water from manifolds to the risers.I hope that some experts as Don S and Bond O give us their Knowledge.Keep in touch.
 

knightowl

Petty Officer 2nd Class
Joined
Sep 20, 2008
Messages
111
Re: 1979 1980 mercruiser 260hp 350 elbow & exhaust manifold gaskets & metal plate gas

Re: 1979 1980 mercruiser 260hp 350 elbow & exhaust manifold gaskets & metal plate gas

today went fishing, boat stays around 120 never higher. I will put the plates on.
 

knightowl

Petty Officer 2nd Class
Joined
Sep 20, 2008
Messages
111
Re: 1979 1980 mercruiser 260hp 350 elbow & exhaust manifold gaskets & metal plate gas

Re: 1979 1980 mercruiser 260hp 350 elbow & exhaust manifold gaskets & metal plate gas

Put the plates on, but before i did, i took your advice by drilling the hole just a bit larger.

Yesterday tested the boat in a 85 degree weather... Boat ran around 140 to 160. Wonderful. Thanks a bunch for the tip. Keep in mind i also bought BRAND NEW "log style manifolds" and also installed a brand new impeller!

Thanks a bunch fella's!
 
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